LinuxLists.cc - [PATCH 1/2] sched: Remove update_rq_runnable

2014-07-02 10:34:08

Subject: [PATCH 1/2] sched: Remove update_rq_runnable_avg

The current rq->avg is not made use of anywhere, and the code is in fair
scheduler's critical path, so remove it.

Signed-off-by: Yuyang Du <[email protected]>
---
kernel/sched/debug.c | 8 --------
kernel/sched/fair.c | 24 ++++--------------------
kernel/sched/sched.h | 2 --
3 files changed, 4 insertions(+), 30 deletions(-)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 695f977..4b864c7 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -68,14 +68,6 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group
#define PN(F) \
SEQ_printf(m, " .%-30s: %lld.%06ld\n", #F, SPLIT_NS((long long)F))

- if (!se) {
- struct sched_avg *avg = &cpu_rq(cpu)->avg;
- P(avg->runnable_avg_sum);
- P(avg->runnable_avg_period);
- return;
- }
-
-
PN(se->exec_start);
PN(se->vruntime);
PN(se->sum_exec_runtime);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fea7d33..1a2d04f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2430,18 +2430,12 @@ static inline void __update_group_entity_contrib(struct sched_entity *se)
}
}

-static inline void update_rq_runnable_avg(struct rq *rq, int runnable)
-{
- __update_entity_runnable_avg(rq_clock_task(rq), &rq->avg, runnable);
- __update_tg_runnable_avg(&rq->avg, &rq->cfs);
-}
#else /* CONFIG_FAIR_GROUP_SCHED */
static inline void __update_cfs_rq_tg_load_contrib(struct cfs_rq *cfs_rq,
int force_update) {}
static inline void __update_tg_runnable_avg(struct sched_avg *sa,
struct cfs_rq *cfs_rq) {}
static inline void __update_group_entity_contrib(struct sched_entity *se) {}
-static inline void update_rq_runnable_avg(struct rq *rq, int runnable) {}
#endif /* CONFIG_FAIR_GROUP_SCHED */

static inline void __update_task_entity_contrib(struct sched_entity *se)
@@ -2614,7 +2608,6 @@ static inline void dequeue_entity_load_avg(struct cfs_rq *cfs_rq,
*/
void idle_enter_fair(struct rq *this_rq)
{
- update_rq_runnable_avg(this_rq, 1);
}

/*
@@ -2624,7 +2617,6 @@ void idle_enter_fair(struct rq *this_rq)
*/
void idle_exit_fair(struct rq *this_rq)
{
- update_rq_runnable_avg(this_rq, 0);
}

static int idle_balance(struct rq *this_rq);
@@ -2633,7 +2625,6 @@ static int idle_balance(struct rq *this_rq);

static inline void update_entity_load_avg(struct sched_entity *se,
int update_cfs_rq) {}
-static inline void update_rq_runnable_avg(struct rq *rq, int runnable) {}
static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq,
struct sched_entity *se,
int wakeup) {}
@@ -3936,10 +3927,9 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
update_entity_load_avg(se, 1);
}

- if (!se) {
- update_rq_runnable_avg(rq, rq->nr_running);
+ if (!se)
add_nr_running(rq, 1);
- }
+
hrtick_update(rq);
}

@@ -3997,10 +3987,9 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
update_entity_load_avg(se, 1);
}

- if (!se) {
+ if (!se)
sub_nr_running(rq, 1);
- update_rq_runnable_avg(rq, 1);
- }
+
hrtick_update(rq);
}

@@ -5437,9 +5426,6 @@ static void __update_blocked_averages_cpu(struct task_group *tg, int cpu)
*/
if (!se->avg.runnable_avg_sum && !cfs_rq->nr_running)
list_del_leaf_cfs_rq(cfs_rq);
- } else {
- struct rq *rq = rq_of(cfs_rq);
- update_rq_runnable_avg(rq, rq->nr_running);
}
}

@@ -7352,8 +7338,6 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)

if (numabalancing_enabled)
task_tick_numa(rq, curr);
-
- update_rq_runnable_avg(rq, 1);
}

/*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 31cc02e..a147571 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -542,8 +542,6 @@ struct rq {
#ifdef CONFIG_FAIR_GROUP_SCHED
/* list of leaf cfs_rq on this cpu: */
struct list_head leaf_cfs_rq_list;
-
- struct sched_avg avg;
#endif /* CONFIG_FAIR_GROUP_SCHED */

/*
--
1.7.9.5

2014-07-02 10:34:15

by Yuyang Du

[permalink] [raw]

Subject: [PATCH 2/2] sched: Rewrite per entity runnable load average tracking

The idea of per entity runnable load average (aggregated to cfs_rq and task_group load)
was proposed by Paul Turner, and it is still followed by this rewrite. But this rewrite
is made due to the following ends:

(1). cfs_rq's load average (namely runnable_load_avg and blocked_load_avg) is updated
incrementally by one entity at one time, which means the cfs_rq load average is only
partially updated or asynchronous accross its entities (the entity in question is up
to date and contributes to the cfs_rq, but all other entities are effectively lagging
behind).

(2). cfs_rq load average is different between top rq->cfs_rq and task_group's per CPU
cfs_rqs in whether or not blocked_load_average contributes to the load.

(3). How task_group's load is tracked is very confusing and complex.

Therefore, this rewrite tackles these by:

(1). Combine runnable and blocked load averages for cfs_rq. And track cfs_rq's load average
as a whole (contributed by all runnabled and blocked entities on this cfs_rq).

(2). Only track task load average. Do not track task_group's per CPU entity average, but
track that entity's own cfs_rq's aggregated average.

This rewrite resutls in significantly reduced codes and expected consistency and clarity.
Also, if draw the lines of previous cfs_rq runnable_load_avg and blocked_load_avg and the
new rewritten load_avg, then compare those lines, you can see the new load_avg is much
more continuous (no abrupt jumping ups and downs) and decayed/updated more quickly and
synchronously.

Signed-off-by: Yuyang Du <[email protected]>
---
include/linux/sched.h | 13 +-
kernel/sched/debug.c | 22 +--
kernel/sched/fair.c | 475 +++++++++++--------------------------------------
kernel/sched/proc.c | 2 +-
kernel/sched/sched.h | 17 +-
5 files changed, 115 insertions(+), 414 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 306f4f0..7abdd13 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1069,14 +1069,11 @@ struct load_weight {

struct sched_avg {
/*
- * These sums represent an infinite geometric series and so are bound
- * above by 1024/(1-y). Thus we only need a u32 to store them for all
- * choices of y < 1-2^(-32)*1024.
+ * The load_avg represents an infinite geometric series.
*/
- u32 runnable_avg_sum, runnable_avg_period;
- u64 last_runnable_update;
- s64 decay_count;
- unsigned long load_avg_contrib;
+ u32 load_avg;
+ u32 period_contrib;
+ u64 last_update_time;
};

#ifdef CONFIG_SCHEDSTATS
@@ -1142,7 +1139,7 @@ struct sched_entity {
#endif

#ifdef CONFIG_SMP
- /* Per-entity load-tracking */
+ /* Per task load tracking */
struct sched_avg avg;
#endif
};
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 4b864c7..547a01b 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -85,10 +85,7 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group
#endif
P(se->load.weight);
#ifdef CONFIG_SMP
- P(se->avg.runnable_avg_sum);
- P(se->avg.runnable_avg_period);
- P(se->avg.load_avg_contrib);
- P(se->avg.decay_count);
+ P(se->my_q->avg.load_avg);
#endif
#undef PN
#undef P
@@ -205,19 +202,11 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
SEQ_printf(m, " .%-30s: %d\n", "nr_running", cfs_rq->nr_running);
SEQ_printf(m, " .%-30s: %ld\n", "load", cfs_rq->load.weight);
#ifdef CONFIG_SMP
- SEQ_printf(m, " .%-30s: %ld\n", "runnable_load_avg",
- cfs_rq->runnable_load_avg);
- SEQ_printf(m, " .%-30s: %ld\n", "blocked_load_avg",
- cfs_rq->blocked_load_avg);
+ SEQ_printf(m, " .%-30s: %u\n", "load_avg",
+ cfs_rq->avg.load_avg);
#ifdef CONFIG_FAIR_GROUP_SCHED
- SEQ_printf(m, " .%-30s: %ld\n", "tg_load_contrib",
- cfs_rq->tg_load_contrib);
- SEQ_printf(m, " .%-30s: %d\n", "tg_runnable_contrib",
- cfs_rq->tg_runnable_contrib);
SEQ_printf(m, " .%-30s: %ld\n", "tg_load_avg",
atomic_long_read(&cfs_rq->tg->load_avg));
- SEQ_printf(m, " .%-30s: %d\n", "tg->runnable_avg",
- atomic_read(&cfs_rq->tg->runnable_avg));
#endif
#endif
#ifdef CONFIG_CFS_BANDWIDTH
@@ -624,10 +613,7 @@ void proc_sched_show_task(struct task_struct *p, struct seq_file *m)

P(se.load.weight);
#ifdef CONFIG_SMP
- P(se.avg.runnable_avg_sum);
- P(se.avg.runnable_avg_period);
- P(se.avg.load_avg_contrib);
- P(se.avg.decay_count);
+ P(se.avg.load_avg);
#endif
P(policy);
P(prio);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1a2d04f..9b442cd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -282,9 +282,6 @@ static inline struct cfs_rq *group_cfs_rq(struct sched_entity *grp)
return grp->my_q;
}

-static void update_cfs_rq_blocked_load(struct cfs_rq *cfs_rq,
- int force_update);
-
static inline void list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
{
if (!cfs_rq->on_list) {
@@ -304,8 +301,6 @@ static inline void list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
}

cfs_rq->on_list = 1;
- /* We should have no load, but we need to update last_decay. */
- update_cfs_rq_blocked_load(cfs_rq, 0);
}
}

@@ -667,18 +662,19 @@ static u64 sched_vslice(struct cfs_rq *cfs_rq, struct sched_entity *se)
#ifdef CONFIG_SMP
static unsigned long task_h_load(struct task_struct *p);

-static inline void __update_task_entity_contrib(struct sched_entity *se);
+static void __update_load_avg(u64 now, struct sched_avg *sa, unsigned long w);

/* Give new task start runnable values to heavy its load in infant time */
void init_task_runnable_average(struct task_struct *p)
{
u32 slice;
+ struct sched_avg *sa = &p->se.avg;

- p->se.avg.decay_count = 0;
+ sa->last_update_time = 0;
+ sa->period_contrib = 0;
slice = sched_slice(task_cfs_rq(p), &p->se) >> 10;
- p->se.avg.runnable_avg_sum = slice;
- p->se.avg.runnable_avg_period = slice;
- __update_task_entity_contrib(&p->se);
+ sa->load_avg = slice * p->se.load.weight;
+ /* when this task enqueue'ed, it will contribute to its cfs_rq's load_avg */
}
#else
void init_task_runnable_average(struct task_struct *p)
@@ -1504,8 +1500,13 @@ static u64 numa_get_avg_runtime(struct task_struct *p, u64 *period)
delta = runtime - p->last_sum_exec_runtime;
*period = now - p->last_task_numa_placement;
} else {
- delta = p->se.avg.runnable_avg_sum;
- *period = p->se.avg.runnable_avg_period;
+ /*
+ * XXX previous runnable_avg_sum and runnable_avg_period are
+ * only used here. May find a way to better suit NUMA here.
+ */
+
+ delta = p->se.avg.load_avg / p->se.avg.load.weight;
+ *period = LOAD_AVG_MAX;
}

p->last_sum_exec_runtime = runtime;
@@ -2071,13 +2072,9 @@ static inline long calc_tg_weight(struct task_group *tg, struct cfs_rq *cfs_rq)
long tg_weight;

/*
- * Use this CPU's actual weight instead of the last load_contribution
- * to gain a more accurate current total weight. See
- * update_cfs_rq_load_contribution().
+ * Use this CPU's load average instead of actual weight
*/
tg_weight = atomic_long_read(&tg->load_avg);
- tg_weight -= cfs_rq->tg_load_contrib;
- tg_weight += cfs_rq->load.weight;

return tg_weight;
}
@@ -2087,7 +2084,7 @@ static long calc_cfs_shares(struct cfs_rq *cfs_rq, struct task_group *tg)
long tg_weight, load, shares;

tg_weight = calc_tg_weight(tg, cfs_rq);
- load = cfs_rq->load.weight;
+ load = cfs_rq->avg.load_avg;

shares = (tg->shares * load);
if (tg_weight)
@@ -2266,22 +2263,21 @@ static u32 __compute_runnable_contrib(u64 n)
* load_avg = u_0` + y*(u_0 + u_1*y + u_2*y^2 + ... )
* = u_0 + u_1*y + u_2*y^2 + ... [re-labeling u_i --> u_{i+1}]
*/
-static __always_inline int __update_entity_runnable_avg(u64 now,
- struct sched_avg *sa,
- int runnable)
+static __always_inline void
+__update_load_avg(u64 now, struct sched_avg *sa, unsigned long w)
{
u64 delta, periods;
- u32 runnable_contrib;
- int delta_w, decayed = 0;
+ u32 contrib;
+ int delta_w;

- delta = now - sa->last_runnable_update;
+ delta = now - sa->last_update_time;
/*
* This should only happen when time goes backwards, which it
* unfortunately does during sched clock init when we swap over to TSC.
*/
if ((s64)delta < 0) {
- sa->last_runnable_update = now;
- return 0;
+ sa->last_update_time = now;
+ return;
}

/*
@@ -2290,14 +2286,14 @@ static __always_inline int __update_entity_runnable_avg(u64 now,
*/
delta >>= 10;
if (!delta)
- return 0;
- sa->last_runnable_update = now;
+ return;
+ sa->last_update_time = now;

/* delta_w is the amount already accumulated against our next period */
- delta_w = sa->runnable_avg_period % 1024;
+ delta_w = sa->period_contrib;
if (delta + delta_w >= 1024) {
- /* period roll-over */
- decayed = 1;
+ /* how much left for next period will start over, we don't know yet */
+ sa->period_contrib = 0;

/*
* Now that we know we're crossing a period boundary, figure
@@ -2305,9 +2301,8 @@ static __always_inline int __update_entity_runnable_avg(u64 now,
* period and accrue it.
*/
delta_w = 1024 - delta_w;
- if (runnable)
- sa->runnable_avg_sum += delta_w;
- sa->runnable_avg_period += delta_w;
+ if (w)
+ sa->load_avg += w * delta_w;

delta -= delta_w;

@@ -2315,290 +2310,77 @@ static __always_inline int __update_entity_runnable_avg(u64 now,
periods = delta / 1024;
delta %= 1024;

- sa->runnable_avg_sum = decay_load(sa->runnable_avg_sum,
- periods + 1);
- sa->runnable_avg_period = decay_load(sa->runnable_avg_period,
- periods + 1);
+ sa->load_avg = decay_load(sa->load_avg, periods + 1);

/* Efficiently calculate \sum (1..n_period) 1024*y^i */
- runnable_contrib = __compute_runnable_contrib(periods);
- if (runnable)
- sa->runnable_avg_sum += runnable_contrib;
- sa->runnable_avg_period += runnable_contrib;
+ contrib = __compute_runnable_contrib(periods);
+ if (w)
+ sa->load_avg += w * contrib;
}

/* Remainder of delta accrued against u_0` */
- if (runnable)
- sa->runnable_avg_sum += delta;
- sa->runnable_avg_period += delta;
-
- return decayed;
-}
-
-/* Synchronize an entity's decay with its parenting cfs_rq.*/
-static inline u64 __synchronize_entity_decay(struct sched_entity *se)
-{
- struct cfs_rq *cfs_rq = cfs_rq_of(se);
- u64 decays = atomic64_read(&cfs_rq->decay_counter);
-
- decays -= se->avg.decay_count;
- if (!decays)
- return 0;
+ if (w)
+ sa->load_avg += w * delta;

- se->avg.load_avg_contrib = decay_load(se->avg.load_avg_contrib, decays);
- se->avg.decay_count = 0;
+ sa->period_contrib += delta;

- return decays;
+ return;
}

#ifdef CONFIG_FAIR_GROUP_SCHED
-static inline void __update_cfs_rq_tg_load_contrib(struct cfs_rq *cfs_rq,
- int force_update)
-{
- struct task_group *tg = cfs_rq->tg;
- long tg_contrib;
-
- tg_contrib = cfs_rq->runnable_load_avg + cfs_rq->blocked_load_avg;
- tg_contrib -= cfs_rq->tg_load_contrib;
-
- if (force_update || abs(tg_contrib) > cfs_rq->tg_load_contrib / 8) {
- atomic_long_add(tg_contrib, &tg->load_avg);
- cfs_rq->tg_load_contrib += tg_contrib;
- }
-}
-
-/*
- * Aggregate cfs_rq runnable averages into an equivalent task_group
- * representation for computing load contributions.
- */
-static inline void __update_tg_runnable_avg(struct sched_avg *sa,
- struct cfs_rq *cfs_rq)
+static inline void synchronize_tg_load_avg(struct cfs_rq *cfs_rq, u32 old)
{
- struct task_group *tg = cfs_rq->tg;
- long contrib;
-
- /* The fraction of a cpu used by this cfs_rq */
- contrib = div_u64((u64)sa->runnable_avg_sum << NICE_0_SHIFT,
- sa->runnable_avg_period + 1);
- contrib -= cfs_rq->tg_runnable_contrib;
+ s32 delta = cfs_rq->avg.load_avg - old;

- if (abs(contrib) > cfs_rq->tg_runnable_contrib / 64) {
- atomic_add(contrib, &tg->runnable_avg);
- cfs_rq->tg_runnable_contrib += contrib;
- }
-}
-
-static inline void __update_group_entity_contrib(struct sched_entity *se)
-{
- struct cfs_rq *cfs_rq = group_cfs_rq(se);
- struct task_group *tg = cfs_rq->tg;
- int runnable_avg;
-
- u64 contrib;
-
- contrib = cfs_rq->tg_load_contrib * tg->shares;
- se->avg.load_avg_contrib = div_u64(contrib,
- atomic_long_read(&tg->load_avg) + 1);
-
- /*
- * For group entities we need to compute a correction term in the case
- * that they are consuming <1 cpu so that we would contribute the same
- * load as a task of equal weight.
- *
- * Explicitly co-ordinating this measurement would be expensive, but
- * fortunately the sum of each cpus contribution forms a usable
- * lower-bound on the true value.
- *
- * Consider the aggregate of 2 contributions. Either they are disjoint
- * (and the sum represents true value) or they are disjoint and we are
- * understating by the aggregate of their overlap.
- *
- * Extending this to N cpus, for a given overlap, the maximum amount we
- * understand is then n_i(n_i+1)/2 * w_i where n_i is the number of
- * cpus that overlap for this interval and w_i is the interval width.
- *
- * On a small machine; the first term is well-bounded which bounds the
- * total error since w_i is a subset of the period. Whereas on a
- * larger machine, while this first term can be larger, if w_i is the
- * of consequential size guaranteed to see n_i*w_i quickly converge to
- * our upper bound of 1-cpu.
- */
- runnable_avg = atomic_read(&tg->runnable_avg);
- if (runnable_avg < NICE_0_LOAD) {
- se->avg.load_avg_contrib *= runnable_avg;
- se->avg.load_avg_contrib >>= NICE_0_SHIFT;
- }
+ if (delta)
+ atomic_long_add(delta, &cfs_rq->tg->load_avg);
}

#else /* CONFIG_FAIR_GROUP_SCHED */
-static inline void __update_cfs_rq_tg_load_contrib(struct cfs_rq *cfs_rq,
- int force_update) {}
-static inline void __update_tg_runnable_avg(struct sched_avg *sa,
- struct cfs_rq *cfs_rq) {}
-static inline void __update_group_entity_contrib(struct sched_entity *se) {}
+static inline void synchronize_tg_load_avg(struct cfs_rq *cfs_rq, u32 old) {}
#endif /* CONFIG_FAIR_GROUP_SCHED */

-static inline void __update_task_entity_contrib(struct sched_entity *se)
-{
- u32 contrib;
-
- /* avoid overflowing a 32-bit type w/ SCHED_LOAD_SCALE */
- contrib = se->avg.runnable_avg_sum * scale_load_down(se->load.weight);
- contrib /= (se->avg.runnable_avg_period + 1);
- se->avg.load_avg_contrib = scale_load(contrib);
-}
-
-/* Compute the current contribution to load_avg by se, return any delta */
-static long __update_entity_load_avg_contrib(struct sched_entity *se)
-{
- long old_contrib = se->avg.load_avg_contrib;
-
- if (entity_is_task(se)) {
- __update_task_entity_contrib(se);
- } else {
- __update_tg_runnable_avg(&se->avg, group_cfs_rq(se));
- __update_group_entity_contrib(se);
- }
-
- return se->avg.load_avg_contrib - old_contrib;
-}
-
-static inline void subtract_blocked_load_contrib(struct cfs_rq *cfs_rq,
- long load_contrib)
-{
- if (likely(load_contrib < cfs_rq->blocked_load_avg))
- cfs_rq->blocked_load_avg -= load_contrib;
- else
- cfs_rq->blocked_load_avg = 0;
-}
-
static inline u64 cfs_rq_clock_task(struct cfs_rq *cfs_rq);

-/* Update a sched_entity's runnable average */
-static inline void update_entity_load_avg(struct sched_entity *se,
- int update_cfs_rq)
+/* Update task/cfs_rq load average */
+static inline void update_load_avg(struct sched_entity *se)
{
struct cfs_rq *cfs_rq = cfs_rq_of(se);
- long contrib_delta;
- u64 now;
+ u64 now = cfs_rq_clock_task(cfs_rq);
+ u32 old_load_avg = cfs_rq->avg.load_avg;

- /*
- * For a group entity we need to use their owned cfs_rq_clock_task() in
- * case they are the parent of a throttled hierarchy.
- */
if (entity_is_task(se))
- now = cfs_rq_clock_task(cfs_rq);
- else
- now = cfs_rq_clock_task(group_cfs_rq(se));
-
- if (!__update_entity_runnable_avg(now, &se->avg, se->on_rq))
- return;
-
- contrib_delta = __update_entity_load_avg_contrib(se);
+ __update_load_avg(now, &se->avg, se->on_rq * se->load.weight);

- if (!update_cfs_rq)
- return;
+ __update_load_avg(now, &cfs_rq->avg, cfs_rq->load.weight);

- if (se->on_rq)
- cfs_rq->runnable_load_avg += contrib_delta;
- else
- subtract_blocked_load_contrib(cfs_rq, -contrib_delta);
+ synchronize_tg_load_avg(cfs_rq, old_load_avg);
}

-/*
- * Decay the load contributed by all blocked children and account this so that
- * their contribution may appropriately discounted when they wake up.
- */
-static void update_cfs_rq_blocked_load(struct cfs_rq *cfs_rq, int force_update)
+/* Add the load generated by se into cfs_rq's load average */
+static inline void enqueue_entity_load_avg(struct sched_entity *se)
{
- u64 now = cfs_rq_clock_task(cfs_rq) >> 20;
- u64 decays;
-
- decays = now - cfs_rq->last_decay;
- if (!decays && !force_update)
- return;
-
- if (atomic_long_read(&cfs_rq->removed_load)) {
- unsigned long removed_load;
- removed_load = atomic_long_xchg(&cfs_rq->removed_load, 0);
- subtract_blocked_load_contrib(cfs_rq, removed_load);
- }
-
- if (decays) {
- cfs_rq->blocked_load_avg = decay_load(cfs_rq->blocked_load_avg,
- decays);
- atomic64_add(decays, &cfs_rq->decay_counter);
- cfs_rq->last_decay = now;
- }
-
- __update_cfs_rq_tg_load_contrib(cfs_rq, force_update);
-}
+ struct sched_avg *sa = &se->avg;
+ struct cfs_rq *cfs_rq = cfs_rq_of(se);
+ u64 now = cfs_rq_clock_task(cfs_rq);
+ u32 old_load_avg = cfs_rq->avg.load_avg;
+ int migrated = 0;

-/* Add the load generated by se into cfs_rq's child load-average */
-static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq,
- struct sched_entity *se,
- int wakeup)
-{
- /*
- * We track migrations using entity decay_count <= 0, on a wake-up
- * migration we use a negative decay count to track the remote decays
- * accumulated while sleeping.
- *
- * Newly forked tasks are enqueued with se->avg.decay_count == 0, they
- * are seen by enqueue_entity_load_avg() as a migration with an already
- * constructed load_avg_contrib.
- */
- if (unlikely(se->avg.decay_count <= 0)) {
- se->avg.last_runnable_update = rq_clock_task(rq_of(cfs_rq));
- if (se->avg.decay_count) {
- /*
- * In a wake-up migration we have to approximate the
- * time sleeping. This is because we can't synchronize
- * clock_task between the two cpus, and it is not
- * guaranteed to be read-safe. Instead, we can
- * approximate this using our carried decays, which are
- * explicitly atomically readable.
- */
- se->avg.last_runnable_update -= (-se->avg.decay_count)
- << 20;
- update_entity_load_avg(se, 0);
- /* Indicate that we're now synchronized and on-rq */
- se->avg.decay_count = 0;
+ if (entity_is_task(se)) {
+ if (sa->last_update_time == 0) {
+ sa->last_update_time = now;
+ migrated = 1;
}
- wakeup = 0;
- } else {
- __synchronize_entity_decay(se);
- }
-
- /* migrated tasks did not contribute to our blocked load */
- if (wakeup) {
- subtract_blocked_load_contrib(cfs_rq, se->avg.load_avg_contrib);
- update_entity_load_avg(se, 0);
+ else
+ __update_load_avg(now, sa, se->on_rq * se->load.weight);
}

- cfs_rq->runnable_load_avg += se->avg.load_avg_contrib;
- /* we force update consideration on load-balancer moves */
- update_cfs_rq_blocked_load(cfs_rq, !wakeup);
-}
+ __update_load_avg(now, &cfs_rq->avg, cfs_rq->load.weight);

-/*
- * Remove se's load from this cfs_rq child load-average, if the entity is
- * transitioning to a blocked state we track its projected decay using
- * blocked_load_avg.
- */
-static inline void dequeue_entity_load_avg(struct cfs_rq *cfs_rq,
- struct sched_entity *se,
- int sleep)
-{
- update_entity_load_avg(se, 1);
- /* we force update consideration on load-balancer moves */
- update_cfs_rq_blocked_load(cfs_rq, !sleep);
+ if (migrated)
+ cfs_rq->avg.load_avg += sa->load_avg;

- cfs_rq->runnable_load_avg -= se->avg.load_avg_contrib;
- if (sleep) {
- cfs_rq->blocked_load_avg += se->avg.load_avg_contrib;
- se->avg.decay_count = atomic64_read(&cfs_rq->decay_counter);
- } /* migrations, e.g. sleep=0 leave decay_count == 0 */
+ synchronize_tg_load_avg(cfs_rq, old_load_avg);
}

/*
@@ -2623,16 +2405,8 @@ static int idle_balance(struct rq *this_rq);

#else /* CONFIG_SMP */

-static inline void update_entity_load_avg(struct sched_entity *se,
- int update_cfs_rq) {}
-static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq,
- struct sched_entity *se,
- int wakeup) {}
-static inline void dequeue_entity_load_avg(struct cfs_rq *cfs_rq,
- struct sched_entity *se,
- int sleep) {}
-static inline void update_cfs_rq_blocked_load(struct cfs_rq *cfs_rq,
- int force_update) {}
+static inline void update_load_avg(struct sched_entity *se) {}
+static inline void enqueue_entity_load_avg(struct sched_entity *se) {}

static inline int idle_balance(struct rq *rq)
{
@@ -2764,7 +2538,7 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
* Update run-time statistics of the 'current'.
*/
update_curr(cfs_rq);
- enqueue_entity_load_avg(cfs_rq, se, flags & ENQUEUE_WAKEUP);
+ enqueue_entity_load_avg(se);
account_entity_enqueue(cfs_rq, se);
update_cfs_shares(cfs_rq);

@@ -2839,7 +2613,8 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
* Update run-time statistics of the 'current'.
*/
update_curr(cfs_rq);
- dequeue_entity_load_avg(cfs_rq, se, flags & DEQUEUE_SLEEP);
+
+ update_load_avg(se);

update_stats_dequeue(cfs_rq, se);
if (flags & DEQUEUE_SLEEP) {
@@ -3028,7 +2803,7 @@ static void put_prev_entity(struct cfs_rq *cfs_rq, struct sched_entity *prev)
/* Put 'current' back into the tree. */
__enqueue_entity(cfs_rq, prev);
/* in !on_rq case, update occurred at dequeue */
- update_entity_load_avg(prev, 1);
+ update_load_avg(prev);
}
cfs_rq->curr = NULL;
}
@@ -3044,8 +2819,7 @@ entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr, int queued)
/*
* Ensure that runnable average is periodically updated.
*/
- update_entity_load_avg(curr, 1);
- update_cfs_rq_blocked_load(cfs_rq, 1);
+ update_load_avg(curr);
update_cfs_shares(cfs_rq);

#ifdef CONFIG_SCHED_HRTICK
@@ -3924,7 +3698,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
break;

update_cfs_shares(cfs_rq);
- update_entity_load_avg(se, 1);
+ update_load_avg(se);
}

if (!se)
@@ -3984,7 +3758,7 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
break;

update_cfs_shares(cfs_rq);
- update_entity_load_avg(se, 1);
+ update_load_avg(se);
}

if (!se)
@@ -3997,7 +3771,7 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
/* Used instead of source_load when we know the type == 0 */
static unsigned long weighted_cpuload(const int cpu)
{
- return cpu_rq(cpu)->cfs.runnable_load_avg;
+ return cpu_rq(cpu)->cfs.avg.load_avg;
}

/*
@@ -4042,7 +3816,7 @@ static unsigned long cpu_avg_load_per_task(int cpu)
{
struct rq *rq = cpu_rq(cpu);
unsigned long nr_running = ACCESS_ONCE(rq->nr_running);
- unsigned long load_avg = rq->cfs.runnable_load_avg;
+ unsigned long load_avg = rq->cfs.avg.load_avg;

if (nr_running)
return load_avg / nr_running;
@@ -4552,17 +4326,9 @@ migrate_task_rq_fair(struct task_struct *p, int next_cpu)
struct sched_entity *se = &p->se;
struct cfs_rq *cfs_rq = cfs_rq_of(se);

- /*
- * Load tracking: accumulate removed load so that it can be processed
- * when we next update owning cfs_rq under rq->lock. Tasks contribute
- * to blocked load iff they have a positive decay-count. It can never
- * be negative here since on-rq tasks have decay-count == 0.
- */
- if (se->avg.decay_count) {
- se->avg.decay_count = -__synchronize_entity_decay(se);
- atomic_long_add(se->avg.load_avg_contrib,
- &cfs_rq->removed_load);
- }
+ /* Update task on old CPU, then ready to go (entity must be off the queue) */
+ __update_load_avg(cfs_rq_clock_task(cfs_rq), &se->avg, 0);
+ se->avg.last_update_time = 0;

/* We have migrated, no longer consider this task hot */
se->exec_start = 0;
@@ -5399,36 +5165,6 @@ next:
}

#ifdef CONFIG_FAIR_GROUP_SCHED
-/*
- * update tg->load_weight by folding this cpu's load_avg
- */
-static void __update_blocked_averages_cpu(struct task_group *tg, int cpu)
-{
- struct sched_entity *se = tg->se[cpu];
- struct cfs_rq *cfs_rq = tg->cfs_rq[cpu];
-
- /* throttled entities do not contribute to load */
- if (throttled_hierarchy(cfs_rq))
- return;
-
- update_cfs_rq_blocked_load(cfs_rq, 1);
-
- if (se) {
- update_entity_load_avg(se, 1);
- /*
- * We pivot on our runnable average having decayed to zero for
- * list removal. This generally implies that all our children
- * have also been removed (modulo rounding error or bandwidth
- * control); however, such cases are rare and we can fix these
- * at enqueue.
- *
- * TODO: fix up out-of-order children on enqueue.
- */
- if (!se->avg.runnable_avg_sum && !cfs_rq->nr_running)
- list_del_leaf_cfs_rq(cfs_rq);
- }
-}
-
static void update_blocked_averages(int cpu)
{
struct rq *rq = cpu_rq(cpu);
@@ -5442,12 +5178,12 @@ static void update_blocked_averages(int cpu)
* list_add_leaf_cfs_rq() for details.
*/
for_each_leaf_cfs_rq(rq, cfs_rq) {
- /*
- * Note: We may want to consider periodically releasing
- * rq->lock about these updates so that creating many task
- * groups does not result in continually extending hold time.
- */
- __update_blocked_averages_cpu(cfs_rq->tg, rq->cpu);
+ u32 old_load_avg = cfs_rq->avg.load_avg;
+
+ __update_load_avg(cfs_rq_clock_task(cfs_rq),
+ &cfs_rq->avg, cfs_rq->load.weight);
+
+ synchronize_tg_load_avg(cfs_rq, old_load_avg);
}

raw_spin_unlock_irqrestore(&rq->lock, flags);
@@ -5477,14 +5213,14 @@ static void update_cfs_rq_h_load(struct cfs_rq *cfs_rq)
}

if (!se) {
- cfs_rq->h_load = cfs_rq->runnable_load_avg;
+ cfs_rq->h_load = cfs_rq->avg.load_avg;
cfs_rq->last_h_load_update = now;
}

while ((se = cfs_rq->h_load_next) != NULL) {
load = cfs_rq->h_load;
- load = div64_ul(load * se->avg.load_avg_contrib,
- cfs_rq->runnable_load_avg + 1);
+ load = div64_ul(load * se->avg.load_avg,
+ cfs_rq->avg.load_avg + 1);
cfs_rq = group_cfs_rq(se);
cfs_rq->h_load = load;
cfs_rq->last_h_load_update = now;
@@ -5496,8 +5232,8 @@ static unsigned long task_h_load(struct task_struct *p)
struct cfs_rq *cfs_rq = task_cfs_rq(p);

update_cfs_rq_h_load(cfs_rq);
- return div64_ul(p->se.avg.load_avg_contrib * cfs_rq->h_load,
- cfs_rq->runnable_load_avg + 1);
+ return div64_ul(p->se.avg.load_avg * cfs_rq->h_load,
+ cfs_rq->avg.load_avg + 1);
}
#else
static inline void update_blocked_averages(int cpu)
@@ -5506,7 +5242,7 @@ static inline void update_blocked_averages(int cpu)

static unsigned long task_h_load(struct task_struct *p)
{
- return p->se.avg.load_avg_contrib;
+ return p->se.avg.load_avg;
}
#endif

@@ -7437,14 +7173,11 @@ static void switched_from_fair(struct rq *rq, struct task_struct *p)

#ifdef CONFIG_SMP
/*
- * Remove our load from contribution when we leave sched_fair
- * and ensure we don't carry in an old decay_count if we
- * switch back.
+ * Remove our load from contribution when we leave cfs_rq.
*/
- if (se->avg.decay_count) {
- __synchronize_entity_decay(se);
- subtract_blocked_load_contrib(cfs_rq, se->avg.load_avg_contrib);
- }
+ cfs_rq->avg.load_avg -= se->avg.load_avg;
+ synchronize_tg_load_avg(cfs_rq, -se->avg.load_avg);
+
#endif
}

@@ -7500,10 +7233,6 @@ void init_cfs_rq(struct cfs_rq *cfs_rq)
#ifndef CONFIG_64BIT
cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime;
#endif
-#ifdef CONFIG_SMP
- atomic64_set(&cfs_rq->decay_counter, 1);
- atomic_long_set(&cfs_rq->removed_load, 0);
-#endif
}

#ifdef CONFIG_FAIR_GROUP_SCHED
@@ -7548,13 +7277,11 @@ static void task_move_group_fair(struct task_struct *p, int on_rq)
cfs_rq = cfs_rq_of(se);
se->vruntime += cfs_rq->min_vruntime;
#ifdef CONFIG_SMP
- /*
- * migrate_task_rq_fair() will have removed our previous
- * contribution, but we must synchronize for ongoing future
- * decay.
- */
- se->avg.decay_count = atomic64_read(&cfs_rq->decay_counter);
- cfs_rq->blocked_load_avg += se->avg.load_avg_contrib;
+ /* synchronize task with its new cfs_rq */
+ __update_load_avg(cfs_rq->avg.last_update_time, &p->se.avg, 0);
+
+ cfs_rq->avg.load_avg += p->se.avg.load_avg;
+ synchronize_tg_load_avg(cfs_rq, p->se.avg.load_avg);
#endif
}
}
diff --git a/kernel/sched/proc.c b/kernel/sched/proc.c
index 16f5a30..8f547fe 100644
--- a/kernel/sched/proc.c
+++ b/kernel/sched/proc.c
@@ -504,7 +504,7 @@ static void __update_cpu_load(struct rq *this_rq, unsigned long this_load,
#ifdef CONFIG_SMP
static inline unsigned long get_rq_runnable_load(struct rq *rq)
{
- return rq->cfs.runnable_load_avg;
+ return rq->cfs.avg.load_avg;
}
#else
static inline unsigned long get_rq_runnable_load(struct rq *rq)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index a147571..d68f069 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -210,7 +210,6 @@ struct task_group {

#ifdef CONFIG_SMP
atomic_long_t load_avg;
- atomic_t runnable_avg;
#endif
#endif

@@ -331,21 +330,13 @@ struct cfs_rq {

#ifdef CONFIG_SMP
/*
- * CFS Load tracking
- * Under CFS, load is tracked on a per-entity basis and aggregated up.
- * This allows for the description of both thread and group usage (in
- * the FAIR_GROUP_SCHED case).
+ * CFS load tracking
+ * XXX as load.weight could be large, the avg.load_avg may overflow
+ * its u32
*/
- unsigned long runnable_load_avg, blocked_load_avg;
- atomic64_t decay_counter;
- u64 last_decay;
- atomic_long_t removed_load;
+ struct sched_avg avg;

#ifdef CONFIG_FAIR_GROUP_SCHED
- /* Required to track per-cpu representation of a task_group */
- u32 tg_runnable_contrib;
- unsigned long tg_load_contrib;
-
/*
* h_load = weight * f(tg)
*
--
1.7.9.5

2014-07-07 10:07:55

by Peter Zijlstra

[permalink] [raw]

Subject: Re: [PATCH 2/2] sched: Rewrite per entity runnable load average tracking

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2014-07-07 10:46:56

On Mon, Jul 07, 2014 at 03:25:07PM -0700, [email protected] wrote:
> >> +static inline void enqueue_entity_load_avg(struct sched_entity *se)
> >> {
> >> + struct sched_avg *sa = &se->avg;
> >> + struct cfs_rq *cfs_rq = cfs_rq_of(se);
> >> + u64 now = cfs_rq_clock_task(cfs_rq);
> >> + u32 old_load_avg = cfs_rq->avg.load_avg;
> >> + int migrated = 0;
> >>
> >> + if (entity_is_task(se)) {
> >> + if (sa->last_update_time == 0) {
> >> + sa->last_update_time = now;
> >> + migrated = 1;
> >> }
> >> + else
> >> + __update_load_avg(now, sa, se->on_rq * se->load.weight);
> >> }
> >>
> >> + __update_load_avg(now, &cfs_rq->avg, cfs_rq->load.weight);
> >>
> >> + if (migrated)
> >> + cfs_rq->avg.load_avg += sa->load_avg;
> >>
> >> + synchronize_tg_load_avg(cfs_rq, old_load_avg);
> >> }
> >
> > So here you add the task to the cfs_rq avg when its got migrate in,
> > however:
> >
> >> @@ -4552,17 +4326,9 @@ migrate_task_rq_fair(struct task_struct *p, int next_cpu)
> >> struct sched_entity *se = &p->se;
> >> struct cfs_rq *cfs_rq = cfs_rq_of(se);
> >>
> >> + /* Update task on old CPU, then ready to go (entity must be off the queue) */
> >> + __update_load_avg(cfs_rq_clock_task(cfs_rq), &se->avg, 0);
> >> + se->avg.last_update_time = 0;
> >>
> >> /* We have migrated, no longer consider this task hot */
> >> se->exec_start = 0;
> >
> > there you don't remove it first..
>
> Yeah, the issue is that you can't remove it, because you don't hold the
> lock. Thus the whole runnable/blocked split iirc. Also the
> cfs_rq_clock_task read is incorrect for the same reason (and while
> rq_clock_task could certainly be fixed min_vruntime-style,
> cfs_rq_clock_task would be harder).
>
> The problem with just working around the clock issue somehow and then using an
> atomic to do this subtraction is that you have no idea when the /cfs_rq/
> last updated - there's no guarantee it is up to date, and if it's not
> then the subtraction is wrong. You can't update it to make it up to date
> like the se->avg, becasue you don't hold any locks. You would need
> decay_counter stuff like the current code, and I'm not certain how well
> that would work out without the runnable/blocked split.

Right; so the current code jumps through a few nasty hoops because of
this. But I think the proposed code got this wrong (understandably).

But yes, we spend a lot of time and effort to remove the rq->lock from
the remote wakeup path, which makes all this very tedious indeed.

Like you said, we can indeed make the time thing work, but the remote
subtraction is going to be messy. Can't seem to come up with anything
sane there either.

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2014-07-08 17:04:26

by Benjamin Segall

[permalink] [raw]

Subject: Re: [PATCH 2/2] sched: Rewrite per entity runnable load average tracking

Yuyang Du <[email protected]> writes:

> Thanks, Ben,
>
> On Mon, Jul 07, 2014 at 03:25:07PM -0700, [email protected] wrote:
>> Also something to note is that cfs_rq->load_avg just takes samples of
>> load.weight every 1us, which seems unfortunate. We thought this was ok
>> for p->se.load.weight, because it isn't really likely for userspace to
>> be calling nice(2) all the time, but wake/sleeps are more frequent,
>> particularly on newer cpus. Still, it might not be /that/ bad.
>
> The sampling of cfs_rq->load.weight should be equivalent to the current
> code in that at the end of day cfs_rq->load.weight worth of runnable would
> contribute to runnable_load_avg/blocked_load_avg for both the current and
> the rewrite.

Yes, but the point is that it looks at load.weight when delta/1024 > 0
and assumes that it has had that load.weight the entire time, when this
might not actually be true. The largest error I can think of is if you
have a very high weight task that tends to run for very short periods of
time, you could end up losing their entire contribution to load. This
may be acceptable, I'm not certain.

>
>> Also, as a nitpick/annoyance this does a lot of
>> if (entity_is_task(se)) __update_load_avg(... se ...)
>> __update_load_avg(... cfs_rq_of(se) ...)
>> which is just a waste of the avg struct on every group se, and all it
>> buys you is the ability to not have a separate rq->avg struct (removed
>> by patch 1) instead of rq->cfs.avg.
>
> I actually struggled on this issue. As we only need a sched_avg for task (not
> entity), and a sched_avg for cfs_rq, I planned to move entity avg to
> task. Good?

Yeah, that can probably be done perfectly fine.

>
> So left are the migrate_task_rq_fair() not holding lock issue and cfs_rq->avg.load_avg
> overflow issue. I need some time to study them.
>
> Overall, I think none of these issues are originally caused by combination/split
> of runnable and blocked. It is just a matter of how synchronized we want to be
> (this rewrite is the most synchronized), and the workaround I need to borrow from the
> current codes.

If you can manage to find a to deal with the migration issue without it,
that would be great, I'm just pretty sure that's why we did the split.

>
> Yuyang

2014-07-09 09:10:50

by Yuyang Du

[permalink] [raw]

Subject: Re: [PATCH 2/2] sched: Rewrite per entity runnable load average tracking

Thanks, Ben.

On Tue, Jul 08, 2014 at 10:04:22AM -0700, [email protected] wrote:

> > The sampling of cfs_rq->load.weight should be equivalent to the current
> > code in that at the end of day cfs_rq->load.weight worth of runnable would
> > contribute to runnable_load_avg/blocked_load_avg for both the current and
> > the rewrite.
>
> Yes, but the point is that it looks at load.weight when delta/1024 > 0
> and assumes that it has had that load.weight the entire time, when this
> might not actually be true. The largest error I can think of is if you
> have a very high weight task that tends to run for very short periods of
> time, you could end up losing their entire contribution to load. This
> may be acceptable, I'm not certain.

That I believe is not a problem. It is a matter of when cfs_rq->load.weight
changes and when we look at it to contribute to the cfs_rq's load_avg.
Fortunately, we will not miss any change of cfs_rq->load.weight, always
contributing to the load_avg the right amount. Put another way, we always
use the right cfs_rq->load.weight.

> > So left are the migrate_task_rq_fair() not holding lock issue and cfs_rq->avg.load_avg
> > overflow issue. I need some time to study them.
> >
> > Overall, I think none of these issues are originally caused by combination/split
> > of runnable and blocked. It is just a matter of how synchronized we want to be
> > (this rewrite is the most synchronized), and the workaround I need to borrow from the
> > current codes.
>
> If you can manage to find a to deal with the migration issue without it,
> that would be great, I'm just pretty sure that's why we did the split.

After thinking about it the whole morning, this issue is a killer... :)

But I think, even by spliting and by using atomic operations, the current code
does not substract the migrating task's load absolutely synchronized:

When se migrating, you atomic_read cfs_rq->decay_counter (say t1), and then atomic_add
the se's load_avg_contrib to removed_load. However, when updating the cfs_rq->blocked_load_avg
it is not guranteed when substracting the removed_load from the blocked_load_avg,
the actual decay_counter (say t2) is the same as t1. Because at t1 time (when you
atomic_read it), the decay_counter can be being updated (before you atomic_add to
removed_load). Atomic operation does not help synchronize them. Right?

So essentially, to perfectly substract the right amount, we must first synchronize
the migrating task with its cfs_rq (catch up), and then substract it right away.
Those two steps must be synchronized (with lock) or atomically done at once. Considering
migrating and load averaging are not sequential (strict interleaving). Separating the
two steps but synchronizing them is not enough, they must be done atomically.

That is chalenging... Can someone (Peter) grant us a lock of the remote rq? :)

Thanks,
Yuyang

2014-07-09 17:08:47

by Benjamin Segall

[permalink] [raw]

Subject: Re: [PATCH 2/2] sched: Rewrite per entity runnable load average tracking

Yuyang Du <[email protected]> writes:

> Thanks, Ben.
>
> On Tue, Jul 08, 2014 at 10:04:22AM -0700, [email protected] wrote:
>
>> > The sampling of cfs_rq->load.weight should be equivalent to the current
>> > code in that at the end of day cfs_rq->load.weight worth of runnable would
>> > contribute to runnable_load_avg/blocked_load_avg for both the current and
>> > the rewrite.
>>
>> Yes, but the point is that it looks at load.weight when delta/1024 > 0
>> and assumes that it has had that load.weight the entire time, when this
>> might not actually be true. The largest error I can think of is if you
>> have a very high weight task that tends to run for very short periods of
>> time, you could end up losing their entire contribution to load. This
>> may be acceptable, I'm not certain.
>
> That I believe is not a problem. It is a matter of when cfs_rq->load.weight
> changes and when we look at it to contribute to the cfs_rq's load_avg.
> Fortunately, we will not miss any change of cfs_rq->load.weight, always
> contributing to the load_avg the right amount. Put another way, we always
> use the right cfs_rq->load.weight.

You always call __update_load_avg with every needed load.weight, but if
now - sa->last_update_time < 1024, then it will not do anything with
that weight, and the next actual update may be with a different weight.

>
>> > So left are the migrate_task_rq_fair() not holding lock issue and cfs_rq->avg.load_avg
>> > overflow issue. I need some time to study them.
>> >
>> > Overall, I think none of these issues are originally caused by combination/split
>> > of runnable and blocked. It is just a matter of how synchronized we want to be
>> > (this rewrite is the most synchronized), and the workaround I need to borrow from the
>> > current codes.
>>
>> If you can manage to find a to deal with the migration issue without it,
>> that would be great, I'm just pretty sure that's why we did the split.
>
> After thinking about it the whole morning, this issue is a killer... :)
>
> But I think, even by spliting and by using atomic operations, the current code
> does not substract the migrating task's load absolutely synchronized:
>
> When se migrating, you atomic_read cfs_rq->decay_counter (say t1), and then atomic_add
> the se's load_avg_contrib to removed_load. However, when updating the cfs_rq->blocked_load_avg
> it is not guranteed when substracting the removed_load from the blocked_load_avg,
> the actual decay_counter (say t2) is the same as t1. Because at t1 time (when you
> atomic_read it), the decay_counter can be being updated (before you atomic_add to
> removed_load). Atomic operation does not help synchronize them. Right?
>
> So essentially, to perfectly substract the right amount, we must first synchronize
> the migrating task with its cfs_rq (catch up), and then substract it right away.
> Those two steps must be synchronized (with lock) or atomically done at once. Considering
> migrating and load averaging are not sequential (strict interleaving). Separating the
> two steps but synchronizing them is not enough, they must be done
> atomically.

Yeah, I don't think it is actually entirely safe, and looking at it
again, there's not even any guarantee that the decay_counter read is
anything close to current, even on something like x86, which is... bad.
I know this is why subtract_blocked_load_contrib makes sure to clamp at
zero.

To do this actually correctly, you'd need cmpxchg16b or locking. Given
that we end up pulling a cacheline to do the removed_load add RMW
anyway, it might be reasonable to do something like that, I don't know.

>
> That is chalenging... Can someone (Peter) grant us a lock of the remote rq? :)
>
> Thanks,
> Yuyang

2014-07-09 18:45:56

On Fri, Jul 11, 2014 at 10:47:09AM +0200, Peter Zijlstra wrote:
> On Fri, Jul 11, 2014 at 07:22:07AM +0800, Yuyang Du wrote:
> > On Thu, Jul 10, 2014 at 12:08:59PM +0200, Peter Zijlstra wrote:
> > >
> > > Since clock_task is the regular clock minus some local amount, the
> > > difference between two regular clock reads is always a strict upper
> > > bound on clock_task differences.
> > >
> > This is inspiring. Regarding the clock source in load avg tracking,
> > should we simply use rq_clock_task instead of cfs_rq_clock_task.
>
> Oh *groan* I forgot about that thing. But no, it obviously doesn't
> matter for running time, because if you're throttled you're nor running
> and therefore it all doesn't matter, but it can make a huge difference
> for blocked time accounting I suppose.
>
> > For the bandwidth control case, just update/increase the last_update_time when
> > unthrottled by this throttled time, so the time would look like freezed. Am I
> > understanding right?
>
> Yes, it stops the clock when throttled.
>
> > Not sure how much bandwidth control is used, but even not used, every time
> > we read cfs_rq_clock_task, will burn useless cycles here.
>
> Yep, nothing much you can do about that.
>
> In any case, it is still the case that a normal clock difference is an
> upper bound.

I meant, not for this migrating case. But completely don't use cfs_rq_clock_task
in the entire load avg tracking (and specially compensate the throttle case). No?

Thanks,
Yuyang

2014-07-11 10:04:20

by Yuyang Du

[permalink] [raw]

Subject: Re: [PATCH 2/2] sched: Rewrite per entity runnable load average tracking

On Fri, Jul 11, 2014 at 08:52:01AM +0800, Yuyang Du wrote:
> On Fri, Jul 11, 2014 at 10:47:09AM +0200, Peter Zijlstra wrote:
> > On Fri, Jul 11, 2014 at 07:22:07AM +0800, Yuyang Du wrote:
> > > On Thu, Jul 10, 2014 at 12:08:59PM +0200, Peter Zijlstra wrote:
> > > >
> > > > Since clock_task is the regular clock minus some local amount, the
> > > > difference between two regular clock reads is always a strict upper
> > > > bound on clock_task differences.
> > > >
> > > This is inspiring. Regarding the clock source in load avg tracking,
> > > should we simply use rq_clock_task instead of cfs_rq_clock_task.
> >
> > Oh *groan* I forgot about that thing. But no, it obviously doesn't
> > matter for running time, because if you're throttled you're nor running
> > and therefore it all doesn't matter, but it can make a huge difference
> > for blocked time accounting I suppose.
> >
> > > For the bandwidth control case, just update/increase the last_update_time when
> > > unthrottled by this throttled time, so the time would look like freezed. Am I
> > > understanding right?
> >
> > Yes, it stops the clock when throttled.
> >
> > > Not sure how much bandwidth control is used, but even not used, every time
> > > we read cfs_rq_clock_task, will burn useless cycles here.
> >
> > Yep, nothing much you can do about that.
> >
> > In any case, it is still the case that a normal clock difference is an
> > upper bound.
>
> I meant, not for this migrating case. But completely don't use cfs_rq_clock_task
> in the entire load avg tracking (and specially compensate the throttle case). No?
>
Oh, there seems no way to change every task's last_update_time when unthrottled.

Still, need cfs_rq_clock_task.

Thanks,
Yuyang