2017-08-22 08:30:31

by Byungchul Park

[permalink] [raw]
Subject: [PATCH v9 0/2] Make find_later_rq() choose a closer cpu in topology

Change from v8
-. add suggested-by Peterz
-. add several comments

Change from v7
-. fix a trivial typo
-. modify commit messages to explain what it does more clearly
-. simplify code with an existing macro

Change from v6
-. add a comment about selection of fallback_cpu incase more than one exist
-. modify a comment explaining what we do wrt PREFER_SIBLING

Change from v5
-. exclude two patches already picked up by peterz
(sched/deadline: Make find_later_rq() choose a closer cpu in topology)
(sched/deadline: Change return value of cpudl_find())
-. apply what peterz fixed for 'prefer sibling', into deadline and rt

Change from v4
-. remove a patch that might cause huge lock contention
(by spin lock(&cpudl.lock) in a hot path of scheduler)

Change from v3
-. rename closest_cpu to best_cpu so that it align with rt
-. protect referring cpudl.elements with cpudl.lock
-. change return value of cpudl_find() to bool

Change from v2
-. add support for SD_PREFER_SIBLING

Change from v1
-. clean up the patch

Byungchul Park (2):
sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()
sched/rt: Add support for SD_PREFER_SIBLING on find_lowest_rq()

kernel/sched/deadline.c | 80 ++++++++++++++++++++++++++++++++++++++++++++-----
kernel/sched/rt.c | 78 +++++++++++++++++++++++++++++++++++++++++++++--
2 files changed, 148 insertions(+), 10 deletions(-)

--
1.9.1


2017-08-22 08:30:35

by Byungchul Park

[permalink] [raw]
Subject: [PATCH v9 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()

It would be better to try to check other siblings first if
SD_PREFER_SIBLING is flaged when pushing tasks - migration.

Signed-off-by: Byungchul Park <[email protected]>
Suggested-by: Peter Zijlstra <[email protected]>
---
kernel/sched/deadline.c | 80 ++++++++++++++++++++++++++++++++++++++++++++-----
1 file changed, 73 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 0223694..b6b3855 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1319,12 +1319,35 @@ static struct task_struct *pick_earliest_pushable_dl_task(struct rq *rq, int cpu

static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask_dl);

+/*
+ * Find the first cpu in: mask & sd & ~prefer
+ */
+static int find_cpu(const struct cpumask *mask,
+ const struct sched_domain *sd,
+ const struct sched_domain *prefer)
+{
+ const struct cpumask *sds = sched_domain_span(sd);
+ const struct cpumask *ps = prefer ? sched_domain_span(prefer) : NULL;
+ int cpu;
+
+ for_each_cpu(cpu, mask) {
+ if (!cpumask_test_cpu(cpu, sds))
+ continue;
+ if (ps && cpumask_test_cpu(cpu, ps))
+ continue;
+ break;
+ }
+
+ return cpu;
+}
+
static int find_later_rq(struct task_struct *task)
{
- struct sched_domain *sd;
+ struct sched_domain *sd, *prefer = NULL;
struct cpumask *later_mask = this_cpu_cpumask_var_ptr(local_cpu_mask_dl);
int this_cpu = smp_processor_id();
int cpu = task_cpu(task);
+ int fallback_cpu = -1;

/* Make sure the mask is initialized first */
if (unlikely(!later_mask))
@@ -1376,15 +1399,35 @@ static int find_later_rq(struct task_struct *task)
return this_cpu;
}

- best_cpu = cpumask_first_and(later_mask,
- sched_domain_span(sd));
/*
- * Last chance: if a cpu being in both later_mask
- * and current sd span is valid, that becomes our
- * choice. Of course, the latest possible cpu is
- * already under consideration through later_mask.
+ * If a cpu being in later_mask & current sd &
+ * ~prefer sd is valid, that becomes our choice.
+ * Of course, the latest possible cpu is already
+ * under consideration through later_mask.
*/
+ best_cpu = find_cpu(later_mask, sd, prefer);
+
if (best_cpu < nr_cpu_ids) {
+ /*
+ * If current domain is SD_PREFER_SIBLING
+ * flaged, we have to try to check other
+ * siblings first.
+ */
+ if (sd->flags & SD_PREFER_SIBLING) {
+ prefer = sd;
+
+ /*
+ * fallback_cpu should be one
+ * in the closest domain among
+ * SD_PREFER_SIBLING domains,
+ * in case that more than one
+ * SD_PREFER_SIBLING domains
+ * exist in the hierachy.
+ */
+ if (fallback_cpu == -1)
+ fallback_cpu = best_cpu;
+ continue;
+ }
rcu_read_unlock();
return best_cpu;
}
@@ -1393,6 +1436,29 @@ static int find_later_rq(struct task_struct *task)
rcu_read_unlock();

/*
+ * If fallback_cpu is valid, all our guesses failed *except* for
+ * SD_PREFER_SIBLING domain. Now, we can return the fallback cpu.
+ *
+ * XXX: Consider the following example, 4 cores SMT2 system:
+ *
+ * LLC [0 - 7]
+ * SMT [0 1][2 3][4 5][6 7]
+ * o x o x x x x x
+ *
+ * where 'o': occupied and 'x': empty.
+ *
+ * A wakeup on cpu0 will exclude cpu1 and choose cpu3, since
+ * cpu1 is in a SD_PREFER_SIBLING sd and cpu3 is not. However,
+ * in this case, we have to choose cpu4 for better work, instead
+ * of cpu3 that is fully loaded.
+ *
+ * We have to do the best if possible, but choose the second
+ * best here since that is too expensive to adopt.
+ */
+ if (fallback_cpu != -1)
+ return fallback_cpu;
+
+ /*
* At this point, all our guesses failed, we just return
* 'something', and let the caller sort the things out.
*/
--
1.9.1

2017-08-22 08:30:54

by Byungchul Park

[permalink] [raw]
Subject: [PATCH v9 2/2] sched/rt: Add support for SD_PREFER_SIBLING on find_lowest_rq()

Hello Steven,

I added several comments so I'm not sure if I could add your reviewed-by.

----->8-----
>From f0710d99759ed28c1409a527166780899f00d236 Mon Sep 17 00:00:00 2001
From: Byungchul Park <[email protected]>
Date: Tue, 22 Aug 2017 17:19:21 +0900
Subject: [PATCH v9 2/2] sched/rt: Add support for SD_PREFER_SIBLING on
find_lowest_rq()

It would be better to try to check other siblings first if
SD_PREFER_SIBLING is flaged when pushing tasks - migration.

Signed-off-by: Byungchul Park <[email protected]>
Suggested-by: Peter Zijlstra <[email protected]>
---
kernel/sched/rt.c | 78 ++++++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 75 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 979b734..196ffc7 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1618,12 +1618,35 @@ static struct task_struct *pick_highest_pushable_task(struct rq *rq, int cpu)

static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask);

+/*
+ * Find the first cpu in: mask & sd & ~prefer
+ */
+static int find_cpu(const struct cpumask *mask,
+ const struct sched_domain *sd,
+ const struct sched_domain *prefer)
+{
+ const struct cpumask *sds = sched_domain_span(sd);
+ const struct cpumask *ps = prefer ? sched_domain_span(prefer) : NULL;
+ int cpu;
+
+ for_each_cpu(cpu, mask) {
+ if (!cpumask_test_cpu(cpu, sds))
+ continue;
+ if (ps && cpumask_test_cpu(cpu, ps))
+ continue;
+ break;
+ }
+
+ return cpu;
+}
+
static int find_lowest_rq(struct task_struct *task)
{
- struct sched_domain *sd;
+ struct sched_domain *sd, *prefer = NULL;
struct cpumask *lowest_mask = this_cpu_cpumask_var_ptr(local_cpu_mask);
int this_cpu = smp_processor_id();
int cpu = task_cpu(task);
+ int fallback_cpu = -1;

/* Make sure the mask is initialized first */
if (unlikely(!lowest_mask))
@@ -1668,9 +1691,35 @@ static int find_lowest_rq(struct task_struct *task)
return this_cpu;
}

- best_cpu = cpumask_first_and(lowest_mask,
- sched_domain_span(sd));
+ /*
+ * If a cpu being in lowest_mask & current sd &
+ * ~prefer sd is valid, that becomes our choice.
+ * Of course, the lowest possible cpu is already
+ * under consideration through lowest_mask.
+ */
+ best_cpu = find_cpu(lowest_mask, sd, prefer);
+
if (best_cpu < nr_cpu_ids) {
+ /*
+ * If current domain is SD_PREFER_SIBLING
+ * flaged, we have to try to check other
+ * siblings first.
+ */
+ if (sd->flags & SD_PREFER_SIBLING) {
+ prefer = sd;
+
+ /*
+ * fallback_cpu should be one
+ * in the closest domain among
+ * SD_PREFER_SIBLING domains,
+ * in case that more than one
+ * SD_PREFER_SIBLING domains
+ * exist in the hierachy.
+ */
+ if (fallback_cpu == -1)
+ fallback_cpu = best_cpu;
+ continue;
+ }
rcu_read_unlock();
return best_cpu;
}
@@ -1679,6 +1728,29 @@ static int find_lowest_rq(struct task_struct *task)
rcu_read_unlock();

/*
+ * If fallback_cpu is valid, all our guesses failed *except* for
+ * SD_PREFER_SIBLING domain. Now, we can return the fallback cpu.
+ *
+ * XXX: Consider the following example, 4 cores SMT2 system:
+ *
+ * LLC [0 - 7]
+ * SMT [0 1][2 3][4 5][6 7]
+ * o x o x x x x x
+ *
+ * where 'o': occupied and 'x': empty.
+ *
+ * A wakeup on cpu0 will exclude cpu1 and choose cpu3, since
+ * cpu1 is in a SD_PREFER_SIBLING sd and cpu3 is not. However,
+ * in this case, we have to choose cpu4 for better work, instead
+ * of cpu3 that is fully loaded.
+ *
+ * We have to do the best if possible, but choose the second
+ * best here since that is too expensive to adopt.
+ */
+ if (fallback_cpu != -1)
+ return fallback_cpu;
+
+ /*
* And finally, if there were no matches within the domains
* just give the caller *something* to work with from the compatible
* locations.
--
1.9.1

2017-08-22 09:25:48

by Juri Lelli

[permalink] [raw]
Subject: Re: [PATCH v9 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()

Hi,

On 22/08/17 17:30, Byungchul Park wrote:
> It would be better to try to check other siblings first if
> SD_PREFER_SIBLING is flaged when pushing tasks - migration.
>
> Signed-off-by: Byungchul Park <[email protected]>
> Suggested-by: Peter Zijlstra <[email protected]>
^
This has to come before your SoB.

> ---
> kernel/sched/deadline.c | 80 ++++++++++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 73 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 0223694..b6b3855 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1319,12 +1319,35 @@ static struct task_struct *pick_earliest_pushable_dl_task(struct rq *rq, int cpu
>
> static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask_dl);
>
> +/*
> + * Find the first cpu in: mask & sd & ~prefer
> + */
> +static int find_cpu(const struct cpumask *mask,
> + const struct sched_domain *sd,
> + const struct sched_domain *prefer)
> +{
> + const struct cpumask *sds = sched_domain_span(sd);
> + const struct cpumask *ps = prefer ? sched_domain_span(prefer) : NULL;
> + int cpu;
> +
> + for_each_cpu(cpu, mask) {
> + if (!cpumask_test_cpu(cpu, sds))
> + continue;
> + if (ps && cpumask_test_cpu(cpu, ps))
> + continue;
> + break;
> + }
> +
> + return cpu;
> +}
> +
> static int find_later_rq(struct task_struct *task)
> {
> - struct sched_domain *sd;
> + struct sched_domain *sd, *prefer = NULL;
> struct cpumask *later_mask = this_cpu_cpumask_var_ptr(local_cpu_mask_dl);
> int this_cpu = smp_processor_id();
> int cpu = task_cpu(task);
> + int fallback_cpu = -1;
>
> /* Make sure the mask is initialized first */
> if (unlikely(!later_mask))
> @@ -1376,15 +1399,35 @@ static int find_later_rq(struct task_struct *task)
> return this_cpu;
> }
>
> - best_cpu = cpumask_first_and(later_mask,
> - sched_domain_span(sd));
> /*
> - * Last chance: if a cpu being in both later_mask
> - * and current sd span is valid, that becomes our
> - * choice. Of course, the latest possible cpu is
> - * already under consideration through later_mask.
> + * If a cpu being in later_mask & current sd &
> + * ~prefer sd is valid, that becomes our choice.
> + * Of course, the latest possible cpu is already
> + * under consideration through later_mask.
> */
> + best_cpu = find_cpu(later_mask, sd, prefer);
> +
> if (best_cpu < nr_cpu_ids) {
> + /*
> + * If current domain is SD_PREFER_SIBLING
> + * flaged, we have to try to check other
> + * siblings first.
> + */
> + if (sd->flags & SD_PREFER_SIBLING) {
> + prefer = sd;
> +
> + /*
> + * fallback_cpu should be one
> + * in the closest domain among
> + * SD_PREFER_SIBLING domains,
> + * in case that more than one
> + * SD_PREFER_SIBLING domains
> + * exist in the hierachy.
> + */
> + if (fallback_cpu == -1)
> + fallback_cpu = best_cpu;
> + continue;
> + }
> rcu_read_unlock();
> return best_cpu;
> }
> @@ -1393,6 +1436,29 @@ static int find_later_rq(struct task_struct *task)
> rcu_read_unlock();
>
> /*
> + * If fallback_cpu is valid, all our guesses failed *except* for
> + * SD_PREFER_SIBLING domain. Now, we can return the fallback cpu.
> + *
> + * XXX: Consider the following example, 4 cores SMT2 system:
> + *
> + * LLC [0 - 7]
> + * SMT [0 1][2 3][4 5][6 7]
> + * o x o x x x x x
> + *
> + * where 'o': occupied and 'x': empty.
> + *
> + * A wakeup on cpu0 will exclude cpu1 and choose cpu3, since
> + * cpu1 is in a SD_PREFER_SIBLING sd and cpu3 is not. However,
> + * in this case, we have to choose cpu4 for better work, instead

... in this case cpu4 would have been a better choice, since cpu3 is a
(SMT) thread of an already loaded core.

> + * of cpu3 that is fully loaded.
> + *
> + * We have to do the best if possible, but choose the second
> + * best here since that is too expensive to adopt.
> + */

I'd also modify this last paragraph with something like:

"Doing it 'right' is difficult and expensive. The current solution is an
acceptable approximation."

Apart from these minor points, patch looks ok to me.

Acked-by: Juri Lelli <[email protected]>

Best,

- Juri

2017-08-24 00:52:35

by Byungchul Park

[permalink] [raw]
Subject: Re: [PATCH v9 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()

On Tue, Aug 22, 2017 at 10:25:42AM +0100, Juri Lelli wrote:
> Hi,
>
> On 22/08/17 17:30, Byungchul Park wrote:
> > It would be better to try to check other siblings first if
> > SD_PREFER_SIBLING is flaged when pushing tasks - migration.
> >
> > Signed-off-by: Byungchul Park <[email protected]>
> > Suggested-by: Peter Zijlstra <[email protected]>
> ^
> This has to come before your SoB.

Thank you, I will.

> > ---
> > kernel/sched/deadline.c | 80 ++++++++++++++++++++++++++++++++++++++++++++-----
> > 1 file changed, 73 insertions(+), 7 deletions(-)
> >
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index 0223694..b6b3855 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -1319,12 +1319,35 @@ static struct task_struct *pick_earliest_pushable_dl_task(struct rq *rq, int cpu
> >
> > static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask_dl);
> >
> > +/*
> > + * Find the first cpu in: mask & sd & ~prefer
> > + */
> > +static int find_cpu(const struct cpumask *mask,
> > + const struct sched_domain *sd,
> > + const struct sched_domain *prefer)
> > +{
> > + const struct cpumask *sds = sched_domain_span(sd);
> > + const struct cpumask *ps = prefer ? sched_domain_span(prefer) : NULL;
> > + int cpu;
> > +
> > + for_each_cpu(cpu, mask) {
> > + if (!cpumask_test_cpu(cpu, sds))
> > + continue;
> > + if (ps && cpumask_test_cpu(cpu, ps))
> > + continue;
> > + break;
> > + }
> > +
> > + return cpu;
> > +}
> > +
> > static int find_later_rq(struct task_struct *task)
> > {
> > - struct sched_domain *sd;
> > + struct sched_domain *sd, *prefer = NULL;
> > struct cpumask *later_mask = this_cpu_cpumask_var_ptr(local_cpu_mask_dl);
> > int this_cpu = smp_processor_id();
> > int cpu = task_cpu(task);
> > + int fallback_cpu = -1;
> >
> > /* Make sure the mask is initialized first */
> > if (unlikely(!later_mask))
> > @@ -1376,15 +1399,35 @@ static int find_later_rq(struct task_struct *task)
> > return this_cpu;
> > }
> >
> > - best_cpu = cpumask_first_and(later_mask,
> > - sched_domain_span(sd));
> > /*
> > - * Last chance: if a cpu being in both later_mask
> > - * and current sd span is valid, that becomes our
> > - * choice. Of course, the latest possible cpu is
> > - * already under consideration through later_mask.
> > + * If a cpu being in later_mask & current sd &
> > + * ~prefer sd is valid, that becomes our choice.
> > + * Of course, the latest possible cpu is already
> > + * under consideration through later_mask.
> > */
> > + best_cpu = find_cpu(later_mask, sd, prefer);
> > +
> > if (best_cpu < nr_cpu_ids) {
> > + /*
> > + * If current domain is SD_PREFER_SIBLING
> > + * flaged, we have to try to check other
> > + * siblings first.
> > + */
> > + if (sd->flags & SD_PREFER_SIBLING) {
> > + prefer = sd;
> > +
> > + /*
> > + * fallback_cpu should be one
> > + * in the closest domain among
> > + * SD_PREFER_SIBLING domains,
> > + * in case that more than one
> > + * SD_PREFER_SIBLING domains
> > + * exist in the hierachy.
> > + */
> > + if (fallback_cpu == -1)
> > + fallback_cpu = best_cpu;
> > + continue;
> > + }
> > rcu_read_unlock();
> > return best_cpu;
> > }
> > @@ -1393,6 +1436,29 @@ static int find_later_rq(struct task_struct *task)
> > rcu_read_unlock();
> >
> > /*
> > + * If fallback_cpu is valid, all our guesses failed *except* for
> > + * SD_PREFER_SIBLING domain. Now, we can return the fallback cpu.
> > + *
> > + * XXX: Consider the following example, 4 cores SMT2 system:
> > + *
> > + * LLC [0 - 7]
> > + * SMT [0 1][2 3][4 5][6 7]
> > + * o x o x x x x x
> > + *
> > + * where 'o': occupied and 'x': empty.
> > + *
> > + * A wakeup on cpu0 will exclude cpu1 and choose cpu3, since
> > + * cpu1 is in a SD_PREFER_SIBLING sd and cpu3 is not. However,
> > + * in this case, we have to choose cpu4 for better work, instead
>
> ... in this case cpu4 would have been a better choice, since cpu3 is a
> (SMT) thread of an already loaded core.

Thank you, I will.

> > + * of cpu3 that is fully loaded.
> > + *
> > + * We have to do the best if possible, but choose the second
> > + * best here since that is too expensive to adopt.
> > + */
>
> I'd also modify this last paragraph with something like:
>
> "Doing it 'right' is difficult and expensive. The current solution is an
> acceptable approximation."

Thank you, I will.

> Apart from these minor points, patch looks ok to me.
>
> Acked-by: Juri Lelli <[email protected]>
>
> Best,
>
> - Juri