Igor Raits and Bagas Sanjaya report a RQCF_ACT_SKIP leak warning.
Link: https://lore.kernel.org/all/[email protected]
Commit ebb83d84e49b54 ("sched/core: Avoid multiple
calling update_rq_clock() in __cfsb_csd_unthrottle()")
add RQCF_ACT_SKIP leak warning in rq_clock_start_loop_update().
But this warning is inaccurate and may be triggered
incorrectly in the following situations:
CPU0 CPU1
__schedule()
*rq->clock_update_flags <<= 1;* unregister_fair_sched_group()
pick_next_task_fair+0x4a/0x410 destroy_cfs_bandwidth()
newidle_balance+0x115/0x3e0 for_each_possible_cpu(i) *i=0*
rq_unpin_lock(this_rq, rf) __cfsb_csd_unthrottle()
raw_spin_rq_unlock(this_rq)
rq_lock(*CPU0_rq*, &rf)
rq_clock_start_loop_update()
rq->clock_update_flags & RQCF_ACT_SKIP <--
raw_spin_rq_lock(this_rq)
So we remove this wrong check. Add assert_clock_updated() to
check that rq clock has been updated before calling
rq_clock_start_loop_update(). And we cannot unconditionally set
rq->clock_update_flags to RQCF_ACT_SKIP in rq_clock_start_loop_update().
So we use the variable rq_clock_flags in rq_clock_start_loop_update()
to record the previous state of rq->clock_update_flags.
Correspondingly, restore rq->clock_update_flags through
rq_clock_flags in rq_clock_stop_loop_update() to prevent
losing its previous information.
Fixes: ebb83d84e49b ("sched/core: Avoid multiple calling update_rq_clock() in __cfsb_csd_unthrottle()")
Cc: [email protected]
Reported-by: Igor Raits <[email protected]>
Reported-by: Bagas Sanjaya <[email protected]>
Signed-off-by: Hao Jia <[email protected]>
---
kernel/sched/fair.c | 10 ++++++----
kernel/sched/sched.h | 12 +++++++-----
2 files changed, 13 insertions(+), 9 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8dbff6e7ad4f..a64a002573d9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5679,6 +5679,7 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
#ifdef CONFIG_SMP
static void __cfsb_csd_unthrottle(void *arg)
{
+ unsigned int rq_clock_flags;
struct cfs_rq *cursor, *tmp;
struct rq *rq = arg;
struct rq_flags rf;
@@ -5691,7 +5692,7 @@ static void __cfsb_csd_unthrottle(void *arg)
* Do it once and skip the potential next ones.
*/
update_rq_clock(rq);
- rq_clock_start_loop_update(rq);
+ rq_clock_start_loop_update(rq, &rq_clock_flags);
/*
* Since we hold rq lock we're safe from concurrent manipulation of
@@ -5712,7 +5713,7 @@ static void __cfsb_csd_unthrottle(void *arg)
rcu_read_unlock();
- rq_clock_stop_loop_update(rq);
+ rq_clock_stop_loop_update(rq, &rq_clock_flags);
rq_unlock(rq, &rf);
}
@@ -6230,6 +6231,7 @@ static void __maybe_unused update_runtime_enabled(struct rq *rq)
/* cpu offline callback */
static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
{
+ unsigned int rq_clock_flags;
struct task_group *tg;
lockdep_assert_rq_held(rq);
@@ -6239,7 +6241,7 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
* set_rq_offline(), so we should skip updating
* the rq clock again in unthrottle_cfs_rq().
*/
- rq_clock_start_loop_update(rq);
+ rq_clock_start_loop_update(rq, &rq_clock_flags);
rcu_read_lock();
list_for_each_entry_rcu(tg, &task_groups, list) {
@@ -6264,7 +6266,7 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
}
rcu_read_unlock();
- rq_clock_stop_loop_update(rq);
+ rq_clock_stop_loop_update(rq, &rq_clock_flags);
}
bool cfs_task_bw_constrained(struct task_struct *p)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 04846272409c..ff2864f202f5 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1558,20 +1558,22 @@ static inline void rq_clock_cancel_skipupdate(struct rq *rq)
* when using list_for_each_entry_*)
* rq_clock_start_loop_update() can be called after updating the clock
* once and before iterating over the list to prevent multiple update.
+ * And use @rq_clock_flags to record the previous state of rq->clock_update_flags.
* After the iterative traversal, we need to call rq_clock_stop_loop_update()
- * to clear RQCF_ACT_SKIP of rq->clock_update_flags.
+ * to restore rq->clock_update_flags through @rq_clock_flags.
*/
-static inline void rq_clock_start_loop_update(struct rq *rq)
+static inline void rq_clock_start_loop_update(struct rq *rq, unsigned int *rq_clock_flags)
{
lockdep_assert_rq_held(rq);
- SCHED_WARN_ON(rq->clock_update_flags & RQCF_ACT_SKIP);
+ assert_clock_updated(rq);
+ *rq_clock_flags = rq->clock_update_flags;
rq->clock_update_flags |= RQCF_ACT_SKIP;
}
-static inline void rq_clock_stop_loop_update(struct rq *rq)
+static inline void rq_clock_stop_loop_update(struct rq *rq, unsigned int *rq_clock_flags)
{
lockdep_assert_rq_held(rq);
- rq->clock_update_flags &= ~RQCF_ACT_SKIP;
+ rq->clock_update_flags = *rq_clock_flags;
}
struct rq_flags {
--
2.39.2
On Wed, Sep 13, 2023 at 04:24:24PM +0800, Hao Jia wrote:
> Igor Raits and Bagas Sanjaya report a RQCF_ACT_SKIP leak warning.
> Link: https://lore.kernel.org/all/[email protected]
>
> Commit ebb83d84e49b54 ("sched/core: Avoid multiple
> calling update_rq_clock() in __cfsb_csd_unthrottle()")
> add RQCF_ACT_SKIP leak warning in rq_clock_start_loop_update().
> But this warning is inaccurate and may be triggered
> incorrectly in the following situations:
>
> CPU0 CPU1
>
> __schedule()
> *rq->clock_update_flags <<= 1;* unregister_fair_sched_group()
> pick_next_task_fair+0x4a/0x410 destroy_cfs_bandwidth()
> newidle_balance+0x115/0x3e0 for_each_possible_cpu(i) *i=0*
> rq_unpin_lock(this_rq, rf) __cfsb_csd_unthrottle()
if (rq->clock_update_flags > RQCF_ACT_SKIP)
rf->clock_update_flags = RQCF_UPDATED;
so that preserves all flags, but only stores UPDATED.
> raw_spin_rq_unlock(this_rq)
> rq_lock(*CPU0_rq*, &rf)
rq_pin_lock()
rq->clock_update_flags &= (REQ_SKIP|ACT_SKIP);
rf->clock_update_flags = 0;
IOW, we preserve ACT_SKIP from CPU0
> rq_clock_start_loop_update()
> rq->clock_update_flags & RQCF_ACT_SKIP <--
And go SPLAT
>
> raw_spin_rq_lock(this_rq)
rq_repin_lock()
rq->clock_update_flags |= rf->clock_update_flags;
which restores UPDATED, even though in reality time could have moved on
quite significantly.
Anyway....
the purpose of ACT_SKIP is to skip the update (clue in name etc), but
the update is very early in __schedule(), but we clear *_SKIP very late,
causing it to span that gap above.
Going by the commits that put it there, the thinking was to clear
clock_skip_update before unlock, but AFAICT we can clear SKIP flags
right after the update_rq_clock() we're wanting to skip, no?
That is, would not something like the below make more sense?
---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d8fd29d66b24..bfd2ab4b95da 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5357,8 +5357,6 @@ context_switch(struct rq *rq, struct task_struct *prev,
/* switch_mm_cid() requires the memory barriers above. */
switch_mm_cid(rq, prev, next);
- rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
-
prepare_lock_switch(rq, next, rf);
/* Here we just switch the register state and the stack. */
@@ -6596,6 +6594,8 @@ static void __sched notrace __schedule(unsigned int sched_mode)
/* Promote REQ to ACT */
rq->clock_update_flags <<= 1;
update_rq_clock(rq);
+ rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
+
switch_count = &prev->nivcsw;
@@ -6675,8 +6675,6 @@ static void __sched notrace __schedule(unsigned int sched_mode)
/* Also unlocks the rq: */
rq = context_switch(rq, prev, next, &rf);
} else {
- rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
-
rq_unpin_lock(rq, &rf);
__balance_callbacks(rq);
raw_spin_rq_unlock_irq(rq);
On 2023/9/28 Peter Zijlstra wrote:
> On Wed, Sep 13, 2023 at 04:24:24PM +0800, Hao Jia wrote:
>> Igor Raits and Bagas Sanjaya report a RQCF_ACT_SKIP leak warning.
>> Link: https://lore.kernel.org/all/[email protected]
>>
>> Commit ebb83d84e49b54 ("sched/core: Avoid multiple
>> calling update_rq_clock() in __cfsb_csd_unthrottle()")
>> add RQCF_ACT_SKIP leak warning in rq_clock_start_loop_update().
>> But this warning is inaccurate and may be triggered
>> incorrectly in the following situations:
>>
>> CPU0 CPU1
>>
>> __schedule()
>> *rq->clock_update_flags <<= 1;* unregister_fair_sched_group()
>> pick_next_task_fair+0x4a/0x410 destroy_cfs_bandwidth()
>> newidle_balance+0x115/0x3e0 for_each_possible_cpu(i) *i=0*
>> rq_unpin_lock(this_rq, rf) __cfsb_csd_unthrottle()
> if (rq->clock_update_flags > RQCF_ACT_SKIP)
> rf->clock_update_flags = RQCF_UPDATED;
>
> so that preserves all flags, but only stores UPDATED.
>
>> raw_spin_rq_unlock(this_rq)
>> rq_lock(*CPU0_rq*, &rf)
> rq_pin_lock()
> rq->clock_update_flags &= (REQ_SKIP|ACT_SKIP);
> rf->clock_update_flags = 0;
>
> IOW, we preserve ACT_SKIP from CPU0
>
>> rq_clock_start_loop_update()
>> rq->clock_update_flags & RQCF_ACT_SKIP <--
>
> And go SPLAT
>
>>
>> raw_spin_rq_lock(this_rq)
> rq_repin_lock()
> rq->clock_update_flags |= rf->clock_update_flags;
>
> which restores UPDATED, even though in reality time could have moved on
> quite significantly.
>
>
> Anyway....
>
> the purpose of ACT_SKIP is to skip the update (clue in name etc), but
> the update is very early in __schedule(), but we clear *_SKIP very late,
> causing it to span that gap above.
>
> Going by the commits that put it there, the thinking was to clear
> clock_skip_update before unlock, but AFAICT we can clear SKIP flags
> right after the update_rq_clock() we're wanting to skip, no?
>
Thanks for your review, and I am very sorry to reply to you so late. I
just came back from a long vacation.
> That is, would not something like the below make more sense?
If we understand correctly, this may not work.
After applying this patch, the following situation will trigger the
rq->clock_update_flags < RQCF_ACT_SKIP warning.
If rq_clock_skip_update() is called before __schedule(), so
RQCF_REQ_SKIP of rq->clock_update_flags is set.
__schedule() {
rq_lock(rq, &rf); [rq->clock_update_flags is RQCF_REQ_SKIP]
rq->clock_update_flags <<= 1;
update_rq_clock(rq); [rq->clock_update_flags is RQCF_ACT_SKIP]
+ rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
* At this time, rq->clock_update_flags = 0; *
pick_next_task_fair
set_next_entity
update_load_avg
assert_clock_updated() <---
}
assert_clock_updated() will determine whether rq->clock_update_flags is
less than RQCF_ACT_SKIP. If we clear RQCF_ACT_SKIP prematurely, this
assert may be triggered later.
>
> ---
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index d8fd29d66b24..bfd2ab4b95da 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5357,8 +5357,6 @@ context_switch(struct rq *rq, struct task_struct *prev,
> /* switch_mm_cid() requires the memory barriers above. */
> switch_mm_cid(rq, prev, next);
>
> - rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
> -
> prepare_lock_switch(rq, next, rf);
>
> /* Here we just switch the register state and the stack. */
> @@ -6596,6 +6594,8 @@ static void __sched notrace __schedule(unsigned int sched_mode)
> /* Promote REQ to ACT */
> rq->clock_update_flags <<= 1;
> update_rq_clock(rq);
> + rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
> +
>
> switch_count = &prev->nivcsw;
>
> @@ -6675,8 +6675,6 @@ static void __sched notrace __schedule(unsigned int sched_mode)
> /* Also unlocks the rq: */
> rq = context_switch(rq, prev, next, &rf);
> } else {
> - rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
> -
> rq_unpin_lock(rq, &rf);
> __balance_callbacks(rq);
> raw_spin_rq_unlock_irq(rq);
On Sat, Oct 07, 2023 at 04:44:46PM +0800, Hao Jia wrote:
> > That is, would not something like the below make more sense?
>
> If we understand correctly, this may not work.
>
> After applying this patch, the following situation will trigger the
> rq->clock_update_flags < RQCF_ACT_SKIP warning.
>
> If rq_clock_skip_update() is called before __schedule(), so RQCF_REQ_SKIP of
> rq->clock_update_flags is set.
>
>
>
>
> __schedule() {
> rq_lock(rq, &rf); [rq->clock_update_flags is RQCF_REQ_SKIP]
> rq->clock_update_flags <<= 1;
> update_rq_clock(rq); [rq->clock_update_flags is RQCF_ACT_SKIP]
> + rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
> * At this time, rq->clock_update_flags = 0; *
Fixed easily enough, just change to:
rq->clock_updated_flags = RQCF_UPDATED;
>
> pick_next_task_fair
> set_next_entity
> update_load_avg
> assert_clock_updated() <---
> }
---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a0a582c8cf8c..cf9eb1a26c22 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5357,8 +5357,6 @@ context_switch(struct rq *rq, struct task_struct *prev,
/* switch_mm_cid() requires the memory barriers above. */
switch_mm_cid(rq, prev, next);
- rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
-
prepare_lock_switch(rq, next, rf);
/* Here we just switch the register state and the stack. */
@@ -6596,6 +6594,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)
/* Promote REQ to ACT */
rq->clock_update_flags <<= 1;
update_rq_clock(rq);
+ rq->clock_update_flags = RQCF_UPDATED;
switch_count = &prev->nivcsw;
@@ -6675,8 +6674,6 @@ static void __sched notrace __schedule(unsigned int sched_mode)
/* Also unlocks the rq: */
rq = context_switch(rq, prev, next, &rf);
} else {
- rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
-
rq_unpin_lock(rq, &rf);
__balance_callbacks(rq);
raw_spin_rq_unlock_irq(rq);
On 2023/10/10 Peter Zijlstra wrote:
> On Sat, Oct 07, 2023 at 04:44:46PM +0800, Hao Jia wrote:
>
>>> That is, would not something like the below make more sense?
>>
>> If we understand correctly, this may not work.
>>
>> After applying this patch, the following situation will trigger the
>> rq->clock_update_flags < RQCF_ACT_SKIP warning.
>>
>> If rq_clock_skip_update() is called before __schedule(), so RQCF_REQ_SKIP of
>> rq->clock_update_flags is set.
>>
>>
>>
>>
>> __schedule() {
>> rq_lock(rq, &rf); [rq->clock_update_flags is RQCF_REQ_SKIP]
>> rq->clock_update_flags <<= 1;
>> update_rq_clock(rq); [rq->clock_update_flags is RQCF_ACT_SKIP]
>> + rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
>> * At this time, rq->clock_update_flags = 0; *
>
> Fixed easily enough, just change to:
>
> rq->clock_updated_flags = RQCF_UPDATED;
>
Thanks for your suggestions and help, I revised the commit message and
sent patch v2.
https://lore.kernel.org/all/[email protected]/
Please review again.
Thanks,
Hao
>>
>> pick_next_task_fair
>> set_next_entity
>> update_load_avg
>> assert_clock_updated() <---
>> }
>