[PATCH v2] sched/core: Fix RQCF_ACT_SKIP leak

[PATCH v2] sched/core: Fix RQCF_ACT_SKIP leak

[Hao Jia]

Igor Raits and Bagas Sanjaya report a RQCF_ACT_SKIP leak warning.
Link: https://lore.kernel.org/all/a5dd536d-041a-2ce9-f4b7-64d8d85c86dc@gmail.com

This warning may be triggered 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)

The purpose of RQCF_ACT_SKIP is to skip the update rq clock,
but the update is very early in __schedule(), but we clear
RQCF_*_SKIP very late, causing it to span that gap above
and triggering this warning.

In __schedule() we can clear the RQCF_*_SKIP flag immediately
after update_rq_clock() to avoid this RQCF_ACT_SKIP leak warning.
And set rq->clock_update_flags to RQCF_UPDATED to avoid
rq->clock_update_flags < RQCF_ACT_SKIP warning that may be triggered later.

Fixes: ebb83d84e49b ("sched/core: Avoid multiple calling update_rq_clock() in __cfsb_csd_unthrottle()")
Cc: stable@vger.kernel.org
Reported-by: Igor Raits <igor.raits@gmail.com>
Reported-by: Bagas Sanjaya <bagasdotme@gmail.com>
Closes: https://lore.kernel.org/all/20230913082424.73252-1-jiahao.os@bytedance.com
Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Hao Jia <jiahao.os@bytedance.com>
---
 kernel/sched/core.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 802551e0009b..afb8d213155b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5374,8 +5374,6 @@ 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. */
@@ -6615,6 +6613,7 @@ 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;
 
@@ -6694,8 +6693,6 @@ 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);
-- 
2.39.2

Re: [PATCH v2] sched/core: Fix RQCF_ACT_SKIP leak

[Peter Zijlstra]

On Thu, Oct 12, 2023 at 05:00:03PM +0800, Hao Jia wrote:
> Igor Raits and Bagas Sanjaya report a RQCF_ACT_SKIP leak warning.
> Link: https://lore.kernel.org/all/a5dd536d-041a-2ce9-f4b7-64d8d85c86dc@gmail.com
> 
> This warning may be triggered 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)
> 
> The purpose of RQCF_ACT_SKIP is to skip the update rq clock,
> but the update is very early in __schedule(), but we clear
> RQCF_*_SKIP very late, causing it to span that gap above
> and triggering this warning.
> 
> In __schedule() we can clear the RQCF_*_SKIP flag immediately
> after update_rq_clock() to avoid this RQCF_ACT_SKIP leak warning.
> And set rq->clock_update_flags to RQCF_UPDATED to avoid
> rq->clock_update_flags < RQCF_ACT_SKIP warning that may be triggered later.
> 

Thanks!