Re: [PATCH] sched/core: Fix wrong warning check in rq_clock_start_loop_update()

[Peter Zijlstra <peterz@infradead.org>]

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/a5dd536d-041a-2ce9-f4b7-64d8d85c86dc@gmail.com
> 
> 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);