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

[Hao Jia <jiahao.os@bytedance.com>]

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/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?
>

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);