Re: [PATCH] sched/stats: Fix run_delay over-count for proxy-migrated blocked donors

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Oct 27, 2026 at 03:15:40PM +0530, K Prateek Nayak wrote:
> On 10/27/2026 12:22 PM, Kayra Cizmeci wrote:
> > I think this fixes the issue. BTW I think this should skip curr, no?
> > On a case like this:
> > 
> > C holds M1 in CPU0, D waits that mutex in CPU1. A holds mutex M2 in
> > CPU2. B is the working task on CPU1 but it wants mutex M2 and we enter
> > __schedule(). B gets is_blocked. D gets chosen. We open the rq lock to
> > get D to CPU0. While this happens A drops the mutex and wakes up B.
> > we enter ttwu_runnable() and B is still curr. And we start the timer.
> 
> Good catch that!
> 
> > Or am I MISSSING something?
> 
> I can't believe something like wait time accounting can get this
> complicated! 

:-)

> Anyhow, We can perhaps call sched_info_enqueue() in
> ttwu_runnable() instead of proxy_needs_return() like:
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 04b62c16fa8c..d5758b78b741 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3956,6 +3956,8 @@ static int ttwu_runnable(struct task_struct *p, int wake_flags)
>  		 * it should preempt the task that is current now.
>  		 */
>  		wakeup_preempt(rq, p, wake_flags | WF_TTWU_RQ);
> +		/* Start accounting wait time. */
> +		sched_info_enqueue(rq, p);
>  	}
>  	ttwu_do_wakeup(p);
>  	return 1;

So the normal wakeup path goes through enqueue_task() which gets the
sched_info_enqueue() callback there. But yes, ttwu_runnable() does not
go through that, and needs this additional call.

There is one more site in ttwu() that also doesn't go through
enqueue_task() and that is the p==current thing at the top.