Re: [PATCH v33 1/7] sched/psi: Preserve sleep-persistent PSI flags on off-CPU sleeping dequeues

[K Prateek Nayak]

Hello Peter,

On 10/24/2026 5:14 PM, Peter Zijlstra wrote:
> On Fri, Oct 23, 2026 at 10:04:12PM +0000, John Stultz wrote:
>> diff --git a/kernel/sched/stats.h b/kernel/sched/stats.h
>> index ebe0a7765f98e..20820fe1021a1 100644
>> --- a/kernel/sched/stats.h
>> +++ b/kernel/sched/stats.h
>> @@ -175,12 +175,28 @@ static inline void psi_dequeue(struct task_struct *p, int flags)
>>  	 * Do nothing here.
>>  	 *
>>  	 * In the SCHED_PROXY_EXECUTION case we may do sleeping
>> -	 * dequeues that are not followed by a task switch, so check
>> -	 * TSK_ONCPU is set to ensure the task switch is imminent.
>> -	 * Otherwise clear the flags as usual.
>> +	 * dequeues that are not followed by a task switch. If TSK_ONCPU
>> +	 * is not set (meaning we already switched away while the task
>> +	 * stayed on the rq as a blocked donor), handle the sleeping
>> +	 * state transition here so sleep-persistent states (TSK_MEMSTALL,
>> +	 * TSK_IOWAIT) are preserved.
>>  	 */
>> -	if ((flags & DEQUEUE_SLEEP) && (p->psi_flags & TSK_ONCPU))
>> +	if (flags & DEQUEUE_SLEEP) {
>> +		int clear = TSK_RUNNING, set = 0;
>> +
>> +		if (p->psi_flags & TSK_ONCPU)
>> +			return;
>> +		if (p->se.sched_delayed)
>> +			return;
> 
> Must this really be sched_delayed, that is, cannot p->is_blocked be used
> here? I'm trying to push sched_delayed into fair.c if at all possible.

In this case, blocked donors going to sleep would need
to set the PSI flags accordingly (do what psi_sched_switch()
missed).

"p->se.sched_delayed" tasks would have already set those indicators
but can still arrive here from dequeue_task(SLEEP | DELAYED) of
sched_change_begin() - in all those cases, the dequeue flags will
also have DEQUEUE_DELAYED which can be used instead to bail out
early here.

Would that be okay?

> 
>> +
>> +		if (p->in_memstall)
>> +			clear |= TSK_MEMSTALL_RUNNING;
>> +		if (p->in_iowait)
>> +			set |= TSK_IOWAIT;
>> +
>> +		psi_task_change(p, clear, set);
>>  		return;
>> +	}
>>  
>>  	/*
>>  	 * When migrating a task to another CPU, clear all psi
>> -- 
>> 2.56.0.257.gf7497c7d27-goog
>>

-- 
Thanks and Regards,
Prateek