linux-kernel - Re: [PATCH v2] sched: Fix psi

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Message-ID: <9622a5ec-c754-4066-9ab9-683e98e91fcf@163.com>
Date: Fri, 28 Nov 2025 17:35:09 +0800
From: Haiyue Wang <haiyuewa@....com>
To: John Stultz <jstultz@...gle.com>, LKML <linux-kernel@...r.kernel.org>
Cc: K Prateek Nayak <kprateek.nayak@....com>,
 Johannes Weiner <hannes@...xchg.org>, Joel Fernandes
 <joelagnelf@...dia.com>, Qais Yousef <qyousef@...alina.io>,
 Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Valentin Schneider <vschneid@...hat.com>,
 Suren Baghdasaryan <surenb@...gle.com>, Steven Rostedt
 <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Zimuzo Ezeozue <zezeozue@...gle.com>, Mel Gorman <mgorman@...e.de>,
 Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>,
 Boqun Feng <boqun.feng@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>,
 Metin Kaya <Metin.Kaya@....com>, Xuewen Yan <xuewen.yan94@...il.com>,
 Thomas Gleixner <tglx@...utronix.de>,
 Daniel Lezcano <daniel.lezcano@...aro.org>,
 Suleiman Souhlal <suleiman@...gle.com>, kuyo chang
 <kuyo.chang@...iatek.com>, hupu <hupu.gm@...il.com>, kernel-team@...roid.com
Subject: Re: [PATCH v2] sched: Fix psi_dequeue for Proxy Execution


On 11/22/2025 3:01 AM, John Stultz wrote:
> Currently, if the sleep flag is set, psi_dequeue() doesn't
> change any of the psi_flags.
> 
> This is because psi_switch_task() will clear TSK_ONCPU as well

psi_task_switch()

> as other potential flags (TSK_RUNNING), and the assumption is
> that a voluntary sleep always consists of a task being dequeued
> followed shortly there after with a psi_sched_switch() call.
> 
> Proxy Execution changes this expectation, as mutex-blocked tasks
> that would normally sleep stay on the runqueue. But in the case
> where the mutex-owning task goes to sleep, or the owner is on a
> remote cpu, we will then deactivate the blocked task shortly
> after.
> 
> In that situation, the mutex-blocked task will have had its
> TSK_ONCPU cleared when it was switched off the cpu, but it will
> stay TSK_RUNNING. Then if we later dequeue it (as currently done
> if we hit a case find_proxy_task() can't yet handle, such as the
> case of the owner being on another rq or a sleeping owner)
> psi_dequeue() won't change any state (leaving it TSK_RUNNING),
> as it incorrectly expects a psi_task_switch() call to
> immediately follow.
> 
> Later on when the task get woken/re-enqueued, and psi_flags are
> set for TSK_RUNNING, we hit an error as the task is already
> TSK_RUNNING:
>    psi: inconsistent task state! task=188:kworker/28:0 cpu=28 psi_flags=4 clear=0 set=4
> 
> To resolve this, extend the logic in psi_dequeue() so that
> if the sleep flag is set, we also check if psi_flags have
> TSK_ONCPU set (meaning the psi_task_switch is imminent) before
> we do the shortcut return.
> 
> If TSK_ONCPU is not set, that means we've already switched away,
> and this psi_dequeue call needs to clear the flags.
> 
> Fixes: be41bde4c3a8 ("sched: Add an initial sketch of the find_proxy_task() function")
> Reported-by: K Prateek Nayak <kprateek.nayak@....com>
> Closes: https://lore.kernel.org/lkml/20251117185550.365156-1-kprateek.nayak@amd.com/
> Signed-off-by: John Stultz <jstultz@...gle.com>
> Tested-by: K Prateek Nayak <kprateek.nayak@....com>
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> ---
> proxy-v13:
> * Reworked for collision
> proxy-v15:
> * Fixed commit message typo noticed by Todd Kjos
> v1 (separate from proxy series):
> * Reworded commit message in response to K Prateek pointing
>    out this issue can affect us earlier in the full proxy
>    series then I had anticipated.
> v2 (separate from proxy series):
> * Minor tweak to comment suggested by Johannes
> 
> Cc: Joel Fernandes <joelagnelf@...dia.com>
> Cc: Qais Yousef <qyousef@...alina.io>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Juri Lelli <juri.lelli@...hat.com>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Valentin Schneider <vschneid@...hat.com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Suren Baghdasaryan <surenb@...gle.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ben Segall <bsegall@...gle.com>
> Cc: Zimuzo Ezeozue <zezeozue@...gle.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Will Deacon <will@...nel.org>
> Cc: Waiman Long <longman@...hat.com>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: "Paul E. McKenney" <paulmck@...nel.org>
> Cc: Metin Kaya <Metin.Kaya@....com>
> Cc: Xuewen Yan <xuewen.yan94@...il.com>
> Cc: K Prateek Nayak <kprateek.nayak@....com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
> Cc: Suleiman Souhlal <suleiman@...gle.com>
> Cc: kuyo chang <kuyo.chang@...iatek.com>
> Cc: hupu <hupu.gm@...il.com>
> Cc: kernel-team@...roid.com
> ---
>   kernel/sched/stats.h | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 

Tested-by: Haiyue Wang <haiyuewa@....com>

>   
>   	/*