linux-kernel - Re: [PATCH v2] sched/core: Fix RQCF_ACT

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Message-ID: <20231020104601.GA33965@noisy.programming.kicks-ass.net>
Date:   Fri, 20 Oct 2023 12:46:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Hao Jia <jiahao.os@...edance.com>
Cc:     mingo@...hat.com, mingo@...nel.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Igor Raits <igor.raits@...il.com>,
        Bagas Sanjaya <bagasdotme@...il.com>
Subject: Re: [PATCH v2] sched/core: Fix RQCF_ACT_SKIP leak

On Thu, Oct 12, 2023 at 05:00:03PM +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
> 
> This warning may be triggered 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()
>       raw_spin_rq_unlock(this_rq)
>                                       rq_lock(*CPU0_rq*, &rf)
>                                       rq_clock_start_loop_update()
>                                       rq->clock_update_flags & RQCF_ACT_SKIP <--
>       raw_spin_rq_lock(this_rq)
> 
> The purpose of RQCF_ACT_SKIP is to skip the update rq clock,
> but the update is very early in __schedule(), but we clear
> RQCF_*_SKIP very late, causing it to span that gap above
> and triggering this warning.
> 
> In __schedule() we can clear the RQCF_*_SKIP flag immediately
> after update_rq_clock() to avoid this RQCF_ACT_SKIP leak warning.
> And set rq->clock_update_flags to RQCF_UPDATED to avoid
> rq->clock_update_flags < RQCF_ACT_SKIP warning that may be triggered later.
> 

Thanks!