linux-kernel - Re: [PATCH v2] locking/rtmutex: Limit # of lock stealing for non-RT waiters

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Date:   Tue, 21 Jun 2022 15:38:10 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Juri Lelli <juri.lelli@...hat.com>,
        Mike Stowell <mstowell@...hat.com>
Subject: Re: [PATCH v2] locking/rtmutex: Limit # of lock stealing for non-RT
 waiters

On 6/21/22 15:36, Waiman Long wrote:
> Commit 48eb3f4fcfd3 ("locking/rtmutex: Implement equal priority lock
> stealing") allows unlimited number of lock stealing's for non-RT
> tasks. That can lead to lock starvation of non-RT top waiter tasks if
> there is a constant incoming stream of non-RT lockers. This can cause
> task lockup in PREEMPT_RT kernel. For example,
>
> [ 1249.921363] INFO: task systemd:2178 blocked for more than 622 seconds.
> [ 1872.984225] INFO: task kworker/6:4:63401 blocked for more than 622 seconds.
>
> Avoiding this problem and ensuring forward progress by limiting the
> number of times that a lock can be stolen from each waiter. This patch
> sets a threshold of 10. That number is arbitrary and can be changed
> if needed.
>
> With that change, the task lockups previously observed when running
> stressful workloads on PREEMPT_RT kernel disappeared.
>
> Fixes: 48eb3f4fcfd3 ("locking/rtmutex: Implement equal priority lock stealing")
> Reported-by: Mike Stowell <mstowell@...hat.com>
> Signed-off-by: Waiman Long <longman@...hat.com>

There is no code change in v2. I have only updated the patch description.

Cheers,
Longman