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Date:   Mon, 11 Jul 2022 11:34:57 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Juri Lelli <juri.lelli@redhat.com>,
        Mike Stowell <mstowell@redhat.com>
Subject: Re: [PATCH v3] locking/rtmutex: Limit # of lock stealing for non-RT
 waiters
Message-ID: <YsvuwfeY/dSngl8c@hirez.programming.kicks-ass.net>
References: <20220706135916.980580-1-longman@redhat.com>
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On Wed, Jul 06, 2022 at 09:59:16AM -0400, 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
> rcu_preempt self-detected stall or even task lockup in PREEMPT_RT kernel.
> For example,
> 
> [77107.424943] rcu: INFO: rcu_preempt self-detected stall on CPU
> [ 1249.921363] INFO: task systemd:2178 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 32. That number is arbitrary and can be changed
> if needed.
> 

Why not do the same thing we do for regular mutexes?