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Date: Thu, 7 Jul 2022 12:19:56 -0400 From: Steven Rostedt <rostedt@...dmis.org> To: John Keeping <john@...anate.com> Cc: linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>, Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, Daniel Bristot de Oliveira <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com> Subject: Re: [PATCH] sched/core: Always flush pending blk_plug On Thu, 7 Jul 2022 15:39:02 +0100 John Keeping <john@...anate.com> wrote: > Here the kworker is waiting on msdos_sb_info::s_lock which is held by > tar which is in turn waiting for a buffer which is locked waiting to be > flushed, but this operation is plugged in the kworker. > > The lock is a normal struct mutex, so tsk_is_pi_blocked() will always > return false on !RT and thus the behaviour changes for RT. > > It seems that the intent here is to skip blk_flush_plug() in the case > where a non-preemptible lock (such as a spinlock) has been converted to > a rtmutex on RT, which is the case covered by the SM_RTLOCK_WAIT > schedule flag. But sched_submit_work() is only called from schedule() > which is never called in this scenario, so the check can simply be > deleted. > > Looking at the history of the -rt patchset, in fact this change was > present from v5.9.1-rt20 until being dropped in v5.13-rt1 as it was part > of a larger patch [1] most of which was replaced by commit b4bfa3fcfe3b > ("sched/core: Rework the __schedule() preempt argument"). > Nice investigation. So basically what you are saying is that commit b4bfa3fcfe3b was the implementation of [1], but left out the removal of the tsk_is_pi_blocked(), and that what you are seeing is the problem that is described in [1]. Can you add this in the change log: "As described in [1]: The schedule process must distinguish between blocking on a regular sleeping lock (rwsem and mutex) and a RT-only sleeping lock (spinlock and rwlock): - rwsem and mutex must flush block requests (blk_schedule_flush_plug()) even if blocked on a lock. This can not deadlock because this also happens for non-RT. There should be a warning if the scheduling point is within a RCU read section. - spinlock and rwlock must not flush block requests. This will deadlock if the callback attempts to acquire a lock which is already acquired. Similarly to being preempted, there should be no warning if the scheduling point is within a RCU read section. and with the tsk_is_pi_blocked() in the scheduler path, we hit the first issue." Reviewed-by: Steven Rostedt (Google) <rostedt@...dmis.org> -- Steve
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