linux-kernel - Re: Question on task_blocks_on_rt

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Message-ID: <20200904172432.wgxn6x5tpnt4lcln@linux-p48b>
Date:   Fri, 4 Sep 2020 10:24:32 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     peterz@...radead.org, mingo@...hat.com, will@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Question on task_blocks_on_rt_mutex()

On Thu, 03 Sep 2020, Paul E. McKenney wrote:

>commit d93a64389f4d544ded241d0ba30b2586497f5dc0
>Author: Paul E. McKenney <paulmck@...nel.org>
>Date:   Tue Sep 1 16:58:41 2020 -0700
>
>    torture: Periodically pause in stutter_wait()
>
>    Running locktorture scenario LOCK05 results in hangs:
>
>    tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --torture lock --duration 3 --configs LOCK05
>
>    The lock_torture_writer() kthreads set themselves to MAX_NICE while
>    running SCHED_OTHER.  Other locktorture kthreads run at default niceness,
>    also SCHED_OTHER.  This results in these other locktorture kthreads
>    indefinitely preempting the lock_torture_writer() kthreads.  Note that
>    the cond_resched() in the stutter_wait() function's loop is ineffective
>    because this scenario is built with CONFIG_PREEMPT=y.
>
>    It is not clear that such indefinite preemption is supposed to happen, but
>    in the meantime this commit prevents kthreads running in stutter_wait()
>    from being completely CPU-bound, thus allowing the other threads to get
>    some CPU in a timely fashion.  This commit also uses hrtimers to provide
>    very short sleeps to avoid degrading the sudden-on testing that stutter
>    is supposed to provide.
>
>    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

Reviwed-by: Davidlohr Bueso <dbueso@...e.de>