linux-kernel - Re: Question on task_blocks_on_rt

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Message-ID: <20200902015128.wsulcxhbo7dutcjz@linux-p48b>
Date:   Tue, 1 Sep 2020 18:51:28 -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 Tue, 01 Sep 2020, Paul E. McKenney wrote:

>And it appears that a default-niced CPU-bound SCHED_OTHER process is
>not preempted by a newly awakened MAX_NICE SCHED_OTHER process.  OK,
>OK, I never waited for more than 10 minutes, but on my 2.2GHz that is
>close enough to a hang for most people.
>
>Which means that the patch below prevents the hangs.  And maybe does
>other things as well, firing rcutorture up on it to check.
>
>But is this indefinite delay expected behavior?
>
>This reproduces for me on current mainline as follows:
>
>tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --torture lock --duration 3 --configs LOCK05
>
>This hangs within a minute of boot on my setup.  Here "hangs" is defined
>as stopping the per-15-second console output of:
>	Writes:  Total: 569906696 Max/Min: 81495031/63736508   Fail: 0

Ok this doesn't seem to be related to lockless wake_qs then. fyi there have
been missed wakeups in the past where wake_q_add() fails the cmpxchg because
the task is already pending a wakeup leading to the actual wakeup ocurring
before its corresponding wake_up_q(). This is why we have wake_q_add_safe().
But for rtmutexes, because there is no lock stealing only top-waiter is awoken
as well as try_to_take_rt_mutex() is done under the lock->wait_lock I was not
seeing an actual race here.

Thanks,
Davidlohr