linux-kernel - Re: [PATCH RT v3 3/5] sched: migrate_dis/enable: Use rt_invol

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Date:   Tue, 24 Sep 2019 18:05:54 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Scott Wood <swood@...hat.com>
Cc:     linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>
Subject: Re: [PATCH RT v3 3/5] sched: migrate_dis/enable: Use rt_invol_sleep

On 2019-09-24 10:47:36 [-0500], Scott Wood wrote:
> When the stop machine finishes it will do a wake_up_process() via
> complete().  Since this does not pass WF_LOCK_SLEEPER, saved_state will be
> cleared, and you'll have TASK_RUNNING when you get to other_func() and
> schedule(), regardless of whether CPU1 sends wake_up() -- so this change
> doesn't actually accomplish anything.

True, I completely missed that part.

> While as noted in the other thread I don't think these spurious wakeups are
> a huge problem, we could avoid them by doing stop_one_cpu_nowait() and then
> schedule() without messing with task state.  Since we're stopping our own
> cpu, it should be guaranteed that the stopper has finished by the time we
> exit schedule().

I remember loosing a state can be a problem. Lets say it is not "just"
TASK_UNINTERRUPTIBLE -> TASK_RUNNING which sounds harmless but it is
__TASK_TRACED and you lose it as part of unlocking siglock.

> -Scott

Sebastian