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

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Date:   Tue, 24 Sep 2019 10:47:36 -0500
From:   Scott Wood <swood@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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 Tue, 2019-09-24 at 17:25 +0200, Sebastian Andrzej Siewior wrote:
> On 2019-09-24 08:53:43 [-0500], Scott Wood wrote:
> > As I pointed out in the "[PATCH RT 6/8] sched: migrate_enable: Set state
> > to
> > TASK_RUNNING" discussion, we can get here inside the rtmutex code (e.g.
> > from
> > debug_rt_mutex_print_deadlock) where saved_state is already holding
> > something -- plus, the waker won't have WF_LOCK_SLEEPER and therefore
> > saved_state will get cleared anyway.
> 
> So let me drop the saved_state pieces and get back to it once I get to
> the other thread (which you replied and I didn't realised until now).
> 
> Regarding the WF_LOCK_SLEEPER part. I think this works as expected.
> Imagine:
> 
> CPU0						CPU1
> spin_lock();
> set_current_state(TASK_UNINTERRUPTIBLE);
> …
> spin_unlock()
>  -> migrate_enable();
>    -> stop_one_cpu();				<-- A)
> other_func();					<-- B)
> schedule();
> 
> So. With only CPU0 we enter schedule() with TASK_UNINTERRUPTIBLE because
> the state gets preserved with the change I added (which is expected).
> If CPU1 sends a wake_up() at A) then the saved_state gets overwritten
> and we enter schedule() with TASK_RUNNING. Same happens if it is sent at
> B) point which is outside of any migrate/spin lock related code. 
> 
> Was this clear or did I miss the point?

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.

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().

-Scott