From: Scott Wood <swood@redhat.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org,
"Paul E . McKenney" <paulmck@linux.ibm.com>,
Joel Fernandes <joel@joelfernandes.org>,
Thomas Gleixner <tglx@linutronix.de>,
Steven Rostedt <rostedt@goodmis.org>,
Peter Zijlstra <peterz@infradead.org>,
Juri Lelli <juri.lelli@redhat.com>,
Clark Williams <williams@redhat.com>
Subject: Re: [PATCH RT v3 3/5] sched: migrate_dis/enable: Use rt_invol_sleep
Date: Tue, 24 Sep 2019 10:47:36 -0500 [thread overview]
Message-ID: <1a2234884e55e5ee6df5f32f828a99c1b248933f.camel@redhat.com> (raw)
In-Reply-To: <20190924152514.enzeuoo5a6o3mgqu@linutronix.de>
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
next prev parent reply other threads:[~2019-09-24 15:47 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-11 16:57 [PATCH RT v3 0/5] RCU fixes Scott Wood
2019-09-11 16:57 ` [PATCH RT v3 1/5] rcu: Acquire RCU lock when disabling BHs Scott Wood
2019-09-12 22:09 ` Joel Fernandes
2019-09-17 7:44 ` Sebastian Andrzej Siewior
2019-09-17 14:06 ` Scott Wood
2019-09-17 14:42 ` Sebastian Andrzej Siewior
2019-09-17 16:12 ` Scott Wood
2019-09-23 16:41 ` Sebastian Andrzej Siewior
2019-09-11 16:57 ` [PATCH RT v3 2/5] sched: Rename sleeping_lock to rt_invol_sleep Scott Wood
2019-09-17 7:52 ` Sebastian Andrzej Siewior
2019-09-11 16:57 ` [PATCH RT v3 3/5] sched: migrate_dis/enable: Use rt_invol_sleep Scott Wood
2019-09-17 7:59 ` Sebastian Andrzej Siewior
2019-09-17 14:06 ` Scott Wood
2019-09-23 16:59 ` Scott Wood
2019-09-23 17:52 ` Sebastian Andrzej Siewior
2019-09-24 11:21 ` Sebastian Andrzej Siewior
2019-09-24 13:53 ` Scott Wood
2019-09-24 15:25 ` Sebastian Andrzej Siewior
2019-09-24 15:47 ` Scott Wood [this message]
2019-09-24 16:05 ` Sebastian Andrzej Siewior
2019-09-24 16:35 ` Scott Wood
2019-10-04 16:45 ` Sebastian Andrzej Siewior
2019-09-11 16:57 ` [PATCH RT v3 4/5] rcu: Disable use_softirq on PREEMPT_RT Scott Wood
2019-09-12 21:38 ` Joel Fernandes
2019-09-12 22:19 ` Joel Fernandes
2019-09-17 9:31 ` Sebastian Andrzej Siewior
2019-09-17 14:08 ` Scott Wood
2019-09-11 16:57 ` [PATCH RT v3 5/5] rcutorture: Avoid problematic critical section nesting on RT Scott Wood
2019-09-12 22:17 ` Joel Fernandes
2019-09-16 16:55 ` Scott Wood
2019-09-17 10:07 ` Sebastian Andrzej Siewior
2019-09-17 14:36 ` Scott Wood
2019-09-17 14:50 ` Sebastian Andrzej Siewior
2019-09-17 16:32 ` Scott Wood
2019-09-23 16:25 ` Sebastian Andrzej Siewior
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