linux-kernel - Re: timers: Move clearing of base::timer

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Message-ID: <20201208150446.GQ2657@paulmck-ThinkPad-P72>
Date:   Tue, 8 Dec 2020 07:04:46 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Marco Elver <elver@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>
Subject: Re: timers: Move clearing of base::timer_running under base::lock

On Tue, Dec 08, 2020 at 09:50:49AM +0100, Sebastian Andrzej Siewior wrote:
> On 2020-12-07 08:06:48 [-0800], Paul E. McKenney wrote:
> > > Yes, but it triggers frequently. Like `rcuc' is somehow is aligned with
> > > the timeout.
> > 
> > Given that a lot of RCU processing is event-driven based on timers,
> > and given that the scheduling-clock interrupts are synchronized for
> > energy-efficiency reasons on many configs, maybe this alignment is
> > expected behavior?
> 
> No, it is the fact that rcu_preempt has a higher priority than
> ksoftirqd. So immediately after the wakeup (of rcu_preempt) there is a
> context switch and expire_timers() has this:
> 
> |   raw_spin_unlock_irq(&base->lock);
> |   call_timer_fn(timer, fn, baseclk);
> |   raw_spin_lock_irq(&base->lock);
> |   base->running_timer = NULL;
> |   timer_sync_wait_running(base);
> 
> So ->running_timer isn't reset and try_to_del_timer_sync() (that
> del_timer_sync() from schedule_timeout()) returns -1 and then the corner
> case is handled where `expiry_lock' is acquired. So everything goes as
> expected.

Makes sense!  Thank you for the explanation!

							Thanx, Paul