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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	Marco Elver <elver@google.com>,
	kasan-dev <kasan-dev@googlegroups.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Anna-Maria Behnsen <anna-maria@linutronix.de>
Subject: Re: timers: Move clearing of base::timer_running under base::lock
Date: Tue, 8 Dec 2020 09:50:49 +0100	[thread overview]
Message-ID: <20201208085049.vnhudd6qwcsbdepl@linutronix.de> (raw)
In-Reply-To: <20201207160648.GF2657@paulmck-ThinkPad-P72>

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.

> 							Thanx, Paul

Sebastian

  reply	other threads:[~2020-12-08  8:51 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-06 21:40 Thomas Gleixner
2020-12-07  1:10 ` Frederic Weisbecker
2020-12-07 12:25   ` Peter Zijlstra
2020-12-07 12:49     ` Frederic Weisbecker
2020-12-07 13:07 ` Sebastian Andrzej Siewior
2020-12-07 14:29   ` Thomas Gleixner
2020-12-07 15:25     ` Sebastian Andrzej Siewior
2020-12-07 16:06       ` Paul E. McKenney
2020-12-08  8:50         ` Sebastian Andrzej Siewior [this message]
2020-12-08 15:04           ` Paul E. McKenney
2020-12-11 14:36           ` Thomas Gleixner
2020-12-11 15:04             ` Sebastian Andrzej Siewior
2021-07-27 19:00 ` [tip: timers/urgent] timers: Move clearing of base::timer_running under base:: Lock tip-bot2 for Thomas Gleixner

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