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

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Message-ID: <20201208085049.vnhudd6qwcsbdepl@linutronix.de>
Date:   Tue, 8 Dec 2020 09:50:49 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Paul E. McKenney" <paulmck@...nel.org>
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 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