Re: [PATCH V2] clockevents: Fix cpu down race for hrtimer based broadcasting

Ingo Molnar
2015-04-02T10:42:34Z

* Preeti U Murthy <preeti@linux.vnet.ibm.com> wrote:

> It was found when doing a hotplug stress test on POWER, that the machine
> either hit softlockups or rcu_sched stall warnings. The issue was
> traced to commit 7cba160ad789a powernv/cpuidle: Redesign idle states
> management, which exposed the cpu down race with hrtimer based broadcast
> mode(Commit 5d1638acb9f6(tick: Introduce hrtimer based broadcast). This
> is explained below.
>
> Assume CPU1 is the CPU which holds the hrtimer broadcasting duty before
> it is taken down.
>
> CPU0 CPU1
>
> cpu_down() take_cpu_down()
> disable_interrupts()
>
> cpu_die()
>
> while(CPU1 != CPU_DEAD) {
> msleep(100);
> switch_to_idle();
> stop_cpu_timer();
> schedule_broadcast();
> }
>
> tick_cleanup_cpu_dead()
> take_over_broadcast()
>
> So after CPU1 disabled interrupts it cannot handle the broadcast hrtimer
> anymore, so CPU0 will be stuck forever.
>
> Fix this by explicitly taking over broadcast duty before cpu_die().
> This is a temporary workaround. What we really want is a callback in the
> clockevent device which allows us to do that from the dying CPU by
> pushing the hrtimer onto a different cpu. That might involve an IPI and
> is definitely more complex than this immediate fix.

So why not use a suitable CPU_DOWN* notifier for this, instead of open
coding it all into a random place in the hotplug machinery?

Also, I improved the changelog (attached below), but decided against
applying it until these questions are cleared - please use that for
future versions of this patch.

Thanks,

Ingo

===================>
>From 413fbf5193b330c5f478ef7aaeaaee08907a993e Mon Sep 17 00:00:00 2001
From: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Date: Mon, 30 Mar 2015 14:59:19 +0530
Subject: [PATCH] clockevents: Fix cpu_down() race for hrtimer based broadcasting

It was found when doing a hotplug stress test on POWER, that the
machine either hit softlockups or rcu_sched stall warnings. The
issue was traced to commit:

7cba160ad789 ("powernv/cpuidle: Redesign idle states management")

which exposed the cpu_down() race with hrtimer based broadcast mode:

5d1638acb9f6 ("tick: Introduce hrtimer based broadcast")

The race is the following:

Assume CPU1 is the CPU which holds the hrtimer broadcasting duty
before it is taken down.

CPU0 CPU1

cpu_down() take_cpu_down()
disable_interrupts()

cpu_die()

while (CPU1 != CPU_DEAD) {
msleep(100);
switch_to_idle();
stop_cpu_timer();
schedule_broadcast();
}

tick_cleanup_cpu_dead()
take_over_broadcast()

So after CPU1 disabled interrupts it cannot handle the broadcast
hrtimer anymore, so CPU0 will be stuck forever.

Fix this by explicitly taking over broadcast duty before cpu_die().

This is a temporary workaround. What we really want is a callback
in the clockevent device which allows us to do that from the dying
CPU by pushing the hrtimer onto a different cpu. That might involve
an IPI and is definitely more complex than this immediate fix.

Changelog was picked up from:

https://lkml.org/lkml/2015/2/16/213

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Preeti U. Murthy <preeti@linux.vnet.ibm.com>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: mpe@ellerman.id.au
Cc: nicolas.pitre@linaro.org
Cc: peterz@infradead.org
Cc: rjw@rjwysocki.net
Fixes: http://linuxppc.10917.n7.nabble.com/offlining-cpus-breakage-td88619.html