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

[Ingo Molnar]

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