linux-kernel - Re: [PATCH 16/20] sched/idle: Use explicit broadcast oneshot control function

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Date:	Tue, 28 Apr 2015 14:19:11 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Sudeep Holla <sudeep.holla@....com>
Cc:	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>
Subject: Re: [PATCH 16/20] sched/idle: Use explicit broadcast oneshot control function

On Tue, Apr 28, 2015 at 12:42 PM, Sudeep Holla <sudeep.holla@....com> wrote:
>
>
> On 28/04/15 11:34, Daniel Lezcano wrote:
>>
>> On 04/28/2015 12:11 PM, Linus Walleij wrote:
>>>
>>> On Thu, Apr 2, 2015 at 12:22 AM, Rafael J. Wysocki
>>> <rjw@...ysocki.net> wrote:
>>>
>>>> From: Thomas Gleixner <tglx@...utronix.de>
>>>>
>>>> Replace the clockevents_notify() call with an explicit function
>>>> call.
>>>>
>>>> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>>>> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>>>
>>>
>>> For some reason this makes my Ux500 system arbitrarily hang,
>>> especially during boot. Bisected down to this commit. Since the
>>> entire changeset is removing the notifications altogether I can't
>>> just revert it.
>>>
>>> Disabling CONFIG_CPU_IDLE removes the problem.
>>>
>>> Tried registering a stub driver (I just #if 0 all the code in
>>> drivers/cpuidle/cpuidle-ux500.c) it still crashes.
>>>
>>> That makes me think something inside the cpuidle subsystem is
>>> locking up after this, but my other idea is that the timer may be
>>> involved in some way, like this is stressing the timer in some new
>>> yet untested way.
>>>
>>> Has anyone else seen problems with this or is it only ux500?
>>>
>>> I'm looking closer at it but feel a bit clueless...
>>
>>
>> If you keep the #if 0 and remove the CPUIDLE_FLAG_TIMER_STOP flag,
>> does it still crash ?
>>
>
> At-least I observed issue only when I am using hardware broadcast timer.
> It doesn't hang when I am using hrtimer as broadcast timer in which case
> one of the cpu will be not enter deeper idle states that lose timer.
> I will rerun on v4.1-rc1 and post the complete log.

So the bug here is that cpuidle_enter() enables interrupts, so the
assumption about them being not enabled made by
tick_broadcast_oneshot_control() is actually not valid.

It looks like we need to acquire the clockevents_lock at least in this
particular case.  Let me see where to put it and I'll send a patch for
testing.

Rafael
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