linux-kernel - Re: [patch 61/66] timers: Convert to hotplug state machine

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Date:	Tue, 26 Jul 2016 10:20:58 +0100
From:	Jon Hunter <jonathanh@...dia.com>
To:	<rcochran@...utronix.de>
CC:	Anna-Maria Gleixner <anna-maria@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [patch 61/66] timers: Convert to hotplug state machine


On 25/07/16 16:35, rcochran@...utronix.de wrote:
> On Mon, Jul 25, 2016 at 03:56:48PM +0100, Jon Hunter wrote:
>>> There is a hidden dependency between:
>>>
>>> - timers
>>> - Block multiqueue
>>> - rcutree
>>>
>>> If timers_dead_cpu() comes later than blk_mq_queue_reinit_notify()
>>> that latter function causes a RCU stall.
>>
>> After this change is applied I am seeing RCU stalls during suspend
>> on Tegra. I guess I am hitting the case mentioned above? How should
>> this be avoided?
> 
> The problem that I had found was a hidden dependency.  When I
> initially placed the timers callback into the new HP state list, that
> caused a stall because the dependency was broken.  The old code worked
> by luck, based on the order of the notifier registrations.  The new
> code makes the old implicit ordering explicit, so it should work just
> as well as before (famous last words).

I see.

>> Interestingly I am only seeing the above when using the ARM
>> multi_v7_defconfig kernel configuration and not with the tegra_defconfig.
>> One key difference between these is that the multi_v7_defconfig does not
>> have CONFIG_PREEMPT enabled. Initial testing shows enabling CONFIG_PREEMPT
>> for multi_v7_defconfig makes the problem go away.
> 
> Just to be sure, this problem didn't exist before the HP rework, that
> is, suspend worked fine with and without CONFIG_PREEMPT, right?

Correct. I test suspend on Tegra with both multi_v7_defconfig
(CONFIG_PREEMPT disabled) and tegra_defconfig (CONFIG_PREEMPT enabled).
Looking at the git history for these configs I don't see any changes in
this regard since they were added (unless some underlying Kconfig files
have changed).

> I see if I can find a tegra system to test with...

Thanks. I have not tried another ARM based device, but I would be
curious if another ARM device sees this or not.

Cheers
Jon

-- 
nvpublic