linux-kernel - Re: [PATCH V2 Resend 4/4] timer: Migrate running timer

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Date:	Tue, 9 Apr 2013 20:22:45 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	tglx@...utronix.de, Steven Rostedt <rostedt@...dmis.org>
Cc:	pjt@...gle.com, paul.mckenney@...aro.org, tj@...nel.org,
	mingo@...hat.com, peterz@...radead.org, Arvind.Chauhan@....com,
	linaro-dev@...ts.linaro.org, patches@...aro.org,
	pdsw-power-team@....com, linux-kernel@...r.kernel.org,
	linux-rt-users@...r.kernel.org, john stultz <johnstul@...ibm.com>
Subject: Re: [PATCH V2 Resend 4/4] timer: Migrate running timer

[Steven replied to a personal Ping!!, including everybody again]

On 9 April 2013 19:25, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Tue, 2013-04-09 at 14:05 +0530, Viresh Kumar wrote:
>> Ping!!
>>
>
> Remind me again. What problem are you trying to solve?

I was trying to migrate a running timer which arms itself, so that we don't
keep a cpu busy just for servicing this timer.

>> On 20 March 2013 20:43, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>> >
>> > Hi Steven/Thomas,
>> >
>> > I came back to this patch after completing some other stuff and posting
>> > wq part of this patchset separately.
>> >
>> > I got your point and understand how this would fail.
>> >
>> > @Thomas: I need your opinion first. Do you like this concept of migrating
>> > running timer or not? Or you see some basic problem with this concept?
>
> I'll let Thomas answer this, but to me, this sounds really racy.

Sure.

>> > If no (i.e. i can go ahead with another version), then i have some solution to
>> > fix earlier problems reported by Steven:
>> >
>> > The problem lies with del_timer_sync() which just checks
>> > base->running_timer != timer to check if timer is currently running or not.
>> >
>> > What if we add another variable in struct timer_list, that will store if we are
>> > running timer callback or not. And so, before we call callback in timer core,
>> > we will set this variable and will reset it after finishing callback.
>> >
>> > del_timer_sync() will have something like:
>> >
>> > if (base->running_timer != timer)
>> >     remove timer and return;
>
> For example, this didn't fix the issue. You removed the timer when it
> was still running, because base->running_timer did not equal timer.

You are correct and i was stupid. I wanted to write this instead:

del_timer_sync() will have something like:

if (base->running_timer != timer)
    if (timer->running_callback)
        go back to its loop...
    else
        remove timer and return;

i.e. if we aren't running on our base cpu, just check if our callback is
executing somewhere else due to migration.
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