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

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Date:	Mon, 13 May 2013 12:35:30 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Viresh Kumar <viresh.kumar@...aro.org>
cc:	Steven Rostedt <rostedt@...dmis.org>, 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

On Mon, 13 May 2013, Viresh Kumar wrote:
> On 24 April 2013 16:52, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > On 9 April 2013 20:22, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> >> [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.

Which mechanism is migrating the timer away?

> >>>> 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 have no objections to the functionality per se, but the proposed
solution is not going to fly.

Aside of bloating the data structure you're changing the semantics of
__mod_timer(). No __mod_timer() caller can deal with -EBUSY. So you'd
break the world and some more.

Here is a list of questions:

      - Which mechanism migrates timers?

      - How is that mechanism triggered?

      - How does that deal with CPU bound timers?

Thanks,

	tglx



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