linux-kernel - Re: [RFC 3/4] Disable scheduler tick when we are running SCHED

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Message-ID: <87iq2gw46e.fsf@basil.nowhere.org>
Date:	Wed, 08 Sep 2010 19:11:53 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Jan Blunck <jblunck@...ell.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linux-Kernel Mailinglist <linux-kernel@...r.kernel.org>,
	linux-rt-users@...r.kernel.org,
	Sven-Thorsten Dietrich <sdietrich@...ell.com>,
	Michael Galbraith <MGalbraith@...ell.com>
Subject: Re: [RFC 3/4] Disable scheduler tick when we are running SCHED_FIFO tasks

Jan Blunck <jblunck@...ell.com> writes:

> On Wed, Sep 08, Peter Zijlstra wrote:
>
>> On Wed, 2010-09-08 at 14:29 +0200, Jan Blunck wrote:
>> > This patch is disabling the scheduler tick to go off when there is a task
>> > with SCHED_FIFO policy running. Since these tasks are not timesliced anyway
>> > we only care about timers, softirqs and such stuff just like when we disable
>> > the tick during idle periods.
>> > 
>> 
>> Also, doesn't this break any and all jiffies users?
>> 
>
> Sure. It was enough to make FTQ run.
>
>> And you need to restart the tick on call_rcu() and everything else that
>> requires the tick for processing.
>> 
>
> I realized that before as well but somehow forgot about it. Checking for a
> disabled timer tick on syscall entry would help. Like we do on irq entry as
> well.

That doesn't help the gettimeofday/clock_gettime vDSOs
which access jiffie like state in user space.

I guess you could force the vdso to call into the kernel
during such a region, but you cannot force it for an
process that is already context switched in this path
when the option is first enabled.

I guess it would work if you force all the vDSOs
to always do a system call at boot, but that's pretty
costly.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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