Re: [RFC 3/4] Disable scheduler tick when we are running SCHED_FIFO tasks

[Andi Kleen <andi@firstfloor.org>]

Jan Blunck <jblunck@novell.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@linux.intel.com -- Speaking for myself only.