From: "Nikita V. Youshchenko" <yoush@cs.msu.su>
To: Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [rt-preempt / powerpc] Looks like tick_sched_timer() is a major latency source
Date: Wed, 24 Jun 2009 00:01:47 +0400
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Cc: linux-kernel@vger.kernel.org, Alexander Kaliadin <akaliadin@mvista.com>
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> > I tried hard to identify the latency source, and at some moment
> > discovered tick_sched_timer() from kernel/time/tick-sched.c. This
> > routine is called from timer interrupt with hardware interrupts
> > disabled, and may execute for 50 us and more.
> >
> > Could someone please comment on this?
>
> Did you have CONFIG_NOHZ enabled ? If yes, please disable.

I have disabled CONFIG_NOHZ long ago, since it really influences latency.

> > Is it possible to move (part of) tick_sched_timer() call tree out of
> > hardware-interrupts-disabled context without breaking things?
>
> Yes, we can move things out and split the irq disabled regions. One
> particular thing which can be split out is the time update, which we
> already had in the timer softirq at some point, but we moved it back
> as it showed problems with NTP stability. It's one of the things which
> can be revisited. The whole call chain needs some investigation, but
> that's not in my main focus now.

Is that code with time update split out available anywhere?

Nikita
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