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Date: Wed, 8 Sep 2010 16:30:07 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Joakim Tjernlund <joakim.tjernlund@transmode.se>
cc: Eric Dumazet <eric.dumazet@gmail.com>, linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: slow nanosleep?
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On Wed, 8 Sep 2010, Joakim Tjernlund wrote:
> Thomas Gleixner <tglx@linutronix.de> wrote on 2010/09/08 15:52:23:
> > On Wed, 8 Sep 2010, Joakim Tjernlund wrote:
> > > 
> > > Actually, it takes 120 us. The 20 us was when I had Thomas
> > > timeout == 0 fast path patch applied(forgot to remove it).
> > > Without that patch it takes about 115 us. So it seems it takes
> > > 115-20=95 us to turn the timer wheel on my ppc.
> >
> > You might fire up the tracer to look where it spends that time.
> 
> This helps for short(1 ns) nanosleeps, sleeps for 25 us. No idea
> if this is any good, just tossing it out for you to tear apart :)
> 
> diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
> index 5c69e99..e612016 100644
> --- a/kernel/hrtimer.c
> +++ b/kernel/hrtimer.c
> @@ -1545,6 +1545,9 @@ long __sched hrtimer_nanosleep_restart(struct restart_block *restart)
>  				HRTIMER_MODE_ABS);
>  	hrtimer_set_expires_tv64(&t.timer, restart->nanosleep.expires);
> 
> +	if (!hrtimer_active(&t.timer))
> +		goto out;

That actually will return for any expiry time. The timer is armed in
do_nanosleep() not in hrtimer_set_expires_tv64() /
hrtimer_set_expires_range_ns()

Thanks,

	tglx