linux-kernel - Re: [BUG] "perf top" results in "NOHZ: local_softirq

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Date:	Tue, 7 Dec 2010 15:07:36 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Paul Mackerras <paulus@...ba.org>, linux-kernel@...r.kernel.org
Subject: Re: [BUG] "perf top" results in "NOHZ: local_softirq_pending 100"

On Tue, 7 Dec 2010, Peter Zijlstra wrote:
> On Tue, 2010-12-07 at 14:29 +0100, Heiko Carstens wrote:
> > > As far as I could see this function gets called from process context with
> > > a spinlock held and hence we don't have any guarantee that this pending
> > > softirq get executed before the idle task gets scheduled and tries to
> > > disable the tick.
> > > 
> > > The easiest fix would be to set wakeup to one (see patch below), but I guess
> > > there is a reason why its zero. Anybody?
> 
> We can start that hrtimer from within the scheduler function while
> holding the rq->lock, doing a wakeup from there is not sane.
> 
> The best solution would be to fix the hrtimer_start*() interface,
> something Thomas and I have wanted to do for ages but because we've
> procrastinated is now a much larger job than it was :/
> 
> The whole HRTIMER_SOFTIRQ thing should die.. but for that to happen its
> only use-case today must first go.
> 
> The problem is trying to start a timer with already elapsed time.
> Preferably hrtimer_start*() would simply return -ETIME and let the
> caller sort it, sadly the current behaviour is to 'fix' it for the
> caller by enqueueing the timer onto the softirq list and raising the
> softirq.
> 
> I guess we could make hrtimer_start*(.wakeup=false) return the -ENOTIME
> thing and audit those few use-cases.

That would be sensible anyway.
 
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