linux-kernel - Re: [patch 1/2] idle profile hits with NOHZ

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Message-ID: <20090529145614.77694e20@skybase>
Date:	Fri, 29 May 2009 14:56:14 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org, Rob van der Heij <rvdheij@...il.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>, john stultz <johnstul@...ibm.com>
Subject: Re: [patch 1/2] idle profile hits with NOHZ

On Thu, 28 May 2009 22:19:44 +0200 (CEST)
Thomas Gleixner <tglx@...utronix.de> wrote:

> On Thu, 28 May 2009, Martin Schwidefsky wrote:
> > --- quilt-2.6.orig/kernel/time/tick-sched.c
> > +++ quilt-2.6/kernel/time/tick-sched.c
> > @@ -166,6 +166,7 @@ static void tick_nohz_stop_idle(int cpu)
> >  		ts->idle_lastupdate = now;
> >  		ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
> >  		ts->idle_active = 0;
> > +		ts->idle_pc = profile_pc(get_irq_regs());
> 
>   Hmm, tick_nohz_stop_idle() is called from
>   tick_nohz_restart_sched_tick() as well in the context of the idle
>   task.
> 
>   I think there is no guarantee that get_irq_regs() will return
>   anything useful in thread context.
> 
>   So get_irq_regs() might return a NULL pointer which will explode
>   some of the profile_pc() implementations. If not it can still feed
>   total nonsense to the profile_hits() call.

Drat, but if it is called from idle we should be able to just skip the
assignment to ts->idle_pc, no? Then a simple in_interrupt() test is
missing.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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