linux-kernel - Re: [PATCH] avoid using smp_processor_id() in preemptible code (nr_iowait

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Date:	Tue, 15 Jun 2010 17:50:29 +0300
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Jiri Slaby <jslaby@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] avoid using smp_processor_id() in preemptible code
 (nr_iowait_cpu)

On (06/15/10 07:24), Arjan van de Ven wrote:
> On Tue, 15 Jun 2010 09:19:27 +0300
> Sergey Senozhatsky <sergey.senozhatsky@...il.com> wrote:
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index 1d7b9bc..101e8aa 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -159,10 +159,12 @@ update_ts_time_stats(struct tick_sched *ts,
> > ktime_t now, u64 *last_update_time) ktime_t delta;
> >  
> >  	if (ts->idle_active) {
> > +		int cpu = get_cpu();
> >  		delta = ktime_sub(now, ts->idle_entrytime);
> >  		ts->idle_sleeptime = ktime_add(ts->idle_sleeptime,
> > delta);
> > -		if (nr_iowait_cpu() > 0)
> > +		if (nr_iowait_cpu(cpu) > 0)
> >  			ts->iowait_sleeptime =
> > ktime_add(ts->iowait_sleeptime, delta);
> > +		put_cpu();
> >  		ts->idle_entrytime = now;
> >  	}
> 
> 
> hmm this part is wrong
> you pick the current cpu, rather than the one denoted by ts.....
> 

Hmm. Thanks, good catch. 
Well, there is something I'm missing. How can I match given *ts and cpu 
in update_ts_time_stats (except for introducing update_ts_time_stats(..., int cpu)) ?

> they will normally be the same, except for the case where you get your
> warning...
> (and this is also why you can't move the get_cpu() inside
> nr_iowait_cpu() ... )
> 


	Sergey
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