linux-kernel - Re: [patch 1/2] sched, timers: move calc

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Date:	Thu, 14 May 2009 22:40:03 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dimitri Sivanich <sivanich@....com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [patch 1/2] sched, timers: move calc_load() to scheduler

On Thu, 14 May 2009, Peter Zijlstra wrote:
> On Thu, 2009-05-14 at 11:21 +0000, Thomas Gleixner wrote:
> > plain text document attachment (move-calc-load-to-scheduler-v1.patch)
> 
> > +/*
> > + * calc_load - update the avenrun load estimates 10 ticks after the
> > + * CPUs have updated calc_load_tasks.
> > + */
> > +void calc_global_load(void)
> > +{
> > +	unsigned long upd = calc_load_update + 10;
> > +	long active;
> > +
> > +	if (time_before(jiffies, upd))
> > +		return;
> >  
> > +	active = atomic_long_read(&calc_load_tasks);
> > +	active = active > 0 ? active * FIXED_1 : 0;
> >  
> > +	avenrun[0] = calc_load(avenrun[0], EXP_1, active);
> > +	avenrun[1] = calc_load(avenrun[1], EXP_5, active);
> > +	avenrun[2] = calc_load(avenrun[2], EXP_15, active);
> > +
> > +	calc_load_update += LOAD_FREQ;
> > +}
> 
> > @@ -1211,7 +1160,8 @@ static inline void update_times(unsigned
> >  void do_timer(unsigned long ticks)
> >  {
> >  	jiffies_64 += ticks;
> > -	update_times(ticks);
> > +	update_wall_time();
> > +	calc_global_load();
> >  }
> 
> I can see multiple cpus fall into calc_global_load() concurrently, which
> would 'age' the load faster than expected.
> 
> Should we plug that hole?

They can't. do_timer() is called by exactly one CPU under xtime
lock. What we removed is the loop over all online CPUs to retrieve the
number of active tasks.

Thanks,

	tglx
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