linux-kernel - Re: [PATCH v2] Move calc_load call out from xtime

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Date:	Sat, 11 Apr 2009 18:53:45 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Dimitri Sivanich <sivanich@....com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, john stultz <johnstul@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2] Move calc_load call out from xtime_lock protection

On Sat, 11 Apr 2009, Dimitri Sivanich wrote:

> On Sat, Apr 11, 2009 at 11:51:36AM +0200, Thomas Gleixner wrote:
> > On Fri, 10 Apr 2009, Dimitri Sivanich wrote:
> > 
> > > The xtime_lock is being held for long periods on larger systems due
> > > to an extensive amount of time being spent in calc_load(),
> > > specifically here:
> > >   do_timer->update_times->calc_load->count_active_tasks->nr_active()
> > > 
> > > On a 64 cpu system I've seen this take approximately 55 usec.
> > > Presumably it would be worse on larger systems.  This causes other
> > > cpus to be held off in places such as
> > > scheduler_tick->sched_clock_tick waiting for the xtime_lock to be
> > > released.
> > 
> > I thought more about that. Why don't we move the calc_load() call into
> > the timer softirq context and avoid fiddling with all the call sites ?
> > Also moving calc_load out of the timer interrupt context reduces the
> > interrupts off section as well.
> > 
> Sounds reasonable to me.

Ok. I looked once more and I wonder if it's possible to avoid the
whole for_each_online_cpu() business by moving the accounting into the
scheduler and be a bit more clever than just blindly running over all
cpus everytime. 

/me goes back to stare a bit more at the avenrun stuff

Thanks,

	tglx



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