linux-kernel - Re: vmstat: On demand vmstat workers V4

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Date:	Fri, 9 May 2014 00:18:30 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Christoph Lameter <cl@...ux.com>,
	Gilad Ben-Yossef <gilad@...yossef.com>,
	Tejun Heo <tj@...nel.org>, John Stultz <johnstul@...ibm.com>,
	Mike Frysinger <vapier@...too.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, hughd@...gle.com,
	viresh.kumar@...aro.org
Subject: Re: vmstat: On demand vmstat workers V4

On Thu, 8 May 2014, Andrew Morton wrote:
> On Thu, 8 May 2014 10:35:15 -0500 (CDT) Christoph Lameter <cl@...ux.com> wrote:
> > --- linux.orig/kernel/time/tick-common.c	2014-05-06 10:51:19.711239813 -0500
> > +++ linux/kernel/time/tick-common.c	2014-05-06 10:51:19.711239813 -0500
> > @@ -222,6 +222,24 @@
> >  		tick_setup_oneshot(newdev, handler, next_event);
> >  }
> > 
> > +/*
> > + * Return a cpu number that may be used to run housekeeping
> > + * tasks. This is usually the timekeeping cpu unless that
> > + * is not available. Then we simply fall back to the current
> > + * cpu.
> > + */
> 
> This comment is unusably vague.  What the heck is a "housekeeping
> task"?  Why would anyone call this and what is special about the CPU
> number it returns?
> 
> 
> > +int tick_get_housekeeping_cpu(void)
> > +{
> > +	int cpu;
> > +
> > +	if (system_state < SYSTEM_RUNNING || tick_do_timer_cpu < 0)
> > +		cpu = raw_smp_processor_id();

That's completely bogus. The system state check is pointless and
tick_do_timer_cpu even more so because if you call that code from a
worker thread tick_do_timer_cpu should be assigned to some cpu.

Aside of that I'm having a hard time to understand why this stuff
wants to move around at all.

I think we agreed long ago, that for the whole HPC FULL_NOHZ stuff you
have to sacrify at least one CPU for housekeeping purposes of all
kinds, timekeeping, statistics and whatever.

So if you have a housekeeper, then it makes absolutely no sense at all
to move it around in circles.

Can you please enlighten me why we need this at all?

Thanks,

	tglx
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