linux-kernel - Re: One of these things (CONFIG

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Date:	Tue, 22 Jan 2013 01:18:08 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Matt Sealey <matt@...esi-usa.com>
Cc:	John Stultz <john.stultz@...aro.org>,
	Arnd Bergmann <arnd@...db.de>,
	Linux ARM Kernel ML <linux-arm-kernel@...ts.infradead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: One of these things (CONFIG_HZ) is not like the others..

On Mon, Jan 21, 2013 at 07:06:59PM -0600, Matt Sealey wrote:
> On Mon, Jan 21, 2013 at 6:51 PM, John Stultz <john.stultz@...aro.org> wrote:
> > On 01/21/2013 02:54 PM, Matt Sealey wrote:
> >>
> >> On Mon, Jan 21, 2013 at 4:36 PM, John Stultz <john.stultz@...aro.org>
> >> wrote:
> >>>
> >>> On 01/21/2013 01:14 PM, Matt Sealey wrote:
> >
> > As far as jiffies rating, from jiffies.c:
> >     .rating        = 1, /* lowest valid rating*/
> >
> > So I'm not sure what you mean by "the debug on the kernel log is telling me
> > it has a higher resolution".
> 
> Oh, it is just if I actually don't run setup_sched_clock on my
> platform, it gives a little message (with #define DEBUG 1 in
> sched_clock.c)

sched_clock() has nothing to do with time keeping, and that
HZ/NO_HZ/HRTIMERS don't affect it (when it isn't being derived from
jiffies).

Now, sched_clock() is there to give the scheduler a _fast_ to access,
higher resolution clock than is available from other sources, so that
there's ways of accurately measuring the amount of time processes run
for, and other such measurements - and it uses that to determine how
to schedule a particular task and when to preempt it.

Not providing it means you get those measurements at HZ-based resolution,
which is suboptimal for tasks which run often for sub-HZ periods (which
can end up accumulating zero run time.)
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