linux-kernel - Re: [PATCH 4/4] sched,time: only call account_{user,sys,guest,idle}

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Date:	Tue, 9 Feb 2016 18:11:40 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	riel@...hat.com
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...nel.org,
	luto@...capital.net, peterz@...radead.org, clark@...hat.com,
	eric.dumazet@...il.com
Subject: Re: [PATCH 4/4] sched,time: only call
 account_{user,sys,guest,idle}_time once a jiffy

On Tue, Feb 02, 2016 at 12:19:46PM -0500, riel@...hat.com wrote:
> From: Rik van Riel <riel@...hat.com>
> 
> After removing __acct_update_integrals from the profile,
> native_sched_clock remains as the top CPU user. This can be
> reduced by only calling account_{user,sys,guest,idle}_time
> once per jiffy for long running tasks on nohz_full CPUs.
> 
> This will reduce timing accuracy on nohz_full CPUs to jiffy
> based sampling, just like on normal CPUs.

I wonder if that assumption is actually right.

With tick based sampling, we indeed have a statistical accounting
which precision is based on HZ. Now the time accounted when the tick
fires is always a single unit: 1 jiffy. So we have a well distributed
accounting value because it's constant and based on the probability of
a periodic event.

So for any T_slice being a given cpu timeslice (in secs) executed between
two ring switch (user <-> kernel), we are going to account: 1 * P(T_slice*HZ)
(P() stand for probability here).

Now after this patch, the scenario is rather different. We are accounting the
real time spent in a slice with a similar probablity.
This becomes: T_slice * P(T_slice*HZ).

So it seems it could result into logarithmic accounting: timeslices of 1 second
will be accounted right whereas repeating tiny timeslices may result in much lower
values than expected.

To fix this we should instead account jiffies_to_nsecs(jiffies - t->vtime_jiffies).
Well, that would drop the use of finegrained clock and even the need of nsecs based
cputime. But why not if we still have acceptable result for much more performances.

I don't know if all the above actually makes sense. I suck at maths so I may well be
wrong.