linux-kernel - Re: [PATCH 1/2] sched,time: remove pointless divides from __acct_update

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Date:	Fri, 29 Jan 2016 22:47:41 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...nel.org,
	luto@...capital.net
Subject: Re: [PATCH 1/2] sched,time: remove pointless divides from
 __acct_update_integrals

On 01/29/2016 10:36 PM, Frederic Weisbecker wrote:
> On Sat, Jan 30, 2016 at 12:10:18AM +0100, Peter Zijlstra wrote:
>> On Fri, Jan 29, 2016 at 05:22:59PM -0500, riel@...hat.com wrote:
>>> From: Rik van Riel <riel@...hat.com>
>>>
>>> When running a microbenchmark calling an invalid syscall number
>>> in a loop, on a nohz_full CPU, we spend a full 9% of our CPU
>>> time in __acct_update_integrals.
>>>
>>> This function converts cputime_t to jiffies, to a timeval, only to
>>> convert the timeval back to microseconds before discarding it.
>>>
>>> This patch leaves __acct_update_integrals functionally equivalent,
>>> but speeds things up by about 11%, with 10 million calls to an
>>> invalid syscall number dropping from 3.7 to 3.3 seconds.
>>
>> WTH is this taskstat crap anyway? Who uses it and can't we kill it?
> 
> I have no idea what it's used for, it seems to be related to taskstats
> over netlink. I'm not even sure if it's actually used. There don't seem
> to be a runtime offcase and I bet distros enable it. So that stuff does
> some work every millisecond on millions of machines while it probably
> has very few users.
> 
> SGI introduced it in 2006 and it seems that their last contribution there is
> in 2008. The rest is kernel maintainance and fixes.
> 
> If there are still users of it, then at least we should disable it on runtime by
> default.

With all the non-power-of-2 divides removed, __acct_update_integrals
disappears from the profile, even running many times per millisecond.

At that point, native_sched_clock takes over the top of the profile,
and the only way I can think of getting rid of that one is making
sure it is not called twice for every syscall, irq, and kvm guest
entry/exit.

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