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From: Peter Zijlstra <peterz@infradead.org>
To: Rik van Riel <riel@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
	linux-kernel@vger.kernel.org, tglx@linutronix.de,
	mingo@kernel.org, luto@amacapital.net, clark@redhat.com
Subject: Re: [PATCH 1/4] sched,time: remove non-power-of-two divides from __acct_update_integrals
Date: Mon, 1 Feb 2016 12:30:10 +0100	[thread overview]
Message-ID: <20160201113010.GR6357@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <56ACF885.8070203@redhat.com>

On Sat, Jan 30, 2016 at 12:53:09PM -0500, Rik van Riel wrote:
> >> +		delta = cputime_to_nsecs(dtime);
> > 
> > You might want to add a comment specifying why we don't call cputime_to_usecs()
> > directly (because we optimize if delta < TICK_NSEC).
> > 
> > Although this has a good impact on nohz_full, it might have a tiny bad one on !nohz_full
> > because now we first convert jiffies to nsecs (which implies a multiplication by 1000)
> > that we later divide again by 1000. Now this is ok because I plan to convert tsk->utime/stime
> > to nsecs and thus remove most of the cputime_t use and conversions everywhere.
> 
> Isn't cputime_t in nanoseconds even on !nohz_full systems nowadays,
> due to sched_clock?

Don't think so, we still use jiffy accounting for !NOHZ_FULL.

> Also, a multiplication is essentially instantaneous compared to
> a division, which is why Peter suggested going this way around.

Yep.

  reply	other threads:[~2016-02-01 11:30 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-01-30  3:36 [PATCH 0/2] sched,time: reduce nohz_full syscall overhead 40% riel
2016-01-30  3:36 ` [PATCH 1/4] sched,time: remove non-power-of-two divides from __acct_update_integrals riel
2016-01-30  4:56   ` kbuild test robot
2016-01-30 14:44   ` Frederic Weisbecker
2016-01-30 17:53     ` Rik van Riel
2016-02-01 11:30       ` Peter Zijlstra [this message]
2016-01-30  3:36 ` [PATCH 2/4] acct,time: change indentation in __acct_update_integrals riel
2016-01-30 16:15   ` Frederic Weisbecker
2016-01-30  3:36 ` [PATCH 3/4] time,acct: drop irq save & restore from __acct_update_integrals riel
2016-01-30 16:24   ` Frederic Weisbecker
2016-01-30  3:36 ` [PATCH 4/4] sched,time: only call account_{user,sys,guest,idle}_time once a jiffy riel
2016-02-01  2:12 [PATCH 0/4 v3] sched,time: reduce nohz_full syscall overhead 40% riel
2016-02-01  2:12 ` [PATCH 1/4] sched,time: remove non-power-of-two divides from __acct_update_integrals riel
2016-02-01  4:46   ` kbuild test robot
2016-02-01  8:37   ` Thomas Gleixner
2016-02-01  9:22     ` Peter Zijlstra
2016-02-01  9:31       ` Thomas Gleixner
2016-02-01 13:44       ` Rik van Riel
2016-02-01 13:51         ` Peter Zijlstra
2016-02-01 19:21 [PATCH 0/4 v4] sched,time: reduce nohz_full syscall overhead 40% riel
2016-02-01 19:21 ` [PATCH 1/4] sched,time: remove non-power-of-two divides from __acct_update_integrals riel
2016-02-02 17:19 [PATCH 0/4 v5] sched,time: reduce nohz_full syscall overhead 40% riel
2016-02-02 17:19 ` [PATCH 1/4] sched,time: remove non-power-of-two divides from __acct_update_integrals riel
2016-02-11  1:08 [PATCH 0/4 v6] sched,time: reduce nohz_full syscall overhead 40% riel
2016-02-11  1:08 ` [PATCH 1/4] sched,time: remove non-power-of-two divides from __acct_update_integrals riel

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