From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Andrew Fox <afox@redhat.com>,
Stephen Johnston <sjohnsto@redhat.com>,
linux-kernel@vger.kernel.org,
Linus Torvalds <torvalds@linux-foundation.org>,
Stanislaw Gruszka <sgruszka@redhat.com>
Subject: Re: [PATCH] sched/cputime: make scale_stime() more precise
Date: Fri, 19 Jul 2019 16:37:42 +0200 [thread overview]
Message-ID: <20190719143742.GA32243@redhat.com> (raw)
In-Reply-To: <20190719134727.GV3463@hirez.programming.kicks-ass.net>
On 07/19, Peter Zijlstra wrote:
>
> > > $ ./stime 300000
> > > start=300000000000000
> > > ut(diff)/st(diff): 299994875 ( 0) 300009124 (2000)
> > > ut(diff)/st(diff): 299994875 ( 0) 300011124 (2000)
> > > ut(diff)/st(diff): 299994875 ( 0) 300013124 (2000)
> > > ut(diff)/st(diff): 299994875 ( 0) 300015124 (2000)
> > > ut(diff)/st(diff): 299994875 ( 0) 300017124 (2000)
> > > ut(diff)/st(diff): 299994875 ( 0) 300019124 (2000)
> > > ut(diff)/st(diff): 299994875 ( 0) 300021124 (2000)
> > > ut(diff)/st(diff): 299994875 ( 0) 300023124 (2000)
> > > ut(diff)/st(diff): 299994875 ( 0) 300025124 (2000)
> > > ut(diff)/st(diff): 299994875 ( 0) 300027124 (2000)
> > > ut(diff)/st(diff): 299994875 ( 0) 300029124 (2000)
> > > ut(diff)/st(diff): 299996875 (2000) 300029124 ( 0)
> > > ut(diff)/st(diff): 299998875 (2000) 300029124 ( 0)
> > > ut(diff)/st(diff): 300000875 (2000) 300029124 ( 0)
> > > ut(diff)/st(diff): 300002875 (2000) 300029124 ( 0)
> > > ut(diff)/st(diff): 300004875 (2000) 300029124 ( 0)
> > > ut(diff)/st(diff): 300006875 (2000) 300029124 ( 0)
> > > ut(diff)/st(diff): 300008875 (2000) 300029124 ( 0)
> > > ut(diff)/st(diff): 300010875 (2000) 300029124 ( 0)
> > > ut(diff)/st(diff): 300012055 (1180) 300029944 ( 820)
> > > ut(diff)/st(diff): 300012055 ( 0) 300031944 (2000)
> > > ut(diff)/st(diff): 300012055 ( 0) 300033944 (2000)
> > > ut(diff)/st(diff): 300012055 ( 0) 300035944 (2000)
> > > ut(diff)/st(diff): 300012055 ( 0) 300037944 (2000)
> > >
> > > shows the problem even when sum_exec_runtime is not that big: 300000 secs.
> > >
> > > The new implementation of scale_stime() does the additional div64_u64_rem()
> > > in a loop but see the comment, as long it is used by cputime_adjust() this
> > > can happen only once.
> >
> > That only shows something after long long staring :/ There's no words on
> > what the output actually means or what would've been expected.
> >
> > Also, your example is incomplete; the below is a test for scale_stime();
> > from this we can see that the division results in too large a number,
> > but, important for our use-case in cputime_adjust(), it is a step
> > function (due to loss in precision) and for every plateau we shift
> > runtime into the wrong bucket.
>
> But I'm still confused, since in the long run, it should still end up
> with a proportionally divided user/system, irrespective of some short
> term wobblies.
Why?
Yes, statistically the numbers are proportionally divided.
but you will (probably) never see the real stime == 1000 && utime == 10000
numbers if you watch incrementally.
Just in case... yes I know that these numbers can only "converge" to the
reality, only their sum is correct. But people complain.
Oleg.
next prev parent reply other threads:[~2019-07-19 14:37 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-18 13:18 [PATCH] sched/cputime: make scale_stime() more precise Oleg Nesterov
2019-07-18 13:21 ` Oleg Nesterov
2019-07-18 14:55 ` Oleg Nesterov
2019-07-19 11:03 ` Peter Zijlstra
2019-07-19 13:47 ` Peter Zijlstra
2019-07-19 14:37 ` Oleg Nesterov [this message]
2019-07-22 19:56 ` Peter Zijlstra
2019-07-23 14:00 ` Oleg Nesterov
2019-07-23 14:29 ` Oleg Nesterov
2019-07-19 14:03 ` Oleg Nesterov
2019-07-22 19:45 ` Peter Zijlstra
2019-07-22 10:52 ` Stanislaw Gruszka
2019-07-22 20:00 ` Peter Zijlstra
2019-07-23 9:37 ` Stanislaw Gruszka
2020-01-22 16:46 ` Oleg Nesterov
2020-01-23 13:05 ` Oleg Nesterov
2020-01-24 15:42 ` Oleg Nesterov
2020-01-27 12:28 ` [PATCH v2] " Oleg Nesterov
2020-05-15 17:24 ` Oleg Nesterov
2020-05-19 17:25 ` Peter Zijlstra
2020-05-19 18:33 ` Linus Torvalds
2020-05-19 18:42 ` Peter Zijlstra
2020-05-19 19:11 ` Peter Zijlstra
2020-05-19 19:51 ` Linus Torvalds
2020-05-20 15:24 ` Oleg Nesterov
2020-05-20 15:36 ` Peter Zijlstra
2020-05-20 20:10 ` Peter Zijlstra
2020-05-21 13:26 ` Oleg Nesterov
2020-06-16 12:21 ` [tip: sched/core] sched/cputime: Improve cputime_adjust() tip-bot2 for Oleg Nesterov
-- strict thread matches above, loose matches on Subject: below --
2019-07-18 13:15 [PATCH] sched/cputime: make scale_stime() more precise Oleg Nesterov
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