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: Tue, 23 Jul 2019 16:00:43 +0200 [thread overview]
Message-ID: <20190723140043.GB8994@redhat.com> (raw)
In-Reply-To: <20190722195605.GI6698@worktop.programming.kicks-ass.net>
On 07/22, Peter Zijlstra wrote:
>
> On Fri, Jul 19, 2019 at 04:37:42PM +0200, Oleg Nesterov wrote:
> > On 07/19, Peter Zijlstra wrote:
>
> > > 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.
>
> This; due to the loss in precision the distribution is like a step
> function around the actual s:u ratio line, but on average it still is
> s:u.
You know, I am no longer sure... perhaps it is even worse, I need to recheck.
> Even if it were a perfect function, we'd still see increments in stime even
> if the current program state never does syscalls, simply because it
> needs to stay on that s:u line.
>
> > but you will (probably) never see the real stime == 1000 && utime == 10000
> > numbers if you watch incrementally.
>
> See, there are no 'real' stime and utime numbers. What we have are user
> and system samples -- tick based.
Yes, yes, I know.
> Sure, we take a shortcut, it wobbles a bit, but seriously, the samples
> are inaccurate anyway, so who bloody cares :-)
...
> People always complain, just tell em to go pound sand :-)
I tried ;) this was my initial reaction to this bug report.
However,
> You can construct a program that runs 99% in userspace but has all
> system samples.
Yes, but with the current implementation you do not need to construct
such a program, this is what you can easily get "in practice". And this
confuses people.
They can watch /proc/pid/stat incrementally and (when the numbers are big)
find that a program that runs 100% in userspace somehow spends 10 minutes
almost entirely in kernel. Or at least more in kernel than in userspace.
Even if task->stime doesn't grow at all.
Oleg.
next prev parent reply other threads:[~2019-07-23 14:00 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
2019-07-22 19:56 ` Peter Zijlstra
2019-07-23 14:00 ` Oleg Nesterov [this message]
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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