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Date:   Fri, 19 Jul 2019 16:37:42 +0200
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
Message-ID: <20190719143742.GA32243@redhat.com>
References: <20190718131834.GA22211@redhat.com>
 <20190719110349.GG3419@hirez.programming.kicks-ass.net>
 <20190719134727.GV3463@hirez.programming.kicks-ass.net>
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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.