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Date:   Mon, 22 Jul 2019 12:52:41 +0200
From:   Stanislaw Gruszka <sgruszka@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Oleg Nesterov <oleg@redhat.com>, 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>
Subject: Re: [PATCH] sched/cputime: make scale_stime() more precise
Message-ID: <20190722105240.GA27219@redhat.com>
References: <20190718131834.GA22211@redhat.com>
 <20190719110349.GG3419@hirez.programming.kicks-ass.net>
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On Fri, Jul 19, 2019 at 01:03:49PM +0200, Peter Zijlstra wrote:
> > 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.
> 
> Your proposed function works; but is atrocious, esp. on 32bit. That
> said, before we 'fixed' it, it had similar horrible divisions in, see
> commit 55eaa7c1f511 ("sched: Avoid cputime scaling overflow").
> 
> Included below is also an x86_64 implementation in 2 instructions.
> 
> I'm still trying see if there's anything saner we can do...

I was always proponent of removing scaling and export raw values
and sum_exec_runtime. But that has obvious drawback, reintroduce
'top hiding' issue.

But maybe we can export raw values in separate file i.e.
/proc/[pid]/raw_cpu_times ? So applications that require more precise
cputime values for very long-living processes can use this file.

Stanislaw