Re: [PATCH] sched/cputime: make scale_stime() more precise

[Stanislaw Gruszka <sgruszka@redhat.com>]

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