Messages in this thread | | | Date | Mon, 22 Jul 2019 12:52:41 +0200 | From | Stanislaw Gruszka <> | Subject | Re: [PATCH] sched/cputime: make scale_stime() more precise |
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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
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