On Fri, Jul 19, 2019 at 01:03:49PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 18, 2019 at 03:18:34PM +0200, Oleg Nesterov wrote:
> > People report that utime and stime from /proc/<pid>/stat become very wrong
> > when the numbers are big enough. In particular, the monitored application
> > can run all the time in user-space but only stime grows.
> >
> > This is because scale_stime() is very inaccurate. It tries to minimize the
> > relative error, but the absolute error can be huge.
> >
> > Andrew wrote the test-case:
> >
> > int main(int argc, char **argv)
> > {
> > struct task_cputime c;
> > struct prev_cputime p;
> > u64 st, pst, cst;
> > u64 ut, put, cut;
> > u64 x;
> > int i = -1; // one step not printed
> >
> > if (argc != 2)
> > {
> > printf("usage: %s <start_in_seconds>\n", argv[0]);
> > return 1;
> > }
> > x = strtoull(argv[1], NULL, 0) * SEC;
> > printf("start=%lld\n", x);
> >
> > p.stime = 0;
> > p.utime = 0;
> >
> > while (i++ < NSTEPS)
> > {
> > x += STEP;
> > c.stime = x;
> > c.utime = x;
> > c.sum_exec_runtime = x + x;
> > pst = cputime_to_clock_t(p.stime);
> > put = cputime_to_clock_t(p.utime);
> > cputime_adjust(&c, &p, &ut, &st);
> > cst = cputime_to_clock_t(st);
> > cut = cputime_to_clock_t(ut);
> > if (i)
> > printf("ut(diff)/st(diff): %20lld (%4lld) %20lld
> > (%4lld)\n",
> > cut, cut - put, cst, cst - pst);
> > }
> > }
> >
> > For example,
> >
> > $ ./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.
So please, better articulate the problem.
