Re: [BUG nohz]: wrong user and system time accounting

[Rik van Riel <riel@redhat.com>]

On Wed, 2017-03-29 at 15:04 +0200, Frederic Weisbecker wrote:
> On Thu, Mar 23, 2017 at 04:55:12PM -0400, Luiz Capitulino wrote:
> > 
> > When there are two or more tasks executing in user-space and
> > taking 100% of a nohz_full CPU, top reports 70% system time
> > and 30% user time utilization. Sometimes I'm even able to get
> > 100% system time and 0% user time.
> > 
> > This was reproduced with latest Linus tree (093b995), but I
> > don't believe it's a regression (at least not a recent one)
> > as I can reproduce it with older kernels. Also, I have
> > CONFIG_IRQ_TIME_ACCOUNTING=y and haven't tried to reproduce
> > without it yet.
> > 
> > Below you'll find the steps to reproduce and some initial
> > analysis.
> > 
> > Steps to reproduce
> > ------------------
> > 
> > 1. Set up a CPU for nohz_full with isolcpus= nohz_full=
> > 
> > 2. Pin two tasks that hog the CPU 100% of the time to that CPU
> 
> I failed to reproduce with your config. I'm still getting 99%
> userspace
> cputime. So I'm wondering if the hogging style plays a role.
> 
> I run pure user loops:
> 
>     int main(int argc, char **argv)
>     {
>         for (;;);
>         return 0
>     }
> 
> Does your user program perform syscalls or IOs of some sort?

Luiz's program makes a syscall every millisecond,
if started with the arguments he gave as his
reproducer.