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Date: Wed, 29 Mar 2017 09:14:32 -0400 From: Rik van Riel <riel@...hat.com> To: Frederic Weisbecker <fweisbec@...il.com>, Luiz Capitulino <lcapitulino@...hat.com> Cc: linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org, Wanpeng Li <kernellwp@...il.com> Subject: Re: [BUG nohz]: wrong user and system time accounting 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.
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