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Date: Wed, 29 Mar 2017 15:04:49 +0200 From: Frederic Weisbecker <fweisbec@...il.com> To: Luiz Capitulino <lcapitulino@...hat.com> Cc: riel@...hat.com, 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 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?
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