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

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Message-ID: <20170329130447.GB8306@lerouge>
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?