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

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Date:   Wed, 29 Mar 2017 21:48:07 -0400
From:   Luiz Capitulino <lcapitulino@...hat.com>
To:     Frederic Weisbecker <fweisbec@...il.com>
Cc:     Rik van Riel <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 Wed, 29 Mar 2017 23:12:00 +0200
Frederic Weisbecker <fweisbec@...il.com> wrote:

> On Wed, Mar 29, 2017 at 09:23:57AM -0400, Luiz Capitulino wrote:
> > 
> > There are various reproducers actually. I started off with the simple
> > loop above, then wrote the attach program and then wrote the one
> > you're mentioning:
> > 
> >  http://people.redhat.com/~lcapitul/real-time/acct-bug.c
> > 
> > All of them reproduce the issue 100% of the time for me.  
> 
> > #define _GNU_SOURCE
> > #include <stdio.h>
> > #include <unistd.h>
> > #include <stdlib.h>
> > #include <sched.h>
> > #include <sys/types.h>
> > 
> > static int move_to_cpu(int cpu)
> > {
> >         cpu_set_t set;
> > 
> >         CPU_ZERO(&set);
> >         CPU_SET(cpu, &set);
> >         return sched_setaffinity(0, sizeof(set), &set);
> > }
> > 
> > static void loop(void)
> > {
> >         for (;;) ;
> > }
> > 
> > static int fork_hog(int cpu)
> > {
> >         int pid;
> > 
> >         pid = (int) fork();
> >         if (pid == 0) {
> >                 move_to_cpu(cpu);
> >                 loop();
> >                 exit(0);
> >         }
> > 
> >         return pid;
> > }
> > 
> > int main(int argc, char *argv[])
> > {
> >         int i, pid, cpu, nr_procs;
> > 
> > 		if (argc != 3) {
> > 			printf("usage: hog < nr-procs > < CPU >\n");
> > 			exit(1);
> > 		}
> > 
> > 		cpu = atoi(argv[2]);
> > 		nr_procs = atoi(argv[1]);
> > 
> >         for (i = 0; i < nr_procs; i++) {
> >                 pid = fork_hog(cpu);
> >                 fprintf(stderr, "created hog%d pid=%d\n", i, pid);
> >         }
> > 
> >         fprintf(stderr, "pausing...\n");
> >         pause();
> > 
> >         return 0;
> > }  
> 
> I just tried both of these and none seem to show incorrect cputime :-/
> I'm wondering if that bug depends on some hardware.

Are you running on x86? My CPU is:

Intel(R) Xeon(R) CPU E5-2640 v3 @ 2.60GHz

I wonder if this issue depends on the timer used by the hrtimer
subsystem.