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From: "mjbaars1977 at gmail dot com" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug libc/29585] New: sched_getcpu returns invalid results
Date: Mon, 19 Sep 2022 08:22:30 +0000
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X-Bugzilla-Product: glibc
X-Bugzilla-Component: libc
X-Bugzilla-Version: 2.35
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X-Bugzilla-Who: mjbaars1977 at gmail dot com
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D29585

            Bug ID: 29585
           Summary: sched_getcpu returns invalid results
           Product: glibc
           Version: 2.35
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: mjbaars1977 at gmail dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

Hello,

I noticed the following while I was trying SCHED_FIFO, because FIFO process=
es
are not supposed to migrate from processor to processor.

When looking at the manual page of sched_getcpu(), it states that:

The call

           cpu =3D sched_getcpu();

       is equivalent to the following getcpu(2) call:

           int c, s;
           s =3D getcpu(&c, NULL, NULL);
           cpu =3D (s =3D=3D -1) ? s : c;

But when you compare the output of these two in a multi-threaded program (u=
sing
clone()), the two produce very different outcomes. Only the second one will
give you the right answer.

The results can be reproduced with sched_setaffinity() before calling clone=
()
and a call to both methods above from within the child process. My version =
uses
a timer_create() to make this call periodically.

        sched_get_cpu_timer_init(0);
//      core_from     =3D sched_get_cpu() + 1;=20=20=20=20

        double x =3D 1.0;
        double y =3D (double) (random() + 1) * 1.00E-19f;

        for     (long i =3D 0; i < ((long) 1 << 32); i++) x *=3D 1.00f + y;

//      sleep   (1);
//      usleep  (random() % 1000000L);

//      core_upto     =3D sched_get_cpu() + 1;
        sched_get_cpu_timer_free();

Compilation is done with -Ofast -mfpmath=3D387 -fno-toplevel-reorder, for s=
ake of
completeness.

Best regards,
Mischa Baars.

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