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From: "steffen at sdaoden dot eu" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug libc/30135] New: sysconf: _SC_NPROCESSORS*: tweak?
Date: Sat, 18 Feb 2023 20:25:32 +0000
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X-Bugzilla-Version: 2.36
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D30135

            Bug ID: 30135
           Summary: sysconf: _SC_NPROCESSORS*: tweak?
           Product: glibc
           Version: 2.36
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: steffen at sdaoden dot eu
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

#?0|kent:tmp$ cat t.c
#include <stdio.h>
#include <unistd.h>
int main(void){
        long c,o;
        c =3D sysconf(_SC_NPROCESSORS_CONF);
        o =3D sysconf(_SC_NPROCESSORS_ONLN);
        printf("c=3D%ld o=3D%ld\n", c, o);
        return 0;
}


On bare metal:

#?0|kent:tmp$ tcc -run t.c
c=3D8 o=3D4
#?0|kent:tmp$ nproc
4

But in a box with cgroup-limited CPUs:

bash-5.2$ tcc -run t.c
c=3D4 o=3D4
bash-5.2$ nproc
3


Could this be a "bug" in the C library, as this checks
/sys/devices/system/cpu/online, /proc/stat, and only
then (iirc now) uses sched_getaffinity.  It documents
get_nprocs(3) as "number of processors currently
available in the system".

But POSIX standardized these[1] as _SC_NPROCESSORS_CONF
+ _SC_NPROCESSORS_ONLN for sysconf(3) as well as
NPROCESSORS_CONF and NPROCESSORS_ONLN for getconf(1),
saying

  ...
    The maximum number of execution units | _SC_NPROCESSORS_ONLN
    currently available to run threads=E2=80=A0 |
  ....
    =E2=80=A0 The nature of an execution unit and the precise conditions
    under which an execution unit is considered to be available,
    or can be made available, or how many threads it can execute
    in parallel, are implementation-defined.

Which is not the same.  Implementation-defined though.
But i mean if it is easy to give correct results for an execution
context (i would).

  [1] https://austingroupbugs.net/view.php?id=3D339

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