[Bug libc/25817] New: __get_nprocs reads a file rather than calling sched_getaffinity

["ian at airs dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=25817

            Bug ID: 25817
           Summary: __get_nprocs reads a file rather than calling
                    sched_getaffinity
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: ian at airs dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

The function __get_nprocs, which is called by arena_get2 in malloc/arena.c,
reads the file /sys/devices/system/cpu/online.  As far as I can tell,
essentially the same information can be obtained more efficiently and more
accurately by calling sched_getaffinity and counting the number of CPUs
returned.

I would encourage glibc to change the code in malloc/arena.c to call
sched_getaffinity if possible.

(This came up in a lengthy investigation of https://golang.org/issue/25628, in
which a Go program that tried to test that there were no unexpected open file
descriptors would fail because it would occasionally encounter the file
descriptor opened by __get_nprocs.)

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