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Subject: [Bug libc/27651] Performance regression after updating to 2.33
Date: Mon, 12 Apr 2021 22:01:17 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D27651

--- Comment #15 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot g=
nu.org> ---
The master branch has been updated by Adhemerval Zanella
<azanella@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=3Dglibc.git;h=3D9d7c5cc38e58fb0923e=
88901f87174a511b61552

commit 9d7c5cc38e58fb0923e88901f87174a511b61552
Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date:   Wed Mar 31 13:53:34 2021 -0300

    linux: Normalize and return timeout on select (BZ #27651)

    The commit 2433d39b697, which added time64 support to select, changed
    the function to use __NR_pselect6 (or __NR_pelect6_time64) on all
    architectures.  However, on architectures where the symbol was
    implemented with __NR_select the kernel normalizes the passed timeout
    instead of return EINVAL.  For instance, the input timeval
    { 0, 5000000 } is interpreted as { 5, 0 }.

    And as indicated by BZ #27651, this semantic seems to be expected
    and changing it results in some performance issues (most likely
    the program does not check the return code and keeps issuing
    select with unormalized tv_usec argument).

    To avoid a different semantic depending whether which syscall the
    architecture used to issue, select now always normalize the timeout
    input.  This is a slight change for some ABIs (for instance aarch64).

    Checked on x86_64-linux-gnu and i686-linux-gnu.

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