PhysicalCoreID: Use sched_getcpu on aarch64 with glibc >= 2.35 #14224
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The Linux sched_getcpu function was changed to use the glibc "restartable sequences" since glibc 2.35 in in 2022. At this point, most recent Linux distributions have picked this up. The preprocessor macro will conditionally enable it based on the version of glibc, so this should make this faster on newer aarch64 builds, without impacting old ones. I have tested with a microbenchmark that the performance with new glibc is the same as x86-64 (a few ns/call). I added a test to try to check this. It was useful for me in testing this, but it is probably "wrong": without duplicating the check exactly, it is hard to test the support. It is possible we should remove the test. Correct the macro: it should be testing __GLIBC__ and __GLIBC_MINOR__ for the glibc version, rather than __GNUC__ and __GNUC_MINOR__ for the gcc/clang version. Linux distribution support for this feature: RHEL 9 glibc glibc 2.34 released 2022-05-17 (TOO OLD) RHEL 9.1 glibc 2.35 released 2022-11-15 [1] RHEL 10 glibc 2.39 released 2025-05-20 Ubuntu LTS 20.04 glibc 2.31 released 2020-04-23 (TOO OLD) Ubuntu LTS 22.04 glibc 2.35 released 2022-04-21 Ubuntu LTS 24.04 glibc 2.39 released 2024-04-25 Debian 11 glibc 2.31 released 2021-08-14 (TOO OLD) Debian 12 glibc 2.36 released 2023-06-14 Debian 13 glibc 2.41 released 2025-08-09 [1] https://developers.redhat.com/articles/2022/12/22/restartable-sequences-support-glibc-rhel-9
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The Linux sched_getcpu function was changed to use the glibc "restartable sequences" since glibc 2.35 in in 2022. At this point, most recent Linux distributions have picked this up. The preprocessor macro will conditionally enable it based on the version of glibc, so this should make this faster on newer aarch64 builds, without impacting old ones. I have tested with a microbenchmark that the performance with new glibc is the same as x86-64 (a few ns/call).
I added a test to try to check this. It was useful for me in testing this, but it is probably "wrong": without duplicating the check exactly, it is hard to test the support. It is possible we should remove the test.
Correct the macro: it should be testing
__GLIBC__and__GLIBC_MINOR__for the glibc version, rather than__GNUC__and__GNUC_MINOR__for the gcc/clang version.Linux distribution support for this feature:
RHEL 9 glibc glibc 2.34 released 2022-05-17 (TOO OLD)
RHEL 9.1 glibc 2.35 released 2022-11-15 [1]
RHEL 10 glibc 2.39 released 2025-05-20
Ubuntu LTS 20.04 glibc 2.31 released 2020-04-23 (TOO OLD)
Ubuntu LTS 22.04 glibc 2.35 released 2022-04-21
Ubuntu LTS 24.04 glibc 2.39 released 2024-04-25
Debian 11 glibc 2.31 released 2021-08-14 (TOO OLD)
Debian 12 glibc 2.36 released 2023-06-14
Debian 13 glibc 2.41 released 2025-08-09
[1] https://developers.redhat.com/articles/2022/12/22/restartable-sequences-support-glibc-rhel-9