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[Bug libc/21305] clock_gettime(CLOCK_MONOTONIC_RAW) can and should use rdtsc instructions instead of entering kernel through VDSO
- From: "fweimer at redhat dot com" <sourceware-bugzilla at sourceware dot org>
- To: glibc-bugs at sourceware dot org
- Date: Fri, 24 Mar 2017 13:44:39 +0000
- Subject: [Bug libc/21305] clock_gettime(CLOCK_MONOTONIC_RAW) can and should use rdtsc instructions instead of entering kernel through VDSO
- Auto-submitted: auto-generated
- References: <bug-21305-131@http.sourceware.org/bugzilla/>
https://sourceware.org/bugzilla/show_bug.cgi?id=21305
--- Comment #4 from Florian Weimer <fweimer at redhat dot com> ---
(In reply to Jason Vas Dias from comment #2)
> I am using the unmodified Linux 4.10.0 (latest stable git tag).
>
>
> In arch/x86/entry/vdso/vclock_gettime.c,
> clock_gettime(CLOCK_MONOTONIC_RAW,&ts)
> is handled by :
>
> notrace static int __always_inline do_monotonic(struct timespec *ts)
Why do you think the code ends up there? CLOCK_MONOTONIC is not
CLOCK_MONOTONIC_RAW. Profiling matches that—there is no fast path in the vDSO
for CLOCK_MONOTONIC_RAW.
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