Messages in this thread Patch in this message | | | From | Mathieu Desnoyers <> | Subject | [PATCH 2/3] Linux: Use rseq in sched_getcpu if available (v9) | Date | Mon, 22 Jun 2020 14:08:02 -0400 |
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When available, use the cpu_id field from __rseq_abi on Linux to implement sched_getcpu(). Fall-back on the vgetcpu vDSO if unavailable.
Benchmarks:
x86-64: Intel E5-2630 v3@2.40GHz, 16-core, hyperthreading
glibc sched_getcpu(): 13.7 ns (baseline) glibc sched_getcpu() using rseq: 2.5 ns (speedup: 5.5x) inline load cpuid from __rseq_abi TLS: 0.8 ns (speedup: 17.1x)
CC: Carlos O'Donell <carlos@redhat.com> CC: Florian Weimer <fweimer@redhat.com> CC: Joseph Myers <joseph@codesourcery.com> CC: Szabolcs Nagy <szabolcs.nagy@arm.com> CC: Thomas Gleixner <tglx@linutronix.de> CC: Ben Maurer <bmaurer@fb.com> CC: Peter Zijlstra <peterz@infradead.org> CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> CC: Boqun Feng <boqun.feng@gmail.com> CC: Will Deacon <will.deacon@arm.com> CC: Paul Turner <pjt@google.com> CC: libc-alpha@sourceware.org CC: linux-kernel@vger.kernel.org CC: linux-api@vger.kernel.org --- Changes since v1: - rseq is only used if both __NR_rseq and RSEQ_SIG are defined.
Changes since v2: - remove duplicated __rseq_abi extern declaration.
Changes since v3: - update ChangeLog.
Changes since v4: - Use atomic_load_relaxed to load the __rseq_abi.cpu_id field, a consequence of the fact that __rseq_abi is not volatile anymore. - Include atomic.h which provides atomic_load_relaxed.
Changes since v5: - Use __ASSUME_RSEQ to detect rseq availability.
Changes since v6: - Remove use of __ASSUME_RSEQ.
Changes since v7: - Fix incorrect merge with commit d0def09ff6 ("linux: Fix vDSO macros build with time64 interfaces")
Changes since v8: - Update patch title. - Add /* RSEQ_SIG */ for #else and #endif. --- sysdeps/unix/sysv/linux/sched_getcpu.c | 22 ++++++++++++++++++++-- 1 file changed, 20 insertions(+), 2 deletions(-)
diff --git a/sysdeps/unix/sysv/linux/sched_getcpu.c b/sysdeps/unix/sysv/linux/sched_getcpu.c index c019cfb3cf..c0f992e056 100644 --- a/sysdeps/unix/sysv/linux/sched_getcpu.c +++ b/sysdeps/unix/sysv/linux/sched_getcpu.c @@ -18,10 +18,12 @@ #include <errno.h> #include <sched.h> #include <sysdep.h> +#include <atomic.h> #include <sysdep-vdso.h> +#include <sys/rseq.h> -int -sched_getcpu (void) +static int +vsyscall_sched_getcpu (void) { unsigned int cpu; int r = -1; @@ -32,3 +34,19 @@ sched_getcpu (void) #endif return r == -1 ? r : cpu; } + +#ifdef RSEQ_SIG +int +sched_getcpu (void) +{ + int cpu_id = atomic_load_relaxed (&__rseq_abi.cpu_id); + + return cpu_id >= 0 ? cpu_id : vsyscall_sched_getcpu (); +} +#else /* RSEQ_SIG */ +int +sched_getcpu (void) +{ + return vsyscall_sched_getcpu (); +} +#endif /* RSEQ_SIG */ -- 2.17.1
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