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[RFC,5/8] glibc: sched_getcpu(): use rseq cpu_id TLS on Linux (v5)

Message ID 20200108145851.13594-6-mathieu.desnoyers@efficios.com
State Superseded
Headers show

Commit Message

Mathieu Desnoyers Jan. 8, 2020, 2:58 p.m. UTC
When available, use the cpu_id field from __rseq_abi on Linux to
implement sched_getcpu(). Fall-back on the vgetcpu vDSO if unavailable.


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)

	* sysdeps/unix/sysv/linux/sched_getcpu.c: use rseq cpu_id TLS on

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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: Dave Watson <davejwatson@fb.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.
 sysdeps/unix/sysv/linux/sched_getcpu.c | 29 ++++++++++++++++++++++++--
 1 file changed, 27 insertions(+), 2 deletions(-)
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diff --git a/sysdeps/unix/sysv/linux/sched_getcpu.c b/sysdeps/unix/sysv/linux/sched_getcpu.c
index c019cfb3cf..6d3541333d 100644
--- a/sysdeps/unix/sysv/linux/sched_getcpu.c
+++ b/sysdeps/unix/sysv/linux/sched_getcpu.c
@@ -18,10 +18,15 @@ 
 #include <errno.h>
 #include <sched.h>
 #include <sysdep.h>
+#include <atomic.h>
 #include <sysdep-vdso.h>
-sched_getcpu (void)
+static int
+vsyscall_sched_getcpu (void)
   unsigned int cpu;
   int r = -1;
@@ -32,3 +37,23 @@  sched_getcpu (void)
   return r == -1 ? r : cpu;
+#ifdef __NR_rseq
+#include <sys/rseq.h>
+#if defined __NR_rseq && defined RSEQ_SIG
+sched_getcpu (void)
+  int cpu_id = atomic_load_relaxed (&__rseq_abi.cpu_id);
+  return cpu_id >= 0 ? cpu_id : vsyscall_sched_getcpu ();
+sched_getcpu (void)
+  return vsyscall_sched_getcpu ();