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 <car...@redhat.com>
CC: Florian Weimer <fwei...@redhat.com>
CC: Joseph Myers <jos...@codesourcery.com>
CC: Szabolcs Nagy <szabolcs.n...@arm.com>
CC: Thomas Gleixner <t...@linutronix.de>
CC: Ben Maurer <bmau...@fb.com>
CC: Peter Zijlstra <pet...@infradead.org>
CC: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
CC: Boqun Feng <boqun.f...@gmail.com>
CC: Will Deacon <will.dea...@arm.com>
CC: Paul Turner <p...@google.com>
CC: libc-al...@sourceware.org
CC: linux-kernel@vger.kernel.org
CC: linux-...@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.
---
sysdeps/unix/sysv/linux/sched_getcpu.c | 27 ++++++++++++++++++++++++--
1 file changed, 25 insertions(+), 2 deletions(-)
diff --git a/sysdeps/unix/sysv/linux/sched_getcpu.c
b/sysdeps/unix/sysv/linux/sched_getcpu.c
index c019cfb3cf..2269c4f2bd 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>
-int
-sched_getcpu (void)
+#ifdef HAVE_GETCPU_VSYSCALL
+# define HAVE_VSYSCALL
+#endif
+
+static int
+vsyscall_sched_getcpu (void)
{
unsigned int cpu;
int r = -1;
@@ -32,3 +37,21 @@ sched_getcpu (void)
#endif
return r == -1 ? r : cpu;
}
+
+#include <sys/rseq.h>
+
+#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
+int
+sched_getcpu (void)
+{
+ return vsyscall_sched_getcpu ();
+}
+#endif
--
2.17.1
