[PATCH glibc 2/3] Linux: Use rseq in sched_getcpu if available (v9)

From: Mathieu Desnoyers
Date: Wed May 27 2020 - 14:51:43 EST

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@xxxxxxx, 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@xxxxxxxxxx>
CC: Florian Weimer <fweimer@xxxxxxxxxx>
CC: Joseph Myers <joseph@xxxxxxxxxxxxxxxx>
CC: Szabolcs Nagy <szabolcs.nagy@xxxxxxx>
CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CC: Ben Maurer <bmaurer@xxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
CC: Boqun Feng <boqun.feng@xxxxxxxxx>
CC: Will Deacon <will.deacon@xxxxxxx>
CC: Paul Turner <pjt@xxxxxxxxxx>
CC: libc-alpha@xxxxxxxxxxxxxx
CC: linux-kernel@xxxxxxxxxxxxxxx
CC: linux-api@xxxxxxxxxxxxxxx
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>

-sched_getcpu (void)
+static int
+vsyscall_sched_getcpu (void)
unsigned int cpu;
int r = -1;
@@ -32,3 +34,19 @@ sched_getcpu (void)
return r == -1 ? r : cpu;
+#ifdef RSEQ_SIG
+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 */
+sched_getcpu (void)
+ return vsyscall_sched_getcpu ();
+#endif /* RSEQ_SIG */