Re: [PATCH 2/4] glibc: sched_getcpu(): use rseq cpu_id TLS on Linux

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)

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Carlos O'Donell <redacted>
CC: Florian Weimer <redacted>
CC: Joseph Myers <redacted>
CC: Szabolcs Nagy <redacted>
CC: Thomas Gleixner <redacted>
CC: Ben Maurer <redacted>
CC: Peter Zijlstra <peterz@infradead.org>
CC: "Paul E. McKenney" <redacted>
CC: Boqun Feng <redacted>
CC: Will Deacon <redacted>
CC: Dave Watson <redacted>
CC: Paul Turner <redacted>
CC: libc-alpha@sourceware.org
CC: linux-kernel@vger.kernel.org
CC: linux-api@vger.kernel.org
---
  sysdeps/unix/sysv/linux/sched_getcpu.c | 25 +++++++++++++++++++++++--
  1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/sysdeps/unix/sysv/linux/sched_getcpu.c b/sysdeps/unix/sysv/linux/sched_getcpu.c
index fb0d317f83..8bfb03778b 100644
--- a/sysdeps/unix/sysv/linux/sched_getcpu.c
+++ b/sysdeps/unix/sysv/linux/sched_getcpu.c

@@ -24,8 +24,8 @@
  #endif
  #include <sysdep-vdso.h>
  
-int
-sched_getcpu (void)
+static int
+vsyscall_sched_getcpu (void)

  {
  #ifdef __NR_getcpu
    unsigned int cpu;

@@ -37,3 +37,24 @@ sched_getcpu (void)
    return -1;
  #endif
  }
+
+#ifdef __NR_rseq
+#include <linux/rseq.h>
+
+extern __attribute__ ((tls_model ("initial-exec")))
+__thread volatile struct rseq __rseq_abi;

+
+int
+sched_getcpu (void)
+{
+  int cpu_id = __rseq_abi.cpu_id;
+
+  return cpu_id >= 0 ? cpu_id : vsyscall_sched_getcpu ();

+}
+#else
+int
+sched_getcpu (void)
+{
+  return vsyscall_sched_getcpu ();