linux-kernel - [RFC PATCH glibc 2/3] glibc: sched_getcpu(): use rseq cpu

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Message-Id: <20200428171513.22926-2-mathieu.desnoyers@efficios.com>
Date:   Tue, 28 Apr 2020 13:15:12 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Carlos O'Donell <carlos@...hat.com>
Cc:     Florian Weimer <fweimer@...hat.com>,
        Joseph Myers <joseph@...esourcery.com>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        libc-alpha@...rceware.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ben Maurer <bmaurer@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will.deacon@....com>,
        Paul Turner <pjt@...gle.com>, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org
Subject: [RFC PATCH glibc 2/3] glibc: sched_getcpu(): use rseq cpu_id TLS on Linux (v7)

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@...0GHz, 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@...hat.com>
CC: Florian Weimer <fweimer@...hat.com>
CC: Joseph Myers <joseph@...esourcery.com>
CC: Szabolcs Nagy <szabolcs.nagy@....com>
CC: Thomas Gleixner <tglx@...utronix.de>
CC: Ben Maurer <bmaurer@...com>
CC: Peter Zijlstra <peterz@...radead.org>
CC: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
CC: Boqun Feng <boqun.feng@...il.com>
CC: Will Deacon <will.deacon@....com>
CC: Paul Turner <pjt@...gle.com>
CC: libc-alpha@...rceware.org
CC: linux-kernel@...r.kernel.org
CC: linux-api@...r.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