[RFC PATCH glibc 5/8] glibc: sched_getcpu(): use rseq cpu_id TLS on Linux (v6)

From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Carlos O'Donell <carlos@redhat.com>
Cc: Florian Weimer <fweimer@redhat.com>,
	Joseph Myers <joseph@codesourcery.com>,
	Szabolcs Nagy <szabolcs.nagy@arm.com>,
	libc-alpha@sourceware.org,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Thomas Gleixner <tglx@linutronix.de>, Ben Maurer <bmaurer@fb.com>,
	Peter Zijlstra <peterz@infradead.org>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Will Deacon <will.deacon@arm.com>,
	Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
	linux-kernel@vger.kernel.org, linux-api@vger.kernel.org
Subject: [RFC PATCH glibc 5/8] glibc: sched_getcpu(): use rseq cpu_id TLS on Linux (v6)
Date: Thu, 19 Mar 2020 10:41:07 -0400	[thread overview]
Message-ID: <20200319144110.3733-6-mathieu.desnoyers@efficios.com> (raw)
In-Reply-To: <20200319144110.3733-1-mathieu.desnoyers@efficios.com>

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)

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

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.

Changes since v5:
- Use __ASSUME_RSEQ to detect rseq availability.
---
 sysdeps/unix/sysv/linux/sched_getcpu.c | 30 ++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/sysdeps/unix/sysv/linux/sched_getcpu.c b/sysdeps/unix/sysv/linux/sched_getcpu.c
index c019cfb3cf..4f2c5c4b1a 100644
--- a/sysdeps/unix/sysv/linux/sched_getcpu.c
+++ b/sysdeps/unix/sysv/linux/sched_getcpu.c
@@ -18,10 +18,16 @@
 #include <errno.h>
 #include <sched.h>
 #include <sysdep.h>
+#include <atomic.h>
 #include <sysdep-vdso.h>
+#include <kernel-features.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 +38,23 @@ sched_getcpu (void)
 #endif
   return r == -1 ? r : cpu;
 }
+
+#ifdef __ASSUME_RSEQ
+#include <sys/rseq.h>
+#endif
+
+#if defined __ASSUME_RSEQ && defined 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

