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

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Message-Id: <20181102115259.11383-2-mathieu.desnoyers@efficios.com>
Date:   Fri,  2 Nov 2018 07:52:59 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     libc-alpha@...rceware.org
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Carlos O'Donell <carlos@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        Joseph Myers <joseph@...esourcery.com>,
        Szabolcs Nagy <szabolcs.nagy@....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>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Subject: [RFC PATCH 2/2] glibc: sched_getcpu(): use rseq cpu_id TLS on Linux

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)

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
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: Dave Watson <davejwatson@...com>
CC: Paul Turner <pjt@...gle.com>
CC: libc-alpha@...rceware.org
CC: linux-kernel@...r.kernel.org
CC: linux-api@...r.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 b69eeda15c..e1a206075c 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 ();
+}
+#endif
-- 
2.17.1