[2/5] glibc: sched_getcpu(): use rseq cpu_id TLS on Linux (v2)

Message ID 20190416173216.9028-3-mathieu.desnoyers@efficios.com
State Superseded
Headers show
Series
  • Restartable Sequences support for glibc 2.30
Related show

Commit Message

Mathieu Desnoyers April 16, 2019, 5:32 p.m.
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)

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.
---
 sysdeps/unix/sysv/linux/sched_getcpu.c | 27 ++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

-- 
2.17.1

Comments

Szabolcs Nagy April 18, 2019, 3:33 p.m. | #1
On 16/04/2019 18:32, Mathieu Desnoyers wrote:
> --- a/sysdeps/unix/sysv/linux/sched_getcpu.c

> +++ b/sysdeps/unix/sysv/linux/sched_getcpu.c

> @@ -37,3 +37,26 @@ sched_getcpu (void)

>    return -1;

>  #endif

>  }

> +

> +#ifdef __NR_rseq

> +#include <sys/rseq.h>

> +#endif

> +

> +#if defined __NR_rseq && defined RSEQ_SIG

> +extern __attribute__ ((tls_model ("initial-exec")))

> +__thread volatile struct rseq __rseq_abi;


i'd expect sys/rseq.h to provide this declaration.

> +

> +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

>
Mathieu Desnoyers April 18, 2019, 3:45 p.m. | #2
----- On Apr 18, 2019, at 11:33 AM, Szabolcs Nagy Szabolcs.Nagy@arm.com wrote:

> On 16/04/2019 18:32, Mathieu Desnoyers wrote:

>> --- a/sysdeps/unix/sysv/linux/sched_getcpu.c

>> +++ b/sysdeps/unix/sysv/linux/sched_getcpu.c

>> @@ -37,3 +37,26 @@ sched_getcpu (void)

>>    return -1;

>>  #endif

>>  }

>> +

>> +#ifdef __NR_rseq

>> +#include <sys/rseq.h>

>> +#endif

>> +

>> +#if defined __NR_rseq && defined RSEQ_SIG

>> +extern __attribute__ ((tls_model ("initial-exec")))

>> +__thread volatile struct rseq __rseq_abi;

> 

> i'd expect sys/rseq.h to provide this declaration.


And it actually does! Will remove this duplicate.

Thanks,

Mathieu

> 

>> +

>> +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


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

Patch

diff --git a/sysdeps/unix/sysv/linux/sched_getcpu.c b/sysdeps/unix/sysv/linux/sched_getcpu.c
index fb0d317f83..f9466c3b22 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 +37,26 @@  sched_getcpu (void)
   return -1;
 #endif
 }
+
+#ifdef __NR_rseq
+#include <sys/rseq.h>
+#endif
+
+#if defined __NR_rseq && defined RSEQ_SIG
+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