2019-04-16 17:33:30

by Mathieu Desnoyers

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

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 [email protected], 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 <[email protected]>
CC: Carlos O'Donell <[email protected]>
CC: Florian Weimer <[email protected]>
CC: Joseph Myers <[email protected]>
CC: Szabolcs Nagy <[email protected]>
CC: Thomas Gleixner <[email protected]>
CC: Ben Maurer <[email protected]>
CC: Peter Zijlstra <[email protected]>
CC: "Paul E. McKenney" <[email protected]>
CC: Boqun Feng <[email protected]>
CC: Will Deacon <[email protected]>
CC: Dave Watson <[email protected]>
CC: Paul Turner <[email protected]>
CC: [email protected]
CC: [email protected]
CC: [email protected]
---
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(-)

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
--
2.17.1

2019-04-18 15:42:02

by Szabolcs Nagy

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

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
>

2019-04-18 15:47:14

by Mathieu Desnoyers

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

----- On Apr 18, 2019, at 11:33 AM, Szabolcs Nagy [email protected] 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