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Date: Wed, 2 Mar 2016 17:26:27 +0100
From: Nathan Zadoks <nathan@...han7.eu>
To: musl@...ts.openwall.com
Cc: Nathan Zadoks <nathan@...han7.eu>
Subject: [PATCH 2/2] add sched_getcpu vDSO support
This brings the call to an actually usable speed.
Quick unscientific benchmark: 14ns : 102ns :: vDSO : syscall
---
arch/x86_64/syscall_arch.h | 2 ++
src/sched/sched_getcpu.c | 31 +++++++++++++++++++++++++++++++
2 files changed, 33 insertions(+)
diff --git a/arch/x86_64/syscall_arch.h b/arch/x86_64/syscall_arch.h
index a7a7b5a..54e05ff 100644
--- a/arch/x86_64/syscall_arch.h
+++ b/arch/x86_64/syscall_arch.h
@@ -64,3 +64,5 @@ static __inline long __syscall6(long n, long a1, long a2, long a3, long a4, long
#define VDSO_USEFUL
#define VDSO_CGT_SYM "__vdso_clock_gettime"
#define VDSO_CGT_VER "LINUX_2.6"
+#define VDSO_GETCPU_SYM "__vdso_getcpu"
+#define VDSO_GETCPU_VER "LINUX_2.6"
diff --git a/src/sched/sched_getcpu.c b/src/sched/sched_getcpu.c
index 760e4d5..e08cfdf 100644
--- a/src/sched/sched_getcpu.c
+++ b/src/sched/sched_getcpu.c
@@ -1,12 +1,43 @@
#define _GNU_SOURCE
+#include <errno.h>
#include <sched.h>
#include "syscall.h"
+#include "atomic.h"
+
+#ifdef VDSO_GETCPU_SYM
+
+void *__vdsosym(const char *, const char *);
+
+static void *volatile vdso_func;
+
+typedef long (*getcpu_f)(unsigned *, unsigned *, void *);
+
+static long getcpu_init(unsigned *cpu, unsigned *node, void *unused)
+{
+ void *p = __vdsosym(VDSO_GETCPU_VER, VDSO_GETCPU_SYM);
+ getcpu_f f = (getcpu_f)p;
+ a_cas_p(&vdso_func, (void *)getcpu_init, p);
+ return f ? f(cpu, node, unused) : -ENOSYS;
+}
+
+static void *volatile vdso_func = (void *)getcpu_init;
+
+#endif
int sched_getcpu(void)
{
int r;
unsigned cpu;
+#ifdef VDSO_GETCPU_SYM
+ getcpu_f f = (getcpu_f)vdso_func;
+ if (f) {
+ r = f(&cpu, 0, 0);
+ if (!r) return cpu;
+ if (r != -ENOSYS) return __syscall_ret(r);
+ }
+#endif
+
r = __syscall(SYS_getcpu, &cpu, 0, 0);
if (!r) return cpu;
return __syscall_ret(r);
--
2.7.1
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