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Date: Wed, 28 Dec 2011 14:45:36 -0800
From: Arun Sharma <asharma@...com>
To: Andy Lutomirski <luto@...capital.net>
Cc: Arun Sharma <asharma@...com>, Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...e.hu>,
Peter Zijlstra <peterz@...radead.org>,
Kumar Sundararajan <kumar@...com>,
linux-kernel@...r.kernel.org, john stultz <johnstul@...ibm.com>,
Richard Cochran <richardcochran@...il.com>
Subject: Re: [PATCH 1/4] Add clock_gettime_ns syscall
On Wed, Dec 28, 2011 at 12:13:37PM -0800, Andy Lutomirski wrote:
> > How about returning a (signed) long as the time in ns? This way, we save
> a store and a load and the value can be passed in registers.
> >
> > This shouldn't preclude future expansion via extra args.
>
> With an unconditional store to a pointer? If a null pointer is allowed,
> the branch will probably kill any performance gain.
No - please see the patch below.
>
> The downside is that this is probably a non-starter for a syscall on 32-bit
> architectures. I'll see what the i386 ABI says. I wonder if returning a
> struct will use registers for future expansion.
>
I was thinking of doing something similar to lseek() on 32 bit archs
(i.e. by using a type similar to off_t that maps to the right thing for
both 32 and 64 bit).
I used the code below to benchmark the performance of clock_gettime()
vs clock_gettime_ns() when the client is interested in a nanosec based
interface.
gettimespec: 3.19 secs
getns: 2.54 secs (21% faster)
-Arun
PS: I didn't have to delete struct timens. I meant to drop timens.ns
(since its the return value now).
>From 2a9bb81b56c2034f444c3caa6cf8fbfd47f1d888 Mon Sep 17 00:00:00 2001
From: Arun Sharma <asharma@...com>
Date: Wed, 28 Dec 2011 11:10:37 -0800
Subject: [PATCH] Make clock_gettime_ns return nanosecs
This should speed things up a bit, while leaving room for
future expansion (eg: return additional info in a struct
passed in as an extra arg).
Signed-off-by: Arun Sharma <asharma@...com>
---
arch/x86/vdso/vclock_gettime.c | 22 ++++++----------------
include/linux/syscalls.h | 3 +--
include/linux/time.h | 5 -----
kernel/posix-timers.c | 11 ++++-------
4 files changed, 11 insertions(+), 30 deletions(-)
diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index f9c08b2..41f613c 100644
--- a/arch/x86/vdso/vclock_gettime.c
+++ b/arch/x86/vdso/vclock_gettime.c
@@ -219,7 +219,7 @@ notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
int clock_gettime(clockid_t, struct timespec *)
__attribute__((weak, alias("__vdso_clock_gettime")));
-notrace int __vdso_clock_gettime_ns(clockid_t clock, struct timens *t)
+notrace long __vdso_clock_gettime_ns(clockid_t clock)
{
struct timespec ts;
int error;
@@ -227,35 +227,25 @@ notrace int __vdso_clock_gettime_ns(clockid_t clock, struct timens *t)
switch (clock) {
case CLOCK_REALTIME:
if (likely(gtod->clock.vclock_mode != VCLOCK_NONE)) {
- t->ns = do_realtime_ns();
- t->padding = 0;
- return 0;
+ return do_realtime_ns();
}
break;
case CLOCK_MONOTONIC:
if (likely(gtod->clock.vclock_mode != VCLOCK_NONE)) {
- t->ns = do_monotonic_ns();
- t->padding = 0;
- return 0;
+ return do_monotonic_ns();
}
break;
case CLOCK_REALTIME_COARSE:
- t->ns = do_realtime_coarse_ns();
- t->padding = 0;
- return 0;
+ return do_realtime_coarse_ns();
case CLOCK_MONOTONIC_COARSE:
- t->ns = do_monotonic_coarse_ns();
- t->padding = 0;
- return 0;
+ return do_monotonic_coarse_ns();
}
error = vdso_fallback_gettime(clock, &ts);
if (error)
return error;
- t->ns = ts.tv_sec * NSEC_PER_SEC + ts.tv_nsec;
- t->padding = 0;
- return 0;
+ return ts.tv_sec * NSEC_PER_SEC + ts.tv_nsec;
}
notrace int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz)
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index b351ab6..63759aa 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -317,8 +317,7 @@ asmlinkage long sys_clock_settime(clockid_t which_clock,
const struct timespec __user *tp);
asmlinkage long sys_clock_gettime(clockid_t which_clock,
struct timespec __user *tp);
-asmlinkage long sys_clock_gettime_ns(clockid_t which_clock,
- struct timens __user *tp);
+asmlinkage long sys_clock_gettime_ns(clockid_t which_clock);
asmlinkage long sys_clock_adjtime(clockid_t which_clock,
struct timex __user *tx);
asmlinkage long sys_clock_getres(clockid_t which_clock,
diff --git a/include/linux/time.h b/include/linux/time.h
index d4488b1..b306178 100644
--- a/include/linux/time.h
+++ b/include/linux/time.h
@@ -27,11 +27,6 @@ struct timezone {
int tz_dsttime; /* type of dst correction */
};
-struct timens {
- u64 ns; /* nanoseconds since the relevant epoch */
- u64 padding; /* for future expansion (UTC offset? sub-ns?) */
-};
-
#ifdef __KERNEL__
extern struct timezone sys_tz;
diff --git a/kernel/posix-timers.c b/kernel/posix-timers.c
index 1b6ad2d..b87e3dc 100644
--- a/kernel/posix-timers.c
+++ b/kernel/posix-timers.c
@@ -980,8 +980,7 @@ SYSCALL_DEFINE2(clock_gettime, const clockid_t, which_clock,
return error;
}
-SYSCALL_DEFINE2(clock_gettime_ns, const clockid_t, which_clock,
- struct timens __user *, tp)
+SYSCALL_DEFINE1(clock_gettime_ns, const clockid_t, which_clock)
{
/*
* This implementation isn't as fast as it could be, but the syscall
@@ -991,8 +990,8 @@ SYSCALL_DEFINE2(clock_gettime_ns, const clockid_t, which_clock,
struct k_clock *kc = clockid_to_kclock(which_clock);
struct timespec kernel_timespec;
- struct timens timens;
int error;
+ long ns;
if (!kc)
return -EINVAL;
@@ -1000,11 +999,9 @@ SYSCALL_DEFINE2(clock_gettime_ns, const clockid_t, which_clock,
error = kc->clock_get(which_clock, &kernel_timespec);
if (!error) {
- timens.ns = kernel_timespec.tv_sec * NSEC_PER_SEC
+ ns = kernel_timespec.tv_sec * NSEC_PER_SEC
+ kernel_timespec.tv_nsec;
- timens.padding = 0;
-
- error = copy_to_user(tp, &timens, sizeof(timens));
+ return ns;
}
return error;
--
1.7.4
==== getns.c ====
#include <stdio.h>
#include <time.h>
#include <unistd.h>
#include <dlfcn.h>
#include <sys/types.h>
#include <sys/syscall.h>
volatile int sum;
int
main(int argc, char *argv[])
{
unsigned long v;
void *vdso = dlopen("linux-vdso.so.1", RTLD_LAZY | RTLD_LOCAL | RTLD_NOLOAD);
unsigned long (*vdso_func)();
void *vp;
int i;
if (!vdso) {
printf("Warning: failed to find vDSO\n");
return;
}
vdso_func = dlsym(vdso, "__vdso_clock_gettime_ns");
if (!vdso_func) {
printf("Warning: failed to find vdso_func in vDSO\n");
return;
}
v = (*vdso_func)(CLOCK_REALTIME);
printf("%lx %ld\n", v, v);
for (i = 0; i < 100000000; i++) {
sum += (*vdso_func)(CLOCK_REALTIME);
}
v = (*vdso_func)(CLOCK_REALTIME);
printf("%lx %ld\n", v, v);
}
=== gettimespec.c ===
#include <stdio.h>
#include <time.h>
#include <unistd.h>
#include <dlfcn.h>
#include <sys/types.h>
#include <sys/syscall.h>
#define NSECS_PER_SEC 1000000000
volatile int sum;
int
main(int argc, char *argv[])
{
unsigned long v;
void *vdso = dlopen("linux-vdso.so.1", RTLD_LAZY | RTLD_LOCAL | RTLD_NOLOAD);
unsigned long (*vdso_func)();
void *vp;
int i;
struct timespec ts;
if (!vdso) {
printf("Warning: failed to find vDSO\n");
return;
}
vdso_func = dlsym(vdso, "__vdso_clock_gettime");
if (!vdso_func) {
printf("Warning: failed to find vdso_func in vDSO\n");
return;
}
v = (*vdso_func)(CLOCK_REALTIME, &ts);
v = ts.tv_sec * NSECS_PER_SEC + ts.tv_nsec;
printf("%lx %ld\n", v, v);
for (i = 0; i < 100000000; i++) {
(*vdso_func)(CLOCK_REALTIME, &ts);
v = ts.tv_sec * NSECS_PER_SEC + ts.tv_nsec;
sum +=v;
}
v = (*vdso_func)(CLOCK_REALTIME, &ts);
v = ts.tv_sec * NSECS_PER_SEC + ts.tv_nsec;
printf("%lx %ld\n", v, v);
}
--
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