* [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote:
> Inter-CPU monotonicity can not, however, be guaranteed in a vsyscall,
> so vsyscall is not used by default. [...]
note that this is not actually the case. My patch below, ontop of -mm,
implements a fully monotonic gettimeofday as an optional vsyscall
feature.
The 'price' paid for it is lower resolution - but it's still good for
those benchmarking TPC-C runs - and /alot/ simpler. It's also quite a
bit faster than any TSC based vgettimeofday, because it doesnt have to
do an RDTSC (or RDTSCP) instruction nor any approximation of the time.
Ingo
---------------------------->
Subject: [patch] x86_64 GTOD: offer scalable vgettimeofday
From: Ingo Molnar <[EMAIL PROTECTED]>
offer scalable vgettimeofday independently of whether the TSC is
synchronous or not. Off by default. Results in low resolution
gettimefday().
this patch also fixes an SMP bug in sys_vtime(): we should read
__vsyscall_gtod_data.wall_time_tv.tv_sec only once.
Signed-off-by: Ingo Molnar <[EMAIL PROTECTED]>
---
arch/x86_64/kernel/vsyscall.c | 30 +++++++++++++++++++++++++++---
1 file changed, 27 insertions(+), 3 deletions(-)
Index: linux/arch/x86_64/kernel/vsyscall.c
===================================================================
--- linux.orig/arch/x86_64/kernel/vsyscall.c
+++ linux/arch/x86_64/kernel/vsyscall.c
@@ -107,6 +107,22 @@ static __always_inline void do_vgettimeo
cycle_t now, base, mask, cycle_delta;
unsigned long seq, mult, shift, nsec_delta;
cycle_t (*vread)(void);
+
+ if (likely(__vsyscall_gtod_data.sysctl_enabled == 2)) {
+ struct timeval tmp;
+
+ do {
+ barrier();
+ *tv = __vsyscall_gtod_data.wall_time_tv;
+ barrier();
+ tmp = __vsyscall_gtod_data.wall_time_tv;
+
+ } while (tmp.tv_usec != tv->tv_usec ||
+ tmp.tv_sec != tv->tv_sec);
+
+ return;
+ }
+
do {
seq = read_seqbegin(&__vsyscall_gtod_data.lock);
@@ -151,11 +167,19 @@ int __vsyscall(0) vgettimeofday(struct t
* unlikely */
time_t __vsyscall(1) vtime(time_t *t)
{
+ time_t secs;
+
if (!__vsyscall_gtod_data.sysctl_enabled)
return time_syscall(t);
- else if (t)
- *t = __vsyscall_gtod_data.wall_time_tv.tv_sec;
- return __vsyscall_gtod_data.wall_time_tv.tv_sec;
+
+ /*
+ * Make sure that what we return is the same number we
+ * write:
+ */
+ secs = __vsyscall_gtod_data.wall_time_tv.tv_sec;
+ if (t)
+ *t = secs;
+ return secs;
}
/* Fast way to get current CPU and node.
-
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