lkml.org 
[lkml]   [2007]   [Feb]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[-mm patch] x86_64 GTOD: offer scalable vgettimeofday

* jbohac@suse.cz <jbohac@suse.cz> 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 <mingo@elte.hu>

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 <mingo@elte.hu>
---
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.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
\
 
 \ /
  Last update: 2007-02-01 12:51    [W:0.191 / U:9.396 seconds]
©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site