Messages in this thread Patch in this message | | | Date | Thu, 1 Feb 2007 12:46:44 +0100 | From | Ingo Molnar <> | Subject | [-mm patch] x86_64 GTOD: offer scalable vgettimeofday |
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* 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/
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