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SubjectRe: [PATCH RFC V1 0/5] Rationalize time keeping
On Fri, Apr 27, 2012 at 03:49:51PM -0700, John Stultz wrote:
> On 04/27/2012 01:12 AM, Richard Cochran wrote:
> >* Performance Impacts
> >** con
> > - Small extra cost when reading the time (one integer addition plus
> > one integer test).
> This may not be so small when it comes to folks who are very
> concerned about the clock_gettime hotpath.
> Further, the correction will be needed to be made in the vsyscall
> paths, which isn't done with your current patchset (causing userland
> to see different time values then what kernel space calculates).

John, now that you clarified the vDSO thing, I am very confused about
this statement of yours. It appears that the vDSO data are updated
when timekeeping_update() in timekeeper.c calls update_vsyscall().

I think the hunk from patch #5, below, does in fact adjust the time
value correctly before it gets handed off to the arch-specific
update_vsyscall() to be copied into the vDSO page. So I'll make the
claim that:

1. We don't have to touch the vsyscall paths for this.
2. This change does not affect vDSO performance at all.

Would you mind taking another look at the patch?


diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 7941258..6cedf46 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -212,11 +212,14 @@ static inline s64 timekeeping_get_ns_raw(void)
/* must hold write on timekeeper.lock */
static void timekeeping_update(bool clearntp)
+ struct timespec ts;
if (clearntp) {
timekeeper.ntp_error = 0;
- update_vsyscall(&timekeeper.xtime, &timekeeper.wall_to_monotonic,
+ ts.tv_sec = timekeeper_utc_sec();
+ ts.tv_nsec = timekeeper.xtime.tv_nsec;
+ update_vsyscall(&ts, &timekeeper.wall_to_monotonic,
timekeeper.clock, timekeeper.mult);

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