Messages in this thread Patch in this message |  | | | Date | Thu, 3 May 2012 20:21:30 +0200 | | From | Richard Cochran <> | | Subject | Re: [PATCH RFC V1 0/5] Rationalize time keeping |
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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?
Thanks, Richard
--- 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; ntp_clear(); } - 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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