linux-kernel - Re: [PATCH RFC V1 0/5] Rationalize time keeping

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Date:	Thu, 3 May 2012 20:21:30 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [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?

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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