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Message-Id: <1466452585-29379-3-git-send-email-john.stultz@linaro.org>
Date: Mon, 20 Jun 2016 12:56:21 -0700
From: John Stultz <john.stultz@...aro.org>
To: lkml <linux-kernel@...r.kernel.org>
Cc: Thomas Graziadei <thomas.graziadei@...cronenergy.com>,
Prarit Bhargava <prarit@...hat.com>,
Richard Cochran <richardcochran@...il.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...nel.org>,
John Stultz <john.stultz@...aro.org>
Subject: [PATCH 2/6] timekeeping: Fix 1ns/tick drift with GENERIC_TIME_VSYSCALL_OLD
From: Thomas Graziadei <thomas.graziadei@...cronenergy.com>
The user notices the problem in a raw and real time drift, calling
clock_gettime with CLOCK_REALTIME / CLOCK_MONOTONIC_RAW on a system
with no ntp correction taking place (no ntpd or ptp stuff running).
The problem is, that old_vsyscall_fixup adds an extra 1ns even though
xtime_nsec is already held in full nsecs and the remainder in this
case is 0. Do the rounding up buisness only if needed.
Cc: Prarit Bhargava <prarit@...hat.com>
Cc: Richard Cochran <richardcochran@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Thomas Graziadei <thomas.graziadei@...cronenergy.com>
Signed-off-by: John Stultz <john.stultz@...aro.org>
---
kernel/time/timekeeping.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 479d25c..a196e08 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -480,10 +480,12 @@ static inline void old_vsyscall_fixup(struct timekeeper *tk)
* users are removed, this can be killed.
*/
remainder = tk->tkr_mono.xtime_nsec & ((1ULL << tk->tkr_mono.shift) - 1);
- tk->tkr_mono.xtime_nsec -= remainder;
- tk->tkr_mono.xtime_nsec += 1ULL << tk->tkr_mono.shift;
- tk->ntp_error += remainder << tk->ntp_error_shift;
- tk->ntp_error -= (1ULL << tk->tkr_mono.shift) << tk->ntp_error_shift;
+ if (remainder != 0) {
+ tk->tkr_mono.xtime_nsec -= remainder;
+ tk->tkr_mono.xtime_nsec += 1ULL << tk->tkr_mono.shift;
+ tk->ntp_error += remainder << tk->ntp_error_shift;
+ tk->ntp_error -= (1ULL << tk->tkr_mono.shift) << tk->ntp_error_shift;
+ }
}
#else
#define old_vsyscall_fixup(tk)
--
1.9.1
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