Commit 3d88d56c authored by John Stultz's avatar John Stultz Committed by Thomas Gleixner
Browse files

time: Fix CLOCK_MONOTONIC_RAW sub-nanosecond accounting 



Due to how the MONOTONIC_RAW accumulation logic was handled,
there is the potential for a 1ns discontinuity when we do
accumulations. This small discontinuity has for the most part
gone un-noticed, but since ARM64 enabled CLOCK_MONOTONIC_RAW
in their vDSO clock_gettime implementation, we've seen failures
with the inconsistency-check test in kselftest.

This patch addresses the issue by using the same sub-ns
accumulation handling that CLOCK_MONOTONIC uses, which avoids
the issue for in-kernel users.

Since the ARM64 vDSO implementation has its own clock_gettime
calculation logic, this patch reduces the frequency of errors,
but failures are still seen. The ARM64 vDSO will need to be
updated to include the sub-nanosecond xtime_nsec values in its
calculation for this issue to be completely fixed.

Signed-off-by: default avatarJohn Stultz <john.stultz@linaro.org>
Tested-by: default avatarDaniel Mentz <danielmentz@google.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Stephen Boyd <stephen.boyd@linaro.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: "stable #4 . 8+" <stable@vger.kernel.org>
Cc: Miroslav Lichvar <mlichvar@redhat.com>
Link: http://lkml.kernel.org/r/1496965462-20003-3-git-send-email-john.stultz@linaro.org


Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
parent ceea5e37

include/linux/timekeeper_internal.h

+2 −2
Original line number Diff line number Diff line
@@ -57,7 +57,7 @@ struct tk_read_base { 
 *			interval.

 * @xtime_remainder:	Shifted nano seconds left over when rounding

 *			@cycle_interval

 * @raw_interval:	Raw nano seconds accumulated per NTP interval.

 * @raw_interval:	Shifted raw nano seconds accumulated per NTP interval.

 * @ntp_error:		Difference between accumulated time and NTP time in ntp

 *			shifted nano seconds.

 * @ntp_error_shift:	Shift conversion between clock shifted nano seconds and
@@ -99,7 +99,7 @@ struct timekeeper { 
	u64			cycle_interval;

	u64			xtime_interval;

	s64			xtime_remainder;

	u32			raw_interval;

	u64			raw_interval;

	/* The ntp_tick_length() value currently being used.

	 * This cached copy ensures we consistently apply the tick

	 * length for an entire tick, as ntp_tick_length may change

kernel/time/timekeeping.c

+10 −9
Original line number Diff line number Diff line
@@ -280,7 +280,7 @@ static void tk_setup_internals(struct timekeeper *tk, struct clocksource *clock) 
	/* Go back from cycles -> shifted ns */

	tk->xtime_interval = interval * clock->mult;

	tk->xtime_remainder = ntpinterval - tk->xtime_interval;

	tk->raw_interval = (interval * clock->mult) >> clock->shift;

	tk->raw_interval = interval * clock->mult;



	 /* if changing clocks, convert xtime_nsec shift units */

	if (old_clock) {
@@ -1996,7 +1996,7 @@ static u64 logarithmic_accumulation(struct timekeeper *tk, u64 offset, 
				    u32 shift, unsigned int *clock_set)

{

	u64 interval = tk->cycle_interval << shift;

	u64 raw_nsecs;

	u64 snsec_per_sec;



	/* If the offset is smaller than a shifted interval, do nothing */

	if (offset < interval)
@@ -2011,14 +2011,15 @@ static u64 logarithmic_accumulation(struct timekeeper *tk, u64 offset, 
	*clock_set |= accumulate_nsecs_to_secs(tk);



	/* Accumulate raw time */

	raw_nsecs = (u64)tk->raw_interval << shift;

	raw_nsecs += tk->raw_time.tv_nsec;

	if (raw_nsecs >= NSEC_PER_SEC) {

		u64 raw_secs = raw_nsecs;

		raw_nsecs = do_div(raw_secs, NSEC_PER_SEC);

		tk->raw_time.tv_sec += raw_secs;

	}

	tk->raw_time.tv_nsec = raw_nsecs;

	tk->tkr_raw.xtime_nsec += (u64)tk->raw_time.tv_nsec << tk->tkr_raw.shift;

	tk->tkr_raw.xtime_nsec += tk->raw_interval << shift;

	snsec_per_sec = (u64)NSEC_PER_SEC << tk->tkr_raw.shift;

	while (tk->tkr_raw.xtime_nsec >= snsec_per_sec) {

		tk->tkr_raw.xtime_nsec -= snsec_per_sec;

		tk->raw_time.tv_sec++;

	}

	tk->raw_time.tv_nsec = tk->tkr_raw.xtime_nsec >> tk->tkr_raw.shift;

	tk->tkr_raw.xtime_nsec -= (u64)tk->raw_time.tv_nsec << tk->tkr_raw.shift;



	/* Accumulate error between NTP and clock interval */

	tk->ntp_error += tk->ntp_tick << shift;

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