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: John Stultz <john.stultz@linaro.org>
Tested-by: Daniel 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: Thomas Gleixner <tglx@linutronix.de>

diff --git a/include/linux/timekeeper_internal.h b/include/linux/timekeeper_internal.h
index e9834ada4d0c5e9aef79c0334d1f7cb12b339870..f7043ccca81cc65f15c252158e745dc4c2948155 100644 (file)
--- a/include/linux/timekeeper_internal.h
+++ b/include/linux/timekeeper_internal.h
@@ -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

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index eff94cb8e89e7bf45038eeb7aea0daab40380b40..b602c48cb84123890dbdc40b9a3da94439ba3a9d 100644 (file)
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -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->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 = raw_nsecs;
+       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;