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    Subject[tip:timers/urgent] time: Fix CLOCK_MONOTONIC_RAW sub-nanosecond accounting
    Commit-ID:  3d88d56c5873f6eebe23e05c3da701960146b801
    Gitweb: http://git.kernel.org/tip/3d88d56c5873f6eebe23e05c3da701960146b801
    Author: John Stultz <john.stultz@linaro.org>
    AuthorDate: Thu, 8 Jun 2017 16:44:21 -0700
    Committer: Thomas Gleixner <tglx@linutronix.de>
    CommitDate: Tue, 20 Jun 2017 10:41:50 +0200

    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>

    ---
    include/linux/timekeeper_internal.h | 4 ++--
    kernel/time/timekeeping.c | 19 ++++++++++---------
    2 files changed, 12 insertions(+), 11 deletions(-)

    diff --git a/include/linux/timekeeper_internal.h b/include/linux/timekeeper_internal.h
    index e9834ad..f7043cc 100644
    --- 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 eff94cb..b602c48 100644
    --- 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;
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