Messages in this thread Patch in this message | | | From | John Stultz <> | Subject | [PATCH 07/10] time: Try to catch clocksource delta underflows | Date | Fri, 9 Jan 2015 16:34:25 -0800 |
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In the case where there is a broken clocksource where there are multiple actual clocks that aren't perfectly aligned, we may see small "negative" deltas when we subtract now from cycle_last.
The values are actually negative with respect to the clocksource mask value, not necessarily negative if cast to a s64, but we can check by checking the delta see if it is a small (relative to the mask) negative value (again negative relative to the mask).
If so, we assume we jumped backwards somehow and instead use zero for our delta.
Cc: Dave Jones <davej@codemonkey.org.uk> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Richard Cochran <richardcochran@gmail.com> Cc: Prarit Bhargava <prarit@redhat.com> Cc: Stephen Boyd <sboyd@codeaurora.org> Cc: Ingo Molnar <mingo@kernel.org> Cc: Peter Zijlstra <peterz@infradead.org> Signed-off-by: John Stultz <john.stultz@linaro.org> --- kernel/time/timekeeping.c | 7 +++++++ 1 file changed, 7 insertions(+)
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index 9740fd8..4acfc7f 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -224,6 +224,13 @@ static inline s64 timekeeping_get_ns_raw(struct timekeeper *tk) /* calculate the delta since the last update_wall_time: */ delta = clocksource_delta(cycle_now, tk->tkr.cycle_last, tk->tkr.mask); + /* + * Try to catch underflows by checking if we are seeing small + * mask-relative negative values. + */ + if (((~delta+1) & tk->tkr.mask) < (tk->tkr.mask >> 3)) + delta = 0; + /* Cap delta value to the max_cycles values to avoid mult overflows */ delta = min(delta, tk->tkr.clock->max_cycles); -- 1.9.1
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