Messages in this thread Patch in this message | | | From | John Stultz <> | Subject | [PATCH 06/12] time: Try to catch clocksource delta underflows | Date | Wed, 11 Mar 2015 21:16:34 -0700 |
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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 c06fe6e..2ba62fe 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -146,6 +146,13 @@ static inline cycle_t timekeeping_get_delta(struct tk_read_base *tkr) /* calculate the delta since the last update_wall_time */ delta = clocksource_delta(cycle_now, tkr->cycle_last, tkr->mask); + /* + * Try to catch underflows by checking if we are seeing small + * mask-relative negative values. + */ + if (unlikely((~delta & tkr->mask) < (tkr->mask >> 3))) + delta = 0; + /* Cap delta value to the max_cycles values to avoid mult overflows */ if (unlikely(delta > tkr->clock->max_cycles)) delta = tkr->clock->max_cycles; -- 1.9.1
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