From 4ca22c2648f9c1cec0b242f58d7302136f5a4cbb Mon Sep 17 00:00:00 2001 From: John Stultz Date: Wed, 11 Mar 2015 21:16:35 -0700 Subject: [PATCH] timekeeping: Add warnings when overflows or underflows are observed It was suggested that the underflow/overflow protection should probably throw some sort of warning out, rather than just silently fixing the issue. So this patch adds some warnings here. The flag variables used are not protected by locks, but since we can't print from the reading functions, just being able to say we saw an issue in the update interval is useful enough, and can be slightly racy without real consequence. The big complication is that we're only under a read seqlock, so the data could shift under us during our calculation to see if there was a problem. This patch avoids this issue by nesting another seqlock which allows us to snapshot the just required values atomically. So we shouldn't see false positives. I also added some basic rate-limiting here, since on one build machine w/ skewed TSCs it was fairly noisy at bootup. Signed-off-by: John Stultz Cc: Dave Jones Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Prarit Bhargava Cc: Richard Cochran Cc: Stephen Boyd Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/1426133800-29329-8-git-send-email-john.stultz@linaro.org Signed-off-by: Ingo Molnar --- kernel/time/timekeeping.c | 64 ++++++++++++++++++++++++++++++++++----- 1 file changed, 57 insertions(+), 7 deletions(-) diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index 187149be83ea..892f6cbf1e67 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -119,6 +119,20 @@ static inline void tk_update_sleep_time(struct timekeeper *tk, ktime_t delta) } #ifdef CONFIG_DEBUG_TIMEKEEPING +#define WARNING_FREQ (HZ*300) /* 5 minute rate-limiting */ +/* + * These simple flag variables are managed + * without locks, which is racy, but ok since + * we don't really care about being super + * precise about how many events were seen, + * just that a problem was observed. + */ +static int timekeeping_underflow_seen; +static int timekeeping_overflow_seen; + +/* last_warning is only modified under the timekeeping lock */ +static long timekeeping_last_warning; + static void timekeeping_check_update(struct timekeeper *tk, cycle_t offset) { @@ -136,28 +150,64 @@ static void timekeeping_check_update(struct timekeeper *tk, cycle_t offset) printk_deferred(" timekeeping: Your kernel is still fine, but is feeling a bit nervous\n"); } } + + if (timekeeping_underflow_seen) { + if (jiffies - timekeeping_last_warning > WARNING_FREQ) { + printk_deferred("WARNING: Underflow in clocksource '%s' observed, time update ignored.\n", name); + printk_deferred(" Please report this, consider using a different clocksource, if possible.\n"); + printk_deferred(" Your kernel is probably still fine.\n"); + timekeeping_last_warning = jiffies; + } + timekeeping_underflow_seen = 0; + } + + if (timekeeping_overflow_seen) { + if (jiffies - timekeeping_last_warning > WARNING_FREQ) { + printk_deferred("WARNING: Overflow in clocksource '%s' observed, time update capped.\n", name); + printk_deferred(" Please report this, consider using a different clocksource, if possible.\n"); + printk_deferred(" Your kernel is probably still fine.\n"); + timekeeping_last_warning = jiffies; + } + timekeeping_overflow_seen = 0; + } } static inline cycle_t timekeeping_get_delta(struct tk_read_base *tkr) { - cycle_t cycle_now, delta; + cycle_t now, last, mask, max, delta; + unsigned int seq; - /* read clocksource */ - cycle_now = tkr->read(tkr->clock); + /* + * Since we're called holding a seqlock, the data may shift + * under us while we're doing the calculation. This can cause + * false positives, since we'd note a problem but throw the + * results away. So nest another seqlock here to atomically + * grab the points we are checking with. + */ + do { + seq = read_seqcount_begin(&tk_core.seq); + now = tkr->read(tkr->clock); + last = tkr->cycle_last; + mask = tkr->mask; + max = tkr->clock->max_cycles; + } while (read_seqcount_retry(&tk_core.seq, seq)); - /* calculate the delta since the last update_wall_time */ - delta = clocksource_delta(cycle_now, tkr->cycle_last, tkr->mask); + delta = clocksource_delta(now, last, mask); /* * Try to catch underflows by checking if we are seeing small * mask-relative negative values. */ - if (unlikely((~delta & tkr->mask) < (tkr->mask >> 3))) + if (unlikely((~delta & mask) < (mask >> 3))) { + timekeeping_underflow_seen = 1; delta = 0; + } /* Cap delta value to the max_cycles values to avoid mult overflows */ - if (unlikely(delta > tkr->clock->max_cycles)) + if (unlikely(delta > max)) { + timekeeping_overflow_seen = 1; delta = tkr->clock->max_cycles; + } return delta; } -- 2.20.1