timekeeping: Add debugging checks to warn if we see delays

Recently there's been requests for better sanity
checking in the time code, so that it's more clear
when something is going wrong, since timekeeping issues
could manifest in a large number of strange ways in
various subsystems.

Thus, this patch adds some extra infrastructure to
add a check to update_wall_time() to print two new
warnings:

 1) if we see the call delayed beyond the 'max_cycles'
    overflow point,

 2) or if we see the call delayed beyond the clocksource's
    'max_idle_ns' value, which is currently 50% of the
    overflow point.

This extra infrastructure is conditional on
a new CONFIG_DEBUG_TIMEKEEPING option, also
added in this patch - default off.

Tested this a bit by halting qemu for specified
lengths of time to trigger the warnings.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Cc: Dave Jones <davej@codemonkey.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1426133800-29329-5-git-send-email-john.stultz@linaro.org
[ Improved the changelog and the messages a bit. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/time/jiffies.c b/kernel/time/jiffies.c
index a6a5bf53e86d25575f90518399407a4fb65a85ed..7e413902aa6a495b0b51f97b21edd0e35ae1950f 100644 (file)
--- a/kernel/time/jiffies.c
+++ b/kernel/time/jiffies.c
@@ -71,6 +71,7 @@ static struct clocksource clocksource_jiffies = {
        .mask           = 0xffffffff, /*32bits*/
        .mult           = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
        .shift          = JIFFIES_SHIFT,
+       .max_cycles     = 10,
 };
 
 __cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock);

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 91db94136c1062571ba0d0f1bfd1ed687770af3e..acf049144cf63d6cf316969b9c0d11d041ef87ed 100644 (file)
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -118,6 +118,31 @@ static inline void tk_update_sleep_time(struct timekeeper *tk, ktime_t delta)
        tk->offs_boot = ktime_add(tk->offs_boot, delta);
 }
 
+#ifdef CONFIG_DEBUG_TIMEKEEPING
+static void timekeeping_check_update(struct timekeeper *tk, cycle_t offset)
+{
+
+       cycle_t max_cycles = tk->tkr.clock->max_cycles;
+       const char *name = tk->tkr.clock->name;
+
+       if (offset > max_cycles) {
+               printk_deferred("WARNING: timekeeping: Cycle offset (%lld) is larger than allowed by the '%s' clock's max_cycles value (%lld): time overflow\n",
+                               offset, name, max_cycles);
+               printk_deferred("         timekeeping: Your kernel is sick, but tries to cope\n");
+       } else {
+               if (offset > (max_cycles >> 1)) {
+                       printk_deferred("INFO: timekeeping: Cycle offset (%lld) is larger than the the '%s' clock's 50%% safety margin (%lld)\n",
+                                       offset, name, max_cycles >> 1);
+                       printk_deferred("      timekeeping: Your kernel is still fine, but is feeling a bit nervous\n");
+               }
+       }
+}
+#else
+static inline void timekeeping_check_update(struct timekeeper *tk, cycle_t offset)
+{
+}
+#endif
+
 /**
  * tk_setup_internals - Set up internals to use clocksource clock.
  *
@@ -1630,6 +1655,9 @@ void update_wall_time(void)
        if (offset < real_tk->cycle_interval)
                goto out;
 
+       /* Do some additional sanity checking */
+       timekeeping_check_update(real_tk, offset);
+
        /*
         * With NO_HZ we may have to accumulate many cycle_intervals
         * (think "ticks") worth of time at once. To do this efficiently,

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index c5cefb3c009ce9cd51199dc5fef683d7bc9b1bdc..36b6fa88ce5b412f92b15da530772c5a058e5d12 100644 (file)
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -865,6 +865,19 @@ config SCHED_STACK_END_CHECK
          data corruption or a sporadic crash at a later stage once the region
          is examined. The runtime overhead introduced is minimal.
 
+config DEBUG_TIMEKEEPING
+       bool "Enable extra timekeeping sanity checking"
+       help
+         This option will enable additional timekeeping sanity checks
+         which may be helpful when diagnosing issues where timekeeping
+         problems are suspected.
+
+         This may include checks in the timekeeping hotpaths, so this
+         option may have a (very small) performance impact to some
+         workloads.
+
+         If unsure, say N.
+
 config TIMER_STATS
        bool "Collect kernel timers statistics"
        depends on DEBUG_KERNEL && PROC_FS