Commit 3c17ad19 authored by John Stultz's avatar John Stultz Committed by Ingo Molnar
Browse files

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: default avatarJohn 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: default avatarIngo Molnar <mingo@kernel.org>
parent fb82fe2f

kernel/time/jiffies.c

+1 −0
Original line number Diff line number Diff line
@@ -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);

kernel/time/timekeeping.c

+28 −0
Original line number Diff line number Diff line
@@ -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,

lib/Kconfig.debug

+13 −0
Original line number Diff line number Diff line
@@ -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

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