linux-kernel - [PATCH 05/12] time: Add debugging checks to warn if we see delays

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Date:	Fri,  6 Mar 2015 18:49:56 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	lkml <linux-kernel@...r.kernel.org>
Cc:	John Stultz <john.stultz@...aro.org>,
	Dave Jones <davej@...emonkey.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Richard Cochran <richardcochran@...il.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH 05/12] time: Add debugging checks to warn if we see delays

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

Thus, this patch adds some extra infrastructure to
add a check update_wall_time to print warnings if we
see the call delayed beyond the max_cycles overflow
point, or beyond the clocksource max_idle_ns value
which is currently 50% of the overflow point.

This extra infrastructure is conditionalized
behind a new CONFIG_DEBUG_TIMEKEEPING option
also added in this patch.

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

Cc: Dave Jones <davej@...emonkey.org.uk>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Richard Cochran <richardcochran@...il.com>
Cc: Prarit Bhargava <prarit@...hat.com>
Cc: Stephen Boyd <sboyd@...eaurora.org>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: John Stultz <john.stultz@...aro.org>
---
 kernel/time/jiffies.c     |  1 +
 kernel/time/timekeeping.c | 26 ++++++++++++++++++++++++++
 lib/Kconfig.debug         | 12 ++++++++++++
 3 files changed, 39 insertions(+)

diff --git a/kernel/time/jiffies.c b/kernel/time/jiffies.c
index a6a5bf5..7e41390 100644
--- 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 91db941..7e9d433 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -118,6 +118,29 @@ 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("ERROR: cycle offset (%lld) is larger then"
+			" allowed %s max_cycles (%lld)\n",
+			offset, name, max_cycles);
+	else if (offset > (max_cycles >> 1))
+		printk_deferred("WARNING: cycle offset (%lld) is past"
+			" the %s 50%% safety margin (%lld)\n",
+			offset, name, max_cycles>>1);
+}
+#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 +1653,9 @@ 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 c5cefb3..32065f6 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -865,6 +865,18 @@ 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 diagnoising issues where timekeeping
+	  problems are suspected.
+
+	  This may include checks in the timekeeping hotpaths, so this
+	  option may have a performance impact to some workloads.
+
+	  If unsure, say N.
+
 config TIMER_STATS
 	bool "Collect kernel timers statistics"
 	depends on DEBUG_KERNEL && PROC_FS
-- 
1.9.1

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