linux-kernel - [PATCH 05/10] time: Add debugging checks to warn if we see delays

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Date:	Fri,  9 Jan 2015 16:34:23 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Linux Kernel Mailing List <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 05/10] 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.

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 | 18 ++++++++++++++++++
 2 files changed, 19 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 6a93185..0dcceba 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1319,6 +1319,22 @@ static int __init timekeeping_init_ops(void)
 }
 device_initcall(timekeeping_init_ops);
 
+static void timekeeping_check_offset(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("ERROR: cycle offset (%lld) is larger then"
+			" allowed %s max_cycles (%lld)\n",
+			offset, name, max_cycles);
+	else if (offset > (max_cycles >> 1))
+		printk("WARNING: cycle offset (%lld) is past"
+			" the %s 50%% safety margin (%lld)\n",
+			offset, name, max_cycles>>1);
+}
+
 /*
  * Apply a multiplier adjustment to the timekeeper
  */
@@ -1602,6 +1618,8 @@ void update_wall_time(void)
 	if (offset < real_tk->cycle_interval)
 		goto out;
 
+	timekeeping_check_offset(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,
-- 
1.9.1

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