[PATCH v8 clocksource 1/5] clocksource: Provide module parameters to inject delays in watchdog

[Paul E. McKenney]

When the clocksource watchdog marks a clock as unstable, this might be due
to that clock being unstable or it might be due to delays that happen to
occur between the reads of the two clocks.  Yes, interrupts are disabled
across those two reads, but there are no shortage of things that can
delay interrupts-disabled regions of code ranging from SMI handlers to
vCPU preemption.  It would be good to have some indication as to why
the clock was marked unstable.

The first step is a way of injecting such delays.  Therefore, provide
clocksource.inject_delay_freq and clocksource.inject_delay_run kernel boot
parameters that specify that sufficient delay be injected to cause the
clocksource_watchdog() function to mark a clock unstable.  This delay is
injected every Nth set of M calls to clocksource_watchdog(), where N is
the value specified for the inject_delay_freq boot parameter and M is the
value specified for the inject_delay_run boot parameter.  Values of zero
or less for either parameter disable delay injection, and the default for
clocksource.inject_delay_freq is zero, that is, disabled.  The default for
clocksource.inject_delay_run is the value one, that is single-call runs.

This facility is intended for diagnostic use only, and should be avoided
on production systems.

Cc: John Stultz <john.stultz@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Stephen Boyd <sboyd@xxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Mark Rutland <Mark.Rutland@xxxxxxx>
Cc: Marc Zyngier <maz@xxxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
[ paulmck: Apply Rik van Riel feedback. ]
Reported-by: Chris Mason <clm@xxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 .../admin-guide/kernel-parameters.txt         | 22 +++++++++++++++
 kernel/time/clocksource.c                     | 27 +++++++++++++++++++
 2 files changed, 49 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 04545725f187..fc57952dcba0 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -583,6 +583,28 @@
 			loops can be debugged more effectively on production
 			systems.
 
+	clocksource.inject_delay_freq= [KNL]
+			Number of runs of calls to clocksource_watchdog()
+			before delays are injected between reads from the
+			two clocksources.  Values less than or equal to
+			zero disable this delay injection.  These delays
+			can cause clocks to be marked unstable, so use
+			of this parameter should therefore be avoided on
+			production systems.  Defaults to zero (disabled).
+
+	clocksource.inject_delay_run= [KNL]
+			Run lengths of clocksource_watchdog() delay
+			injections.  Specifying the value 8 will result
+			in eight consecutive delays followed by eight
+			times the value specified for inject_delay_freq
+			of consecutive non-delays.
+
+	clocksource.max_read_retries= [KNL]
+			Number of clocksource_watchdog() retries due to
+			external delays before the clock will be marked
+			unstable.  Defaults to three retries, that is,
+			four attempts to read the clock under test.
+
 	clearcpuid=BITNUM[,BITNUM...] [X86]
 			Disable CPUID feature X for the kernel. See
 			arch/x86/include/asm/cpufeatures.h for the valid bit
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index cce484a2cc7c..4be4391aa72f 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -14,6 +14,7 @@
 #include <linux/sched.h> /* for spin_unlock_irq() using preempt_count() m68k */
 #include <linux/tick.h>
 #include <linux/kthread.h>
+#include <linux/delay.h>
 
 #include "tick-internal.h"
 #include "timekeeping_internal.h"
@@ -184,6 +185,31 @@ void clocksource_mark_unstable(struct clocksource *cs)
 	spin_unlock_irqrestore(&watchdog_lock, flags);
 }
 
+static int inject_delay_freq;
+module_param(inject_delay_freq, int, 0644);
+static int inject_delay_run = 1;
+module_param(inject_delay_run, int, 0644);
+static int max_read_retries = 3;
+module_param(max_read_retries, int, 0644);
+
+static void clocksource_watchdog_inject_delay(void)
+{
+	int i;
+	static int injectfail = -1;
+
+	if (inject_delay_freq <= 0 || inject_delay_run <= 0)
+		return;
+	if (injectfail < 0 || injectfail > INT_MAX / 2)
+		injectfail = inject_delay_run;
+	if (!(++injectfail / inject_delay_run % inject_delay_freq)) {
+		pr_warn("%s(): Injecting delay.\n", __func__);
+		for (i = 0; i < 2 * WATCHDOG_THRESHOLD / NSEC_PER_MSEC; i++)
+			udelay(1000);
+		pr_warn("%s(): Done injecting delay.\n", __func__);
+	}
+	WARN_ON_ONCE(injectfail < 0);
+}
+
 static void clocksource_watchdog(struct timer_list *unused)
 {
 	struct clocksource *cs;
@@ -208,6 +234,7 @@ static void clocksource_watchdog(struct timer_list *unused)
 
 		local_irq_disable();
 		csnow = cs->read(cs);
+		clocksource_watchdog_inject_delay();
 		wdnow = watchdog->read(watchdog);
 		local_irq_enable();
 
-- 
2.25.1