[PATCH v11 clocksource 4/6] clocksource: Provide a module parameter to fuzz per-CPU clock checking

["Paul E. McKenney" <paulmck@kernel.org>]

Code that checks for clock desynchronization must itself be tested, so
create a new clocksource.inject_delay_shift_percpu= kernel boot parameter
that adds or subtracts a large value from the check read, using the
specified bit of the CPU ID to determine whether to add or to subtract.

Link: https://lore.kernel.org/lkml/20210425224540.GA1312438@paulmck-ThinkPad-P17-Gen-1/
Link: https://lore.kernel.org/lkml/20210420064934.GE31773@xsang-OptiPlex-9020/
Link: https://lore.kernel.org/lkml/20210106004013.GA11179@paulmck-ThinkPad-P72/
Link: https://lore.kernel.org/lkml/20210414043435.GA2812539@paulmck-ThinkPad-P17-Gen-1/
Link: https://lore.kernel.org/lkml/20210419045155.GA596058@paulmck-ThinkPad-P17-Gen-1/
Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mark Rutland <Mark.Rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Feng Tang <feng.tang@intel.com>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Reported-by: Chris Mason <clm@fb.com>
[ paulmck: Apply Randy Dunlap feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 Documentation/admin-guide/kernel-parameters.txt | 16 ++++++++++++++++
 kernel/time/clocksource.c                       | 10 +++++++++-
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index eb40e5332236..babcc9e80fba 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -604,6 +604,22 @@
 			from, and complained about, and to larger values
 			to force the clock to be marked unstable.
 
+	clocksource.inject_delay_shift_percpu= [KNL]
+			Clocksource delay injection partitions the CPUs
+			into two sets, one whose clocks are moved ahead
+			and the other whose clocks are moved behind.
+			This kernel parameter selects the CPU-number
+			bit that determines which of these two sets the
+			corresponding CPU is placed into.  For example,
+			setting this parameter to the value 4 will result
+			in the first set containing alternating groups
+			of 16 CPUs whose clocks are moved ahead, while
+			the second set will contain the rest of the CPUs,
+			whose clocks are moved behind.
+
+			The default value of -1 disables this type of
+			error injection.
+
 	clocksource.max_read_retries= [KNL]
 			Number of clocksource_watchdog() retries due to
 			external delays before the clock will be marked
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index a8d73e1f9431..584433448226 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -196,6 +196,8 @@ static ulong inject_delay_period;
 module_param(inject_delay_period, ulong, 0644);
 static ulong inject_delay_repeat = 1;
 module_param(inject_delay_repeat, ulong, 0644);
+static int inject_delay_shift_percpu = -1;
+module_param(inject_delay_shift_percpu, int, 0644);
 static ulong max_read_retries = 3;
 module_param(max_read_retries, ulong, 0644);
 
@@ -252,8 +254,14 @@ static cpumask_t cpus_behind;
 static void clocksource_verify_one_cpu(void *csin)
 {
 	struct clocksource *cs = (struct clocksource *)csin;
+	s64 delta = 0;
+	int sign;
 
-	csnow_mid = cs->read(cs);
+	if (inject_delay_shift_percpu >= 0) {
+		sign = ((smp_processor_id() >> inject_delay_shift_percpu) & 0x1) * 2 - 1;
+		delta = sign * NSEC_PER_SEC;
+	}
+	csnow_mid = cs->read(cs) + delta;
 }
 
 static void clocksource_verify_percpu(struct clocksource *cs)
-- 
2.31.1.189.g2e36527f23