[PATCH v9 clocksource 4/6] clocksource: Provide a module parameter to fuzz per-CPU clock checking

[Paul E. McKenney]

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.

Cc: John Stultz <john.stu...@linaro.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Stephen Boyd <sb...@kernel.org>
Cc: Jonathan Corbet <cor...@lwn.net>
Cc: Mark Rutland <mark.rutl...@arm.com>
Cc: Marc Zyngier <m...@kernel.org>
Cc: Andi Kleen <a...@linux.intel.com>
Reported-by: Chris Mason <c...@fb.com>
[ paulmck: Apply Randy Dunlap feedback. ]
Signed-off-by: Paul E. McKenney <paul...@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 7fff95bd5504..03a8c88a6bb9 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -599,6 +599,22 @@
                        will be five delay-free reads followed by three
                        delayed reads.
 
+       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 ee33f14be44d..edcff9fc3f44 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