[PATCH RFC clocksource 4/5] clocksource: Provide a module parameter to fuzz per-CPU clock checking

[paulmck]

From: "Paul E. McKenney" <paul...@kernel.org>

Code that checks for clock desynchronization must itself be tested, so
this commit creates 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>
Reported-by: Chris Mason <c...@fb.com>
Signed-off-by: Paul E. McKenney <paul...@kernel.org>
---
 Documentation/admin-guide/kernel-parameters.txt |  9 +++++++++
 kernel/time/clocksource.c                       | 10 +++++++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt 
b/Documentation/admin-guide/kernel-parameters.txt
index 15aa3fe..613ce32 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -593,6 +593,15 @@
                        times the value specified for inject_delay_freq
                        of consecutive non-delays.
 
+       clocksource.inject_delay_shift_percpu= [KNL]
+                       Shift count to obtain bit from CPU number to
+                       determine whether to shift the time of the per-CPU
+                       clock under test ahead or behind.  For example,
+                       setting this to the value four will result in
+                       alternating groups of 16 CPUs shifting ahead and
+                       the rest of the CPUs shifting behind.  The default
+                       value of -1 disable 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 23bcefe..67cf41c 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -190,6 +190,8 @@ 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 inject_delay_shift_percpu = -1;
+module_param(inject_delay_shift_percpu, int, 0644);
 static int max_read_retries = 3;
 module_param(max_read_retries, int, 0644);
 
@@ -219,8 +221,14 @@ static cpumask_t cpus_behind;
 static void clocksource_verify_one_cpu(void *csin)
 {
        struct clocksource *cs = (struct clocksource *)csin;
+       s64 delta = 0;
+       int sign;
 
-       __this_cpu_write(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;
+       }
+       __this_cpu_write(csnow_mid, cs->read(cs) + delta);
 }
 
 static void clocksource_verify_percpu_wq(struct work_struct *unused)
-- 
2.9.5