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

From: paulmck
Date: Tue Jan 05 2021 - 19:42:39 EST


From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>

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.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>
Reported-by: Chris Mason <clm@xxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
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