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

paulmck
2021-02-17T21:31:29Z

From: "Paul E. McKenney" <paulmck@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.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>
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 9965266..628e87f 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -593,6 +593,22 @@
times the value specified for inject_delay_freq
of consecutive non-delays.

+ 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 663bc53..df48416 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