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    Subject[PATCH v10 clocksource 4/7] clocksource: Provide a module parameter to fuzz per-CPU clock checking
    Date
    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.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 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 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
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