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

[Randy Dunlap]

On 2/2/21 9:06 AM, paulmck@xxxxxxxxxx wrote:
> 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>
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> 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 9965266..f561e94 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,

It's a good think that you give an example -- it helps a little bit.
That sentence above needs to be rewritten...

> +			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.

			            disables

> +
>  	clocksource.max_read_retries= [KNL]
>  			Number of clocksource_watchdog() retries due to
>  			external delays before the clock will be marked


-- 
~Randy