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

Randy Dunlap
2021-02-03T01:33:06Z

On 2/2/21 4:50 PM, Paul E. McKenney wrote:
> On Tue, Feb 02, 2021 at 11:51:02AM -0800, Randy Dunlap wrote:
>> On 2/2/21 9:06 AM, paulmck@kernel.org wrote:
>>> 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>
>>> Signed-off-by: Paul E. McKenney <paulmck@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 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...
>
> That is a bit obscure, now that you mention it.
>
>>> + 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
>
> Good eyes!
>
> So how about like this?

Much better, thanks.

> 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 four will

I know that in "writing," "four" should be written out as you have it,
but IMO using "4" here would be much better. FWIW.

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

thanks!
--
~Randy