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

Paul E. McKenney
2021-02-03T01:41:26Z

On Tue, Feb 02, 2021 at 05:31:55PM -0800, Randy Dunlap wrote:
> 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.

As long as it is "four" and not "fore"! Updated as requested. ;-)

Thanx, Paul

> > 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
>