Re: [PATCH v8 08/14] nohz_full: allow disabling the 1Hz minimum tick at boot

Chris Metcalf
2015-10-20T21:18:38Z

On 10/20/2015 05:03 PM, Frederic Weisbecker wrote:
> On Tue, Oct 20, 2015 at 04:36:06PM -0400, Chris Metcalf wrote:
>> While the current fallback to 1-second tick is still required for
>> a number of kernel accounting tasks (e.g. vruntime, load balancing
>> data, and load accounting), it's useful to be able to disable it
>> for testing purposes. Paul McKenney observed that if we provide
>> a mode where the 1Hz fallback timer is removed, this will provide
>> an environment where new code that relies on that tick will get
>> punished, and we won't forgive such assumptions silently.
>>
>> This option also allows easy testing of nohz_full and task-isolation
>> modes to determine what functionality needs to be implemented,
>> and what possibly-spurious timer interrupts are scheduled when
>> the basic 1Hz tick has been turned off.
>>
>> Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>
> There have been proposals to disable/tune the 1 Hz tick via debugfs which
> I Nacked because once you give such an opportunity to the users, they
> will use that hack and never fix the real underlying issue.
>
> For the same reasons, I'm sorry but I have to Nack this proposal as well.
>
> If this is for development or testing purpose, scheduler_max_tick_deferment() is
> easily commented out.

Fair enough and certainly your prerogative, so don't hesitate to
say "no" to the following argument. :-)

I would tend to differentiate a debugfs proposal from a boot flag
proposal: a boot flag is a more hardcore thing to change, and it's
not like application developers will come along and explain that
you have to boot with different flags to run their app - whereas
if they can just sneak in a modification to a debugfs setting that's
much easier for the app to tweak.

So perhaps a boot flag is an acceptable compromise between
"nothing" and a debugfs tweak? It certainly does make it easier
to hack on the task-isolation code, and likely other things where
people are trying out fixes to subsystems where they are attempting
to remove the reliance on the tick.

--
Chris Metcalf, EZChip Semiconductor