Re: [PATCH v9 2/3] watchdog: add watchdog_cpumask sysctl to assist nohz

[Ulrich Obergfell <uobergfe@redhat.com>]

----- Original Message -----
From: "Chris Metcalf" <cmetcalf@ezchip.com>
[...]
On 04/21/2015 08:32 AM, Ulrich Obergfell wrote:
>> Chris,
>>
>> in v9, smpboot_update_cpumask_percpu_thread() allocates 'tmp' mask dynamically.
>> This allocation can fail and thus the function can now return an error. However,
>> this error is being ignored by proc_watchdog_cpumask().
>
> Yes, I did that intentionally, because it seemed like a pretty extreme 
> corner case (not enough memory to allocate one cpumask), and a relatively
> unproblematic outcome (we don't actually modify the running set of watchdog
> threads the way the /proc knob requested).
>
> The problem with your proposal (to save the old cpumask and put it back on
> failure) is that we will almost certainly not be able to do that either 
> if we can't successfully run smpboot_update_cpumask_percpu_thread(),
> since that's exactly the allocation that we're presuming is going to fail
> internally.
>
> I went down this rathole and decided it wasn't worth worrying about.
> Let me know if you think we need to beat on it some more :-)


Chris,

the other handlers for the watchdog parameters in /proc restore the
original value on failure, so I thought it would be nice to make the
error handling consistent in that regard.

However, on the other hand the 'watchdog_cpumask' parameter is kind of
an exception in terms of when and how it should be used, and thus it's
probably OK if this interface is less 'user-friendly'. As Don commented
in https://lkml.org/lkml/2015/4/22/325 in reply to my suggestion to add
a plausibility check for 'watchdog_cpumask':

 "I am not sure that is necessary.  This was supposed to be a debugging
  interface for nohz (and possibly other technologies). ... Personally,
  I feel anyone who will use this sys interface will need to do so at
  their own risk."

So I think we could apply the same rationale here and ignore a possible
error returned by smpboot_update_cpumask_percpu_thread(). Perhaps you
could add a few comment lines to the code.


Don,

please let us know what you think.


Regards,

Uli