Re: [PATCH v7 1/2] nohz: add tick_nohz_full_add_cpus_to() and _remove_cpus_from() APIs

[Chris Metcalf <cmetcalf@ezchip.com>]

On 4/13/2015 8:33 PM, Frederic Weisbecker wrote:
> On Fri, Apr 10, 2015 at 04:53:51PM -0400, Chris Metcalf wrote:
>> The "removes_cpus_from" API is useful, for example, to modify a cpumask
>> to avoid the nohz cores so that interrupts aren't sent to them.
>>
>> Likewise the "add_cpus_to" API is useful to modify a cpumask indicating
>> some special nohz-type functionality so that the nohz cores are
>> automatically added to that set.
>>
>> Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>
>> ---
>> [...]
>>
>> diff --git a/include/linux/tick.h b/include/linux/tick.h
>> index 9c085dc12ae9..8d1754c0f694 100644
>> --- a/include/linux/tick.h
>> +++ b/include/linux/tick.h
>> @@ -186,6 +186,18 @@ static inline bool tick_nohz_full_cpu(int cpu)
>>   	return cpumask_test_cpu(cpu, tick_nohz_full_mask);
>>   }
>>   
>> +static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask)
>> +{
>> +	if (tick_nohz_full_enabled())
>> +		cpumask_or(mask, mask, tick_nohz_full_mask);
>> +}
>> +
>> +static inline void tick_nohz_full_remove_cpus_from(struct cpumask *mask)
>> +{
>> +	if (tick_nohz_full_enabled())
>> +		cpumask_andnot(mask, mask, tick_nohz_full_mask);
> I would prefer that you don't introduce new APIs if they aren't used in your
> patchset. It seems that's the case for tick_nohz_full_remove_cpus_from().

Yes, it's used in a tile-tree patch to remove nohz_full cpus from the set that
take interrupts from the tilegx network driver.

I can certainly make this patchset have just the _add_cpus_to() variant,
and the other patchset have just the _remove_cpus_from() variant.
It seemed to make sense to include them together as a matched set,
but doing it the way you suggest makes an equal if different amount of sense.

> Also we have housekeeping_affine() that affines a task to CPUs outside the
> range of nohz full. In case you still need tick_nohz_full_remove_cpus_from()
> in a further patchset, housekeeping_affine_cpumask() would extend the existing
> naming.

I like housekeeping_affine(), but it overwrites the affinity mask of
the task.  So I would expect housekeeping_affine_mask() to overwrite
the specified argument cpumask, which it doesn't in my definition.

I don't know that I can think of a good name in the "housekeeping_xxx"
namespace that feels better than the one I proposed.  In context of the
proposed client of the API so far, it's:

        if (!network_cpus_init()) {
                network_cpus_map = *cpu_online_mask;
tick_nohz_full_remove_cpus_from(&network_cpus_map);
        }

If housekeeping_mask were defined for non-nohz_full I could just use
it unconditionally here.  Alternately I could just put in an #ifdef for
CONFIG_NO_HZ_FULL and either use cpu_only_mask or housekeeping_mask
to initialize network_cpus_map, which dodges the bullet of creating
an acceptable API name here for the moment.

Frederic, what's your thought/preference?

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com