Re: [PATCH] swap: remove the struct cpumask has_work

[Chris Metcalf]

On 7/31/2014 9:39 PM, Lai Jiangshan wrote:
> On 08/01/2014 12:09 AM, Chris Metcalf wrote:
> > On 7/31/2014 7:51 AM, Michal Hocko wrote:
> > > On Thu 31-07-14 11:30:19, Lai Jiangshan wrote:
> > > > It is suggested that cpumask_var_t and alloc_cpumask_var() should be used
> > > > instead of struct cpumask.  But I don't want to add this complicity nor
> > > > leave this unwelcome "static struct cpumask has_work;", so I just remove
> > > > it and use flush_work() to perform on all online drain_work.  flush_work()
> > > > performs very quickly on initialized but unused work item, thus we don't
> > > > need the struct cpumask has_work for performance.
> > > Why? Just because there is general recommendation for using
> > > cpumask_var_t rather than cpumask?
> > >
> > > In this particular case cpumask shouldn't matter much as it is static.
> > > Your code will work as well, but I do not see any strong reason to
> > > change it just to get rid of cpumask which is not on stack.
> > The code uses for_each_cpu with a cpumask to avoid waking cpus that don't
> > need to do work.  This is important for the nohz_full type functionality,
> > power efficiency, etc.  So, nack for this change.
> flush_work() on initialized but unused work item just disables irq and
> fetches work->data to test and restores irq and return.
>
> the struct cpumask has_work is just premature optimization.

Yes, I see your point.  I was mistakenly thinking that your patch resulted
in calling schedule_work() on all the online cpus.

Given that, I think your suggestion is reasonable, though like Michal,
I'm not sure it necessarily rises to the level of it being worth changing the
code at this point.  Regardless, I withdraw my nack, and you can add my
Reviewed-by: Chris Metcalf <cmetcalf@xxxxxxxxxx> if the change is taken.

--
Chris Metcalf, Tilera Corp.
http://www.tilera.com

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