Re: [PATCH 1/4] memcg: do not try to drain per-cpu caches without pages

[Michal Hocko <mhocko@suse.cz>]

On Fri 22-07-11 08:44:13, KAMEZAWA Hiroyuki wrote:
> On Thu, 21 Jul 2011 13:36:06 +0200
> Michal Hocko <mhocko@suse.cz> wrote:
> 
> > On Thu 21-07-11 19:12:50, KAMEZAWA Hiroyuki wrote:
> > > On Thu, 21 Jul 2011 09:38:00 +0200
> > > Michal Hocko <mhocko@suse.cz> wrote:
> > > 
> > > > drain_all_stock_async tries to optimize a work to be done on the work
> > > > queue by excluding any work for the current CPU because it assumes that
> > > > the context we are called from already tried to charge from that cache
> > > > and it's failed so it must be empty already.
> > > > While the assumption is correct we can do it by checking the current
> > > > number of pages in the cache. This will also reduce a work on other CPUs
> > > > with an empty stock.
> > > > 
> > > > Signed-off-by: Michal Hocko <mhocko@suse.cz>
> > > 
> > > 
> > > At the first look, when a charge against TransParentHugepage() goes
> > > into the reclaim routine, stock->nr_pages != 0 and this will
> > > call additional kworker.
> > 
> > True. We will drain a charge which could be used by other allocations
> > in the meantime so we have a good chance to reclaim less. But how big
> > problem is that?
> > I mean I can add a new parameter that would force checking the current
> > cpu but it doesn't look nice. I cannot add that condition
> > unconditionally because the code will be shared with the sync path in
> > the next patch and that one needs to drain _all_ cpus.
> > 
> > What would you suggest?
> By 2 methods
> 
>  - just check nr_pages. 

Not sure I understand which nr_pages you mean. The one that comes from
the charging path or stock->nr_pages?
If you mean the first one then we do not have in the reclaim path where
we call drain_all_stock_async.

>  - drain "local stock" without calling schedule_work(). It's fast.

but there is nothing to be drained locally in the paths where we call
drain_all_stock_async... Or do you mean that drain_all_stock shouldn't
use work queue at all?

> 
> Thanks,
> -Kame

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
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190 00 Praha 9    
Czech Republic