Re: [PATCH 12/12] vmscan: Do not writeback pages in direct reclaim

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Tue, 15 Jun 2010 20:29:49 -0400
Rik van Riel <riel@redhat.com> wrote:

> On 06/15/2010 08:17 PM, KAMEZAWA Hiroyuki wrote:
> > On Tue, 15 Jun 2010 15:16:01 +0100
> > Mel Gorman<mel@csn.ul.ie>  wrote:
> 
> >> But in turn, where is mem_cgroup_hierarchical_reclaim called from direct
> >> reclaim? It appears to be only called from the fault path or as a result
> >> of the memcg changing size.
> >>
> > yes. It's only called from
> > 	- page fault
> > 	- add_to_page_cache()
> >
> > I think we'll see no stack problem. Now, memcg doesn't wakeup kswapd for
> > reclaiming memory, it needs direct writeback.
> 
> Of course, a memcg page fault could still be triggered
> from copy_to_user or copy_from_user, with a fairly
> arbitrary stack frame above...
> 

Hmm. But I don't expect copy_from/to_user is called in very deep stack.

Should I prepare a thread for reclaiming memcg pages ?
Because we shouldn't limit kswapd's cpu time by CFS cgroup, waking up
kswapd just because "a memcg hit limits" isn't fun. 

Hmm, or do you recommend no-dirty-page-writeback when a memcg hits limit ?
Maybe we'll see much swaps.

I want to go with this for a while, changing memcg's behavior will took
some amounts of time, there are only a few developpers.

Thanks,
-Kame