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

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

On Tue, 15 Jun 2010 14:54:08 +0100
Mel Gorman <mel@csn.ul.ie> wrote:

> On Tue, Jun 15, 2010 at 09:37:27AM -0400, Christoph Hellwig wrote:
> > On Tue, Jun 15, 2010 at 09:34:18AM -0400, Rik van Riel wrote:
> > > If direct reclaim can overflow the stack, so can direct
> > > memcg reclaim.  That means this patch does not solve the
> > > stack overflow, while admitting that we do need the
> > > ability to get specific pages flushed to disk from the
> > > pageout code.
> > 
> > Can you explain what the hell memcg reclaim is and why it needs
> > to reclaim from random contexts?
> 
> Kamezawa Hiroyuki has the full story here but here is a summary.
> 
Thank you.

> memcg is the Memory Controller cgroup
> (Documentation/cgroups/memory.txt). It's intended for the control of the
> amount of memory usable by a group of processes but its behaviour in
> terms of reclaim differs from global reclaim. It has its own LRU lists
> and kswapd operates on them.

No, we don't use kswapd. But we have some hooks in kswapd for implementing
soft-limit. Soft-limit is for giving a hint for kswapd "please reclaim memory
from this memcg" when global memory exhausts and kswapd runs.

What a memcg use when it his limit is just direct reclaim.
(*) Justfing using a cpu by a kswapd because a memcg hits limit is difficult 
    for me. So, I don't use kswapd until now.
    When direct-reclaim is used, cost-of-reclaim will be charged against
    a cpu cgroup which a thread belongs to.


> What is surprising is that direct reclaim
> for a process in the control group also does not operate within the
> cgroup.
Sorry, I can't understand ....

> 
> Reclaim from a cgroup happens from the fault path. The new page is
> "charged" to the cgroup. If it exceeds its allocated resources, some
> pages within the group are reclaimed in a path that is similar to direct
> reclaim except for its entry point.
> 
yes.

> So, memcg is not reclaiming from a random context, there is a limited
> number of cases where a memcg is reclaiming and it is not expected to
> overflow the stack.
> 

I think so. Especially, we'll never see 1k stack use of select().

> > It seems everything that has a cg in it's name that I stumbled over
> > lately seems to be some ugly wart..
> > 
> 
> The wart in this case is that the behaviour of page reclaim within a
> memcg and globally differ a fair bit.
> 

Sorry. But there has been very long story to reach current implementations.
But don't worry, of memcg is not activated (not mounted), it doesn't affect
the behavior of processes ;)

But Hmm..

>[kamezawa@bluextal mmotm-2.6.35-0611]$ wc -l mm/memcontrol.c
>4705 mm/memcontrol.c

may need some diet :(


Thanks,
-Kame


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