Re: [RFC][PATCH] memcg: remove PCG_ACCT_LRU.

[Hugh Dickins <hughd@google.com>]

On Tue, 6 Dec 2011, KAMEZAWA Hiroyuki wrote:
> On Mon, 5 Dec 2011 23:36:34 -0800 (PST)
> Hugh Dickins <hughd@google.com> wrote:
> 
> Hmm, at first glance at the patch, it seems far complicated than
> I expected

Right, this is just a rollup of assorted changes,
yet to be presented properly as an understandable series.

> and added much checks and hooks to lru path...

Actually, I think it removes more than it adds; while trying not
to increase the overhead of lookup_page_cgroup()s and locking.

> > Okay, here it is: my usual mix of cleanup and functional changes.
> > There's work by Ying and others in here - will apportion authorship
> > more fairly when splitting.  If you're looking through it at all,
> > the place to start would be memcontrol.c's lock_page_lru_irqsave().
> > 
> 
> Thank you. This seems inetersting patch. Hmm...what I think of now is..
> In most case, pages are newly allocated and charged ,and then, added to LRU.
> pc->mem_cgroup never changes while pages are on LRU.
> 
> I have a fix for corner cases as to do
> 
> 	1. lock lru
> 	2. remove-page-from-lru
> 	3. overwrite pc->mem_cgroup
> 	4. add page to lru again
> 	5. unlock lru

That is indeed the sequence which __mem_cgroup_commit_charge() follows
after the patch.

But it optimizes out the majority of cases when no such lru operations
are needed (optimizations best presented in a separate patch), while
being careful about the tricky case when the page is on lru_add_pvecs,
and may get on to an lru at any moment.

And since it uses a separate lock for each memcg-zone's set of lrus,
must take care that both lock and lru in 4 and 5 are different from
those in 1 and 2.

> 
> And blindly believe pc->mem_cgroup regardless of PCG_USED bit at LRU handling.

That's right.  The difficulty comes when Used is cleared while
the page is off lru, or page removed from lru while Used is clear:
once lock is dropped, we have no hold on the memcg, and must move
to root lru lest the old memcg get deleted.

The old Used + AcctLRU + pc->mem_cgroup puppetry used to achieve that
quite cleverly; but in distributing zone lru_locks over memcgs, we went
through a lot of crashes before we understood the subtlety of it; and
in most places were just fighting the way it shifted underneath us.

Now mem_cgroup_move_uncharged_to_root() makes the move explicit,
in just a few places.

> 
> Hm, per-zone-per-memcg lru locking is much easier if
>  - we igonore PCG_USED bit at lru handling

I may or may not agree with you, depending on what you mean!

>  - we never overwrite pc->mem_cgroup if the page is on LRU.

That's not the way I was thinking of it, but I think that's what we're doing.

>  - if page may be added to LRU by pagevec etc.. while we overwrite
>    pc->mem_cgroup, we always take lru_lock. This is our corner case.

Yes, the tricky case I mention above.

> 
> isn't it ? I posted a series of patch. I'm glad if you give me a
> quick review.

I haven't glanced yet, will do so after an hour or two.

Hugh

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