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SubjectRe: [PATCH 6/10] mm/memcg: take care over pc->mem_cgroup
Hugh Dickins wrote:
> On Tue, 21 Feb 2012, KAMEZAWA Hiroyuki wrote:
>> On Mon, 20 Feb 2012 15:34:28 -0800 (PST)
>> Hugh Dickins<hughd@google.com> wrote:
>> return NULL;
>>>
>>> + lruvec = page_lock_lruvec(page);
>>> lock_page_cgroup(pc);
>>>
>>
>> Do we need to take lrulock+irq disable per page in this very very hot path ?
>
> I'm sure we don't want to: I hope you were pleased to find it goes away
> (from most cases) a couple of patches later.
>
> I had lruvec lock nested inside page_cgroup lock in the rollup I sent in
> December, whereas you went for page_cgroup lock nested inside lruvec lock
> in your lrucare patch.
>
> I couldn't find an imperative reason why they should be one way round or
> the other, so I tried hard to stick with your ordering, and it did work
> (in this 6/10). But then I couldn't work out how to get rid of the
> overheads added in doing it this way round, so swapped them back.
>
>>
>> Hmm.... How about adding NR_ISOLATED counter into lruvec ?
>>
>> Then, we can delay freeing lruvec until all conunters goes down to zero.
>> as...
>>
>> bool we_can_free_lruvec = true;
>>
>> lock_lruvec(lruvec->lock);
>> for_each_lru_lruvec(lru)
>> if (!list_empty(&lruvec->lru[lru]))
>> we_can_free_lruvec = false;
>> if (lruvec->nr_isolated)
>> we_can_free_lruvec = false;
>> unlock_lruvec(lruvec)
>> if (we_can_free_lruvec)
>> kfree(lruvec);
>>
>> If compaction, lumpy reclaim free a page taken from LRU,
>> it knows what it does and can decrement lruvec->nr_isolated properly
>> (it seems zone's NR_ISOLATED is decremented at putback.)
>
> At the moment I'm thinking that what we end up with by 9/10 is
> better than adding such a refcount. But I'm not entirely happy with
> mem_cgroup_reset_uncharged_to_root (it adds a further page_cgroup
> lookup just after I got rid of some others), and need yet to think
> about the race which Konstantin posits, so all options remain open.

This lruvec->nr_isolated seem reasonable, and its managegin not very costly.
In move_account() we anyway need to touch old_lruvec->lru_lock after recharge,
to stabilize PageLRU() before adding page to new_lruvec. (because that race)
In migration/compaction this handled automatically, because they always call putback_lru_page() at the end.
Main problem is shrink_page_list() for lumpy-reclaim, but seems like it never used if memory
compaction is enabled, so it can be slow and ineffective with tons of lru_list relocks.

>
> Hugh



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