Re: [PATCH RFC 00/15] mm: memory book keeping and lru_lock splitting

Konstantin Khlebnikov
2012-02-16T11:02:32Z

KAMEZAWA Hiroyuki wrote:
> On Thu, 16 Feb 2012 09:43:52 +0400
> Konstantin Khlebnikov<khlebnikov@openvz.org> wrote:
>
>> KAMEZAWA Hiroyuki wrote:
>>> On Thu, 16 Feb 2012 02:57:04 +0400
>>> Konstantin Khlebnikov<khlebnikov@openvz.org> wrote:
>
>>>> * optimize page to book translations, move it upper in the call stack,
>>>> replace some struct zone arguments with struct book pointer.
>>>>
>>>
>>> a page->book transrater from patch 2/15
>>>
>>> +struct book *page_book(struct page *page)
>>> +{
>>> + struct mem_cgroup_per_zone *mz;
>>> + struct page_cgroup *pc;
>>> +
>>> + if (mem_cgroup_disabled())
>>> + return&page_zone(page)->book;
>>> +
>>> + pc = lookup_page_cgroup(page);
>>> + if (!PageCgroupUsed(pc))
>>> + return&page_zone(page)->book;
>>> + /* Ensure pc->mem_cgroup is visible after reading PCG_USED. */
>>> + smp_rmb();
>>> + mz = mem_cgroup_zoneinfo(pc->mem_cgroup,
>>> + page_to_nid(page), page_zonenum(page));
>>> + return&mz->book;
>>> +}
>>>
>>> What happens when pc->mem_cgroup is rewritten by move_account() ?
>>> Where is the guard for lockless access of this ?
>>
>> Initially this suppose to be protected with lru_lock, in final patch they are protected with rcu.
>
> Hmm, VM_BUG_ON(!PageLRU(page)) ?

Where?

>
> move_account() overwrites pc->mem_cgroup with isolating page from LRU.
> but it doesn't take lru_lock.

There three kinds of lock_page_book() users:
1) caller want to catch page in LRU, it will lock either old or new book and
recheck PageLRU() after locking, if page not it in LRU it don't touch anything.
some of these functions has stable reference to page, some of them not.
[ There actually exist small race, I knew about it, just forget to pick this chunk from old code. See below. ]
2) page is isolated by caller, it want to put it back. book link is stable. no problems.
3) page-release functions. page-counter is zero. no references -- no problems.

race for 1)

catcher switcher

# isolate
old_book = lock_page_book(page)
ClearPageLRU(page)
unlock_book(old_book)
# charge
old_book = lock_page_book(page)
# switch
page->book = new_book
# putback
lock_book(new_book)
SetPageLRU(page)
unlock_book(new_book)
if (PageLRU(page))
oops, page actually in new_book
unlock_book(old_book)

I'll protect "switch" phase with old_book lru-lock:

lock_book(old_book)
page->book = new_book
unlock_book(old_book)

The other option is recheck in "catcher" page book after PageLRU()
maybe there exists some other variants.

> BTW, what amount of perfomance benefit ?

It depends, but usually lru_lock is very-very hot.
This lock splitting can be used without cgroups and containers,
now huge zones can be easily sliced into arbitrary pieces, for example one book per 256Mb.

According to my experience, one of complicated thing there is how to postpone "book" destroying
if some its pages are isolated. For example lumpy reclaim and memory compaction isolates pages
from several books. And they wants to put them back. Currently this can be broken, if someone removes
cgroup in wrong moment. There appears funny races with three players: catcher, switcher and destroyer.
This can be fixed with some extra reference-counting or some other sleepable synchronizing.
In my rhel6-based implementation I uses extra reference-counting, and it looks ugly. So I want to invent something better.
Other option is just never release books, reuse them after rcu grace period for rcu-list iterating.

>
> Thanks,
> -Kame
>