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

[Konstantin Khlebnikov <khlebnikov@openvz.org>]

Konstantin Khlebnikov wrote:
> 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)

I found better solution for switcher sequence:

#isolate
old_book = lock_page_book(page)
ClearPageLRU(page)
unlock_book(old_book)				

#charge

#switch
page->book = new_book
spin_unlock_wait(&old_book->lru_lock)

#putback
lock_book(new_book)
SetPageLRU(page)
unlock_book(new_book)

this spin_unlock_wait() effectively stabilize PageLRU() sign
for potential old_book lock holder.

>
> 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.

Looks like it is not broken, charged page will keep memcg books alive.
To make it completely safe rcu-free callback must wait on spin_unlock_wait(book->lru_lock).

>
>>
>> Thanks,
>> -Kame
>>
>
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