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

KAMEZAWA Hiroyuki
2012-02-16T23:56:00Z

On Thu, 16 Feb 2012 15:02:27 +0400
Konstantin Khlebnikov <khlebnikov@openvz.org> 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?
>

You said this is guarded by lru_lock. So, page should be on LRU.

> >
> > 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:
>
In linex-next, pc->mem_cgroup is modified only when Page is on LRU.

When we need to touch "book", if !PageLRU() ?

> 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.
>
I personally think reducing lock by pagevec works enough well.
So, want to see perforamance on real machine with real apps.

>
> 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.

Thank you for pointing out. Hmm... it can happen ? Currently, at cgroup destroying,
force_empty() works

1. find a page from LRU
2. remove it from LRU
3. move it or reclaim it (you said "switcher")
4. if res.usage != 0 goto 1.

I think "4" will finally keep cgroup from being destroyed.

> 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.
>

Another reference counting is very very bad.

Thanks,
-Kame