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







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