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

Konstantin Khlebnikov
2012-02-16T05:43:57Z

KAMEZAWA Hiroyuki wrote:
> On Thu, 16 Feb 2012 02:57:04 +0400
> Konstantin Khlebnikov<khlebnikov@openvz.org> wrote:
>
>> There should be no logic changes in this patchset, this is only tossing bits around.
>> [ This patchset is on top some memcg cleanup/rework patches,
>> which I sent to linux-mm@ today/yesterday ]
>>
>> Most of things in this patchset are self-descriptive, so here brief plan:
>>
>
> AFAIK, Hugh Dickins said he has per-zone-per-lru-lock and is testing it.
> So, please CC him and Johannes, at least.
>

Ok

>
>> * Transmute struct lruvec into struct book. Like real book this struct will
>> store set of pages for one zone. It will be working unit for reclaimer code.
>> [ If memcg is disabled in config there will only one book embedded into struct zone ]
>>
>
> Why you need to add new structure rahter than enhancing lruvec ?
> "book" means a binder of pages ?
>

I responded to this in the reply to Hugh Dickins.

>
>> * move page-lru counters to struct book
>> [ this adds extra overhead in add_page_to_lru_list()/del_page_from_lru_list() for
>> non-memcg case, but I believe it will be invisible, only one non-atomic add/sub
>> in the same cacheline with lru list ]
>>
>
> This seems straightforward.
>
>> * unify inactive_list_is_low_global() and cleanup reclaimer code
>> * replace struct mem_cgroup_zone with single pointer to struct book
>
> Hm, ok.
>
>> * 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.
After final patch all page_book() calls are collected in [__re]lock_page_book[_irq]() functions.
They pick some book reference, lock its lru and recheck page -> book reference in loop till success.

Currently I found there only one potential problem: free_mem_cgroup_per_zone_info() in "mm: memory bookkeeping core"
maybe should call spin_unlock_wait(&zone->lru_lock), because some guy can pick page_book(pfn_to_page(pfn))
and try to isolate this page. But I not sure, how this is possible. In final patch it is totally fixed with rcu.

>
> Thanks,
> -Kame
>