Re: [RFC][PATCH] memcg: remove PCG_ACCT_LRU.

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Fri, 2 Dec 2011 13:08:49 +0100
Johannes Weiner <hannes@cmpxchg.org> wrote:

> On Fri, Dec 02, 2011 at 07:06:22PM +0900, KAMEZAWA Hiroyuki wrote:
> > I'm now testing this patch, removing PCG_ACCT_LRU, onto mmotm.
> > How do you think ?
> 
> > @@ -1024,18 +1026,8 @@ void mem_cgroup_lru_del_list(struct page *page, enum lru_list lru)
> >  		return;
> >  
> >  	pc = lookup_page_cgroup(page);
> > -	/*
> > -	 * root_mem_cgroup babysits uncharged LRU pages, but
> > -	 * PageCgroupUsed is cleared when the page is about to get
> > -	 * freed.  PageCgroupAcctLRU remembers whether the
> > -	 * LRU-accounting happened against pc->mem_cgroup or
> > -	 * root_mem_cgroup.
> > -	 */
> > -	if (TestClearPageCgroupAcctLRU(pc)) {
> > -		VM_BUG_ON(!pc->mem_cgroup);
> > -		memcg = pc->mem_cgroup;
> > -	} else
> > -		memcg = root_mem_cgroup;
> > +	memcg = pc->mem_cgroup ? pc->mem_cgroup : root_mem_cgroup;
> > +	VM_BUG_ON(memcg != pc->mem_cgroup_lru);
> >  	mz = page_cgroup_zoneinfo(memcg, page);
> >  	/* huge page split is done under lru_lock. so, we have no races. */
> >  	MEM_CGROUP_ZSTAT(mz, lru) -= 1 << compound_order(page);
> 
> Nobody clears pc->mem_cgroup upon uncharge, so this may end up
> mistakenly lru-unaccount a page that was never charged against the
> stale pc->mem_cgroup (e.g. a swap readahead page that has not been
> charged yet gets isolated by reclaim).
> 
> On the other hand, pages that were uncharged just before the lru_del
> MUST be lru-unaccounted against pc->mem_cgroup.
> 
> PageCgroupAcctLRU made it possible to tell those two scenarios apart.
> 
> A possible solution could be to clear pc->mem_cgroup when the page is
> finally freed so that only pages that have been charged since their
> last allocation have pc->mem_cgroup set.  But this means that the page
> freeing hotpath will have to grow a lookup_page_cgroup(), amortizing
> the winnings at least to some extent.
> 

Hmm. IMHO, we have 2 easy ways.

 - Ignore PCG_USED bit at LRU handling.
   2 problems.
   1. memory.stat may show very wrong statistics if swapin is too often.
   2. need careful use of mem_cgroup_charge_lrucare().

 - Clear pc->mem_cgroup at swapin-readahead.
   A problem.
   1. we need a new hook.

I'll try to clear pc->mem_cgroup at swapin. 

Thank you for pointing out.

Regards,
-Kame










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