linux-kernel - Re: [patch] mm: memcontrol: use page lists for uncharge batching

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Date:	Mon, 21 Jul 2014 14:47:00 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] mm: memcontrol: use page lists for uncharge batching

On Fri 18-07-14 11:07:19, Johannes Weiner wrote:
> On Thu, Jul 17, 2014 at 05:29:36PM +0200, Michal Hocko wrote:
> > On Mon 07-07-14 14:55:58, Johannes Weiner wrote:
> > > Pages are now uncharged at release time, and all sources of batched
> > > uncharges operate on lists of pages.  Directly use those lists, and
> > > get rid of the per-task batching state.
> > > 
> > > This also batches statistics accounting, in addition to the res
> > > counter charges, to reduce IRQ-disabling and re-enabling.
> > 
> > It is probably worth noticing that there is a higher chance of missing
> > threshold events now when we can accumulate huge number of uncharges
> > during munmaps. I do not think this is earth shattering and the overall
> > improvement is worth it but changelog should mention it.
> 
> Does this actually matter, though?  We might deliver events a few
> pages later than before, but as I read the threshold code, once
> invoked it catches up from the last delivered threshold to the new
> usage.  So we shouldn't *miss* any events.

You are right. I have completely miss this aspect of threshold
implementation. I should have looked into the code before claiming that
:/ and not focus only on the triggering code. Sorry about that!

> > > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> > 
> > With the follow up fix from
> > http://marc.info/?l=linux-mm&m=140552814228135&w=2
> > 
> > Acked-by: Michal Hocko <mhocko@...e.cz>
> 
> Thanks!
> 
> > > +static void uncharge_list(struct list_head *page_list)
> > > +{
> > > +	struct mem_cgroup *memcg = NULL;
> > > +	unsigned long nr_memsw = 0;
> > > +	unsigned long nr_anon = 0;
> > > +	unsigned long nr_file = 0;
> > > +	unsigned long nr_huge = 0;
> > > +	unsigned long pgpgout = 0;
> > > +	unsigned long nr_mem = 0;
> > > +	struct list_head *next;
> > > +	struct page *page;
> > > +
> > > +	next = page_list->next;
> > > +	do {
> > 
> > I would use list_for_each_entry here which would also save list_empty
> > check in mem_cgroup_uncharge_list
> 
> list_for_each_entry() wouldn't work for the singleton list where we
> pass in page->lru.  That's why it's a do-while that always does the
> first page before checking whether it looped back to the list head.
> 
> Do we need a comment for that?  I'm not convinced, there are only two
> callsites, and the one that passes the singleton page->lru is right
> below this function.

-- 
Michal Hocko
SUSE Labs
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