linux-kernel - Re: [PATCH 2/3] thp: change deferred_split_count() to return number of THP in queue

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Date:	Fri, 22 Jan 2016 17:20:50 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Christoph Lameter <cl@...two.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Steve Capper <steve.capper@...aro.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Jerome Marchand <jmarchan@...hat.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 2/3] thp: change deferred_split_count() to return number
 of THP in queue

On Fri, Jan 22, 2016 at 03:31:27PM +0100, Andrea Arcangeli wrote:
> On Thu, Jan 21, 2016 at 03:09:22PM +0300, Kirill A. Shutemov wrote:
> > @@ -3511,7 +3506,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
> >  	list_splice_tail(&list, &pgdata->split_queue);
> >  	spin_unlock_irqrestore(&pgdata->split_queue_lock, flags);
> >  
> > -	return split * HPAGE_PMD_NR / 2;
> > +	return split;
> >  }
> 
> Looking further at how the caller processes this "split" retval, if
> the list has been fully shrunk by the page freeing, between the
> split_count and split_scan, the caller seems to ignore a 0 value
> returned above and it'll keep calling even if sc->nr_to_scan isn't
> decreasing. The caller won't even check sc->nr_to_scan to notice that
> it isn't decreasing anymore, it's write-only as far as the caller is
> concerned.
> 
> It's also weird we can't return the number of freed pages and break
> the loop with just one invocation of the split_scan, but that's a
> slight inefficiency in the caller interface. The caller also seems to
> forget to set total_scan to 0 if SHRINK_STOP was returned but perhaps
> that's on purpose, however for our purpose it'd be better off if it
> did.
> 
> The split_queue.next is going to be hot in the CPU cache anyway, so
> unless we change the caller, it should be worth it to add a list_empty
> check and return SHRINK_STOP if it was empty. Doing it at the start or
> end doesn't make much difference, at the end lockless it'll deal with
> the split failures too if any.
> 
> 	return split ? : list_empty(&pgdat->split_queue) ? SPLIT_STOP : 0;

Ughh. Shrinker interface is confusing.

>From ed85b527b2ea5c0b8ed93d9d212f9f0d25cae3ab Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Date: Fri, 22 Jan 2016 18:10:59 +0300
Subject: [PATCH] thp: deferred_split_scan(): stop shrinker if the queue is
 empty

If pages on queue were freed under us, deferred_split_scan() would
return zero. It makes caller keep calling deferred_split_scan() without
any result.

Let's return SHRINK_STOP in this situation.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Suggested-by: Andrea Arcangeli <aarcange@...hat.com>
---
 mm/huge_memory.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 298dbc001b07..2ea5e26ce069 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3508,6 +3508,12 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
 	list_splice_tail(&list, &pgdata->split_queue);
 	spin_unlock_irqrestore(&pgdata->split_queue_lock, flags);
 
+	/*
+	 * Stop shrinker, if we didn't split any page, but the queue is empty.
+	 * This can happen if pages were freed under us.
+	 */
+	if (!split && list_empty(&pgdata->split_queue))
+		return SHRINK_STOP;
 	return split;
 }
 
-- 
 Kirill A. Shutemov