linux-kernel - Re: [PATCH 2/5] mm: vmscan: kick flushers when we encounter dirty pages on the LRU

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Message-ID: <20170126174739.GA30636@cmpxchg.org>
Date:   Thu, 26 Jan 2017 12:47:39 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Mel Gorman <mgorman@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH 2/5] mm: vmscan: kick flushers when we encounter dirty
 pages on the LRU

On Thu, Jan 26, 2017 at 09:57:45AM +0000, Mel Gorman wrote:
> On Mon, Jan 23, 2017 at 01:16:38PM -0500, Johannes Weiner wrote:
> > Memory pressure can put dirty pages at the end of the LRU without
> > anybody running into dirty limits. Don't start writing individual
> > pages from kswapd while the flushers might be asleep.
> > 
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> 
> I don't understand the motivation for checking the wb_reason name. Maybe
> it was easier to eyeball while reading ftraces. The comment about the
> flusher not doing its job could also be as simple as the writes took
> place and clean pages were reclaimed before dirty_expire was reached.
> Not impossible if there was a light writer combined with a heavy reader
> or a large number of anonymous faults.

The name change was only because try_to_free_pages() wasn't the only
function doing this flusher wakeup anymore. I associate that name with
direct reclaim rather than reclaim in general, so I figured this makes
more sense. No strong feelings either way, but I doubt this will break
anything in userspace.

The comment on dirty expiration is a good point. Let's add this to the
list of reasons why reclaim might run into dirty data. Fixlet below.

> Acked-by: Mel Gorman <mgorman@...e.de>

Thanks!

---

>From 44c4289ab85c0af66cb06de6d1bb72a5c67fd755 Mon Sep 17 00:00:00 2001
From: Johannes Weiner <hannes@...xchg.org>
Date: Thu, 26 Jan 2017 12:41:39 -0500
Subject: [PATCH] mm: vmscan: kick flushers when we encounter dirty pages on
 the LRU fix

Mention dirty expiration as a condition: we need dirty data that is
too recent for periodic flushing and not large enough for waking up
limit flushing. As per Mel.

Signed-off-by: Johannes Weiner <hannes@...xchg.org>
---
 mm/vmscan.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 56ea8d24041f..ccd4bf952cb3 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1799,15 +1799,14 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
 		/*
 		 * If dirty pages are scanned that are not queued for IO, it
 		 * implies that flushers are not doing their job. This can
-		 * happen when memory pressure pushes dirty pages to the end
-		 * of the LRU without the dirty limits being breached. It can
-		 * also happen when the proportion of dirty pages grows not
-		 * through writes but through memory pressure reclaiming all
-		 * the clean cache. And in some cases, the flushers simply
-		 * cannot keep up with the allocation rate. Nudge the flusher
-		 * threads in case they are asleep, but also allow kswapd to
-		 * start writing pages during reclaim.
+		 * happen when memory pressure pushes dirty pages to the end of
+		 * the LRU before the dirty limits are breached and the dirty
+		 * data has expired. It can also happen when the proportion of
+		 * dirty pages grows not through writes but through memory
+		 * pressure reclaiming all the clean cache. And in some cases,
+		 * the flushers simply cannot keep up with the allocation
+		 * rate. Nudge the flusher threads in case they are asleep, but
+		 * also allow kswapd to start writing pages during reclaim.
 		 */
 		if (stat.nr_unqueued_dirty == nr_taken) {
 			wakeup_flusher_threads(0, WB_REASON_VMSCAN);
-- 
2.11.0