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@xxxxxxxxxxx>
>
> 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@xxxxxxx>
Thanks!
---
>From 44c4289ab85c0af66cb06de6d1bb72a5c67fd755 Mon Sep 17 00:00:00 2001
From: Johannes Weiner <hannes@xxxxxxxxxxx>
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@xxxxxxxxxxx>
---
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
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