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Date: Thu, 21 Mar 2013 14:31:15 +0000
From: Mel Gorman <mgorman@...e.de>
To: Michal Hocko <mhocko@...e.cz>
Cc: Linux-MM <linux-mm@...ck.org>, Jiri Slaby <jslaby@...e.cz>,
Valdis Kletnieks <Valdis.Kletnieks@...edu>,
Rik van Riel <riel@...hat.com>,
Zlatko Calusic <zcalusic@...sync.net>,
Johannes Weiner <hannes@...xchg.org>,
dormando <dormando@...ia.net>,
Satoru Moriya <satoru.moriya@....com>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 02/10] mm: vmscan: Obey proportional scanning
requirements for kswapd
On Thu, Mar 21, 2013 at 03:01:54PM +0100, Michal Hocko wrote:
> On Sun 17-03-13 13:04:08, Mel Gorman wrote:
> > Simplistically, the anon and file LRU lists are scanned proportionally
> > depending on the value of vm.swappiness although there are other factors
> > taken into account by get_scan_count(). The patch "mm: vmscan: Limit
> > the number of pages kswapd reclaims" limits the number of pages kswapd
> > reclaims but it breaks this proportional scanning and may evenly shrink
> > anon/file LRUs regardless of vm.swappiness.
> >
> > This patch preserves the proportional scanning and reclaim. It does mean
> > that kswapd will reclaim more than requested but the number of pages will
> > be related to the high watermark.
> >
> > Signed-off-by: Mel Gorman <mgorman@...e.de>
> > ---
> > mm/vmscan.c | 52 +++++++++++++++++++++++++++++++++++++++++-----------
> > 1 file changed, 41 insertions(+), 11 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 4835a7a..182ff15 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1815,6 +1815,45 @@ out:
> > }
> > }
> >
> > +static void recalculate_scan_count(unsigned long nr_reclaimed,
> > + unsigned long nr_to_reclaim,
> > + unsigned long nr[NR_LRU_LISTS])
> > +{
> > + enum lru_list l;
> > +
> > + /*
> > + * For direct reclaim, reclaim the number of pages requested. Less
> > + * care is taken to ensure that scanning for each LRU is properly
> > + * proportional. This is unfortunate and is improper aging but
> > + * minimises the amount of time a process is stalled.
> > + */
> > + if (!current_is_kswapd()) {
> > + if (nr_reclaimed >= nr_to_reclaim) {
> > + for_each_evictable_lru(l)
> > + nr[l] = 0;
> > + }
> > + return;
>
> Heh, this is nicely cryptically said what could be done in shrink_lruvec
> as
> if (!current_is_kswapd()) {
> if (nr_reclaimed >= nr_to_reclaim)
> break;
> }
>
Pretty much. At one point during development, this function was more
complex and it evolved into this without me rechecking if splitting it
out still made sense.
> Besides that this is not memcg aware which I think it would break
> targeted reclaim which is kind of direct reclaim but it still would be
> good to stay proportional because it starts with DEF_PRIORITY.
>
This does break memcg because it's a special sort of direct reclaim.
> I would suggest moving this back to shrink_lruvec and update the test as
> follows:
I also noticed that we check whether the scan counts need to be
normalised more than once and this reshuffling checks nr_reclaimed
twice. How about this?
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 182ff15..320a2f4 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1815,45 +1815,6 @@ out:
}
}
-static void recalculate_scan_count(unsigned long nr_reclaimed,
- unsigned long nr_to_reclaim,
- unsigned long nr[NR_LRU_LISTS])
-{
- enum lru_list l;
-
- /*
- * For direct reclaim, reclaim the number of pages requested. Less
- * care is taken to ensure that scanning for each LRU is properly
- * proportional. This is unfortunate and is improper aging but
- * minimises the amount of time a process is stalled.
- */
- if (!current_is_kswapd()) {
- if (nr_reclaimed >= nr_to_reclaim) {
- for_each_evictable_lru(l)
- nr[l] = 0;
- }
- return;
- }
-
- /*
- * For kswapd, reclaim at least the number of pages requested.
- * However, ensure that LRUs shrink by the proportion requested
- * by get_scan_count() so vm.swappiness is obeyed.
- */
- if (nr_reclaimed >= nr_to_reclaim) {
- unsigned long min = ULONG_MAX;
-
- /* Find the LRU with the fewest pages to reclaim */
- for_each_evictable_lru(l)
- if (nr[l] < min)
- min = nr[l];
-
- /* Normalise the scan counts so kswapd scans proportionally */
- for_each_evictable_lru(l)
- nr[l] -= min;
- }
-}
-
/*
* This is a basic per-zone page freer. Used by both kswapd and direct reclaim.
*/
@@ -1864,7 +1825,9 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
enum lru_list lru;
unsigned long nr_reclaimed = 0;
unsigned long nr_to_reclaim = sc->nr_to_reclaim;
+ unsigned long min;
struct blk_plug plug;
+ bool scan_adjusted = false;
get_scan_count(lruvec, sc, nr);
@@ -1881,7 +1844,33 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
}
}
- recalculate_scan_count(nr_reclaimed, nr_to_reclaim, nr);
+ if (nr_reclaimed < nr_to_reclaim || scan_adjusted)
+ continue;
+
+ /*
+ * For global direct reclaim, reclaim only the number of pages
+ * requested. Less care is taken to scan proportionally as it
+ * is more important to minimise direct reclaim stall latency
+ * than it is to properly age the LRU lists.
+ */
+ if (global_reclaim(sc) && !current_is_kswapd())
+ break;
+
+ /*
+ * For kswapd and memcg, reclaim at least the number of pages
+ * requested. However, ensure that LRUs shrink by the
+ * proportion requested by get_scan_count() so vm.swappiness
+ * is obeyed. Find the smallest LRU list and normalise the
+ * scan counts so the fewest number of pages are reclaimed
+ * while still maintaining proportionality.
+ */
+ min = ULONG_MAX;
+ for_each_evictable_lru(lru)
+ if (nr[lru] < min)
+ min = nr[lru];
+ for_each_evictable_lru(lru)
+ nr[lru] -= min;
+ scan_adjusted = true;
}
blk_finish_plug(&plug);
sc->nr_reclaimed += nr_reclaimed;
--
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