Re: [PATCH 30/34] mm: vmscan: Do not force kswapd to scan small targets

Mel Gorman
2012-07-25T21:35:14Z

On Wed, Jul 25, 2012 at 12:59:48PM -0700, Greg KH wrote:
> On Mon, Jul 23, 2012 at 02:38:43PM +0100, Mel Gorman wrote:
> > commit ad2b8e601099a23dffffb53f91c18d874fe98854 upstream - WARNING: this is a substitute patch.
> >
> > Stable note: Not tracked in Bugzilla. This is a substitute for an
> > upstream commit addressing a completely different issue that
> > accidentally contained an important fix. The workload this patch
> > helps was memcached when IO is started in the background. memcached
> > should stay resident but without this patch it gets swapped more
> > than it should. Sometimes this manifests as a drop in throughput
> > but mostly it was observed through /proc/vmstat.
> >
> > Commit [246e87a9: memcg: fix get_scan_count() for small targets] was
> > meant to fix a problem whereby small scan targets on memcg were ignored
> > causing priority to raise too sharply. It forced scanning to take place
> > if the target was small, memcg or kswapd.
> >
> > >From the time it was introduced it cause excessive reclaim by kswapd
> > with workloads being pushed to swap that previously would have stayed
> > resident. This was accidentally fixed by commit [ad2b8e60: mm: memcg:
> > remove optimization of keeping the root_mem_cgroup LRU lists empty] but
> > that patchset is not suitable for backporting.
> >
> > The original patch came with no information on what workloads it benefits
> > but the cost of it is obvious in that it forces scanning to take place
> > on lists that would otherwise have been ignored such as small anonymous
> > inactive lists. This patch partially reverts 246e87a9 so that small lists
> > are not force scanned which means that IO-intensive workloads with small
> > amounts of anonymous memory will not be swapped.
> >
> > Signed-off-by: Mel Gorman <mgorman@suse.de>
> > ---
> > mm/vmscan.c | 3 ---
> > 1 file changed, 3 deletions(-)
>
> I don't understand this patch. The original
> ad2b8e601099a23dffffb53f91c18d874fe98854 commit touched the file
> mm/memcontrol.c and seemed to do something quite different from what you
> have done below.
>

The main problem is I'm an idiot and "missed" when copying&paste and followed
through with the mistake. The actual commit of interest was the one after it
[b95a2f2d: mm: vmscan: convert global reclaim to per-memcg LRU lists]

That patch has this hunk in it

@@ -1886,7 +1886,7 @@ static void get_scan_count(struct mem_cgroup_zone *mz, struct scan_control *sc,
* latencies, so it's better to scan a minimum amount there as
* well.
*/
- if (current_is_kswapd())
+ if (current_is_kswapd() && mz->zone->all_unreclaimable)
force_scan = true;
if (!global_reclaim(sc))
force_scan = true;

This change makes it very difficult for kswapd to force scan which was
the fix I was interested in but the series is not suitable for backport.
This has changed again since in 3.5-rc1 due to commit [90126375: mm/vmscan:
push lruvec pointer into get_scan_count()] where this check became

if (current_is_kswapd() && zone->all_unreclaimable)

Superficially that looks ok to backport, but it's not due to a subtle
difference in how zone is looked up in the new context.

Can you use this patch as a replacement? It is functionally much closer
to what happens upstream while still backporting the actual fix of
interest.

---8<---
mm: vmscan: Do not force kswapd to scan small targets

commit b95a2f2d486d0d768a92879c023a03757b9c7e58 upstream - WARNING: this is a substitute patch.

Stable note: Not tracked in Bugzilla. This is a partial backport of an
upstream commit addressing a completely different issue
that accidentally contained an important fix. The workload
this patch helps was memcached when IO is started in the
background. memcached should stay resident but without this patch
it gets swapped. Sometimes this manifests as a drop in throughput
but mostly it was observed through /proc/vmstat.

Commit [246e87a9: memcg: fix get_scan_count() for small targets] was meant
to fix a problem whereby small scan targets on memcg were ignored causing
priority to raise too sharply. It forced scanning to take place if the
target was small, memcg or kswapd.

>From the time it was introduced it caused excessive reclaim by kswapd
with workloads being pushed to swap that previously would have stayed
resident. This was accidentally fixed in commit [b95a2f2d: mm: vmscan:
convert global reclaim to per-memcg LRU lists] by making it harder for
kswapd to force scan small targets but that patchset is not suitable for
backporting. This was later changed again by commit [90126375: mm/vmscan:
push lruvec pointer into get_scan_count()] into a format that looks
like it would be a straight-forward backport but there is a subtle
difference due to the use of lruvecs.

The impact of the accidental fix is to make it harder for kswapd to force
scan small targets by taking zone->all_unreclaimable into account. This
patch is the closest equivalent available based on what is backported.

Signed-off-by: Mel Gorman <mgorman@suse.de>
---
mm/vmscan.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 42d2a5e..e0afff3 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1850,7 +1850,8 @@ static void get_scan_count(struct zone *zone, struct scan_control *sc,
unsigned long nr_force_scan[2];

/* kswapd does zone balancing and needs to scan this zone */
- if (scanning_global_lru(sc) && current_is_kswapd())
+ if (scanning_global_lru(sc) && current_is_kswapd() &&
+ zone->all_unreclaimable)
force_scan = true;
/* memcg may have small limit and need to avoid priority drop */
if (!scanning_global_lru(sc))