Re: isolate_freepages_block and excessive CPU usage by OSD process

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Fri, Nov 28, 2014 at 10:26:15AM +0100, Vlastimil Babka wrote:
> On 28.11.2014 9:03, Joonsoo Kim wrote:
> >On Tue, Nov 25, 2014 at 01:48:42AM +0400, Andrey Korolyov wrote:
> >>On Sun, Nov 23, 2014 at 12:33 PM, Christian Marie <christian@ponies.io> wrote:
> >>>Here's an update:
> >>>
> >>>Tried running 3.18.0-rc5 over the weekend to no avail. A load spike through
> >>>Ceph brings no perceived improvement over the chassis running 3.10 kernels.
> >>>
> >>>Here is a graph of *system* cpu time (not user), note that 3.18 was a005.block:
> >>>
> >>>http://ponies.io/raw/cluster.png
> >>>
> >>>It is perhaps faring a little better that those chassis running the 3.10 in
> >>>that it did not have min_free_kbytes raised to 2GB as the others did, instead
> >>>it was sitting around 90MB.
> >>>
> >>>The perf recording did look a little different. Not sure if this was just the
> >>>luck of the draw in how the fractal rendering works:
> >>>
> >>>http://ponies.io/raw/perf-3.10.png
> >>>
> >>>Any pointers on how we can track this down? There's at least three of us
> >>>following at this now so we should have plenty of area to test.
> >>
> >>Checked against 3.16 (3.17 hanged for an unrelated problem), the issue
> >>is presented for single- and two-headed systems as well. Ceph-users
> >>reported presence of the problem for 3.17, so probably we are facing
> >>generic compaction issue.
> >>
> >Hello,
> >
> >I didn't follow-up this discussion, but, at glance, this excessive CPU
> >usage by compaction is related to following fixes.
> >
> >Could you test following two patches?
> >
> >If these fixes your problem, I will resumit patches with proper commit
> >description.
> >
> >Thanks.
> >
> >-------->8-------------
> > From 079f3f119f1e3cbe9d981e7d0cada94e0c532162 Mon Sep 17 00:00:00 2001
> >From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> >Date: Fri, 28 Nov 2014 16:36:00 +0900
> >Subject: [PATCH 1/2] mm/compaction: fix wrong order check in
> >  compact_finished()
> >
> >What we want to check here is whether there is highorder freepage
> >in buddy list of other migratetype in order to steal it without
> >fragmentation. But, current code just checks cc->order which means
> >allocation request order. So, this is wrong.
> >
> >Without this fix, non-movable synchronous compaction below pageblock order
> >would not stopped until compaction complete, because migratetype of most
> >pageblocks are movable and cc->order is always below than pageblock order
> >in this case.
> >
> >Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> >---
> >  mm/compaction.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> >diff --git a/mm/compaction.c b/mm/compaction.c
> >index b544d61..052194f 100644
> >--- a/mm/compaction.c
> >+++ b/mm/compaction.c
> >@@ -1082,7 +1082,7 @@ static int compact_finished(struct zone *zone, struct compact_control *cc,
> >  			return COMPACT_PARTIAL;
> >  		/* Job done if allocation would set block type */
> >-		if (cc->order >= pageblock_order && area->nr_free)
> >+		if (order >= pageblock_order && area->nr_free)
> >  			return COMPACT_PARTIAL;
> 
> Dang, good catch!
> But I wonder, are MIGRATE_RESERVE pages counted towards area->nr_free?
> Seems to me that they are, so this check can have false positives?
> Hm probably for unmovable allocation, MIGRATE_CMA pages is the same case?
> 

Hello,

Althoth MIGRATE_RESERVE are counted for area->nr_free, at this
moment, there is no freepage on MIGRATE_RESERVE. It would be used
already before triggering compaction.

In case of MIGRATE_CMA, false positives are possible. But, it also
broken on __zone_watermark_ok(). Without area->nr_free_cma, we can't
fix inaccurate check. Please see following link.

https://lkml.org/lkml/2014/6/2/1

Thanks.

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