Re: [PATCH V5] Allow compaction of unevictable pages

From: Vlastimil Babka
Date: Mon Mar 16 2015 - 06:14:11 EST

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On 03/13/2015 09:19 PM, Michal Hocko wrote:
On Fri 13-03-15 15:09:15, Eric B Munson wrote:
On Fri, 13 Mar 2015, Rik van Riel wrote:

On 03/13/2015 01:26 PM, Eric B Munson wrote:

--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1046,6 +1046,8 @@ typedef enum {
ISOLATE_SUCCESS, /* Pages isolated, migrate */
} isolate_migrate_t;

+int sysctl_compact_unevictable;

A comment here would be useful I think, as well as explicit default value. Maybe also __read_mostly although I don't know how much that matters.

I also wonder if it might be confusing that "compact_memory" is a write-only trigger that doesn't even show under "sysctl -a", while "compact_unevictable" is a read/write setting. But I don't have a better suggestion right now.

+
/*
* Isolate all pages that can be migrated from the first suitable block,
* starting at the block pointed to by the migrate scanner pfn within

I suspect that the use cases where users absolutely do not want
unevictable pages migrated are special cases, and it may make
sense to enable sysctl_compact_unevictable by default.

Given that sysctl_compact_unevictable=0 is the way the kernel behaves
now and the push back against always enabling compaction on unevictable
pages, I left the default to be the behavior as it is today.

The question is _why_ we have this behavior now. Is it intentional?

It's there since 748446bb6 ("mm: compaction: memory compaction core"). Commit c53919adc0 ("mm: vmscan: remove lumpy reclaim") changes the comment in __isolate_lru_page() handling of unevictable pages to mention compaction explicitly. It could have been accidental in 748446bb6 though, maybe it just reused __isolate_lru_page() for compaction - it seems that the skipping of unevictable was initially meant to optimize lumpy reclaim.

e46a28790e59 (CMA: migrate mlocked pages) is a precedence in that

Well, CMA and realtime kernels are probably mutually exclusive enough.

direction. Vlastimil has then changed that by edc2ca612496 (mm,
compaction: move pageblock checks up from isolate_migratepages_range()).
There is no mention about mlock pages so I guess it was more an
unintentional side effect of the patch. At least that is my current
understanding. I might be wrong here.

Although that commit did change unintentionally more details that I would have liked (unfortunately), I think you are wrong on this one. ISOLATE_UNEVICTABLE is still passed from isolate_migratepages_range() which is used by CMA, while the compaction variant isolate_migratepages() does not pass it. So it's kept CMA-specific as before.

The thing about RT is that it is not usable with the upstream kernel
without the RT patchset AFAIU. So the default should be reflect what is
better for the standard kernel. RT loads have to tune the system anyway
so it is not so surprising they would disable this option as well. We
should help those guys and do not require them to touch the code but the
knob is reasonable IMHO.

Especially when your changelog suggests that having this enabled by
default is beneficial for the standard kernel.

I agree, but if there's a danger of becoming too of a bikeshed topic, I'm fine with keeping the default same as current behavior and changing it later. Or maybe we should ask some -rt mailing list instead of just Peter and Thomas?
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