Re: [PATCH v2 2/9] mm/compaction: introduce compaction depleted state on zone

Joonsoo Kim
2015-09-30T08:10:53Z

Hello, Vlastimil.

First of all, thanks for review!

On Fri, Sep 25, 2015 at 10:04:49AM +0200, Vlastimil Babka wrote:
> On 08/24/2015 04:19 AM, Joonsoo Kim wrote:
> > Further compaction attempt is deferred when some of compaction attempts
> > already fails. But, after some number of trial are skipped, compaction
> > restarts work to check whether compaction is now possible or not. It
> > scans whole range of zone to determine this possibility and if compaction
> > possibility doesn't recover, this whole range scan is quite big overhead.
> > As a first step to reduce this overhead, this patch implement compaction
> > depleted state on zone.
> >
> > The way to determine depletion of compaction possility is checking number
> > of success on previous compaction attempt. If number of successful
> > compaction is below than specified threshold, we guess that compaction
> > will not successful next time so mark the zone as compaction depleted.
> > In this patch, threshold is choosed by 1 to imitate current compaction
> > deferring algorithm. In the following patch, compaction algorithm will be
> > changed and this threshold is also adjusted to that change.
> >
> > In this patch, only state definition is implemented. There is no action
> > for this new state so no functional change. But, following patch will
> > add some handling for this new state.
> >
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > ---
> > include/linux/mmzone.h | 3 +++
> > mm/compaction.c | 44 +++++++++++++++++++++++++++++++++++++++++---
> > 2 files changed, 44 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index 754c259..700e9b5 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -517,6 +517,8 @@ struct zone {
> > unsigned int compact_considered;
> > unsigned int compact_defer_shift;
> > int compact_order_failed;
> > + unsigned long compact_success;
> > + unsigned long compact_depletion_depth;
> > #endif
> >
> > #if defined CONFIG_COMPACTION || defined CONFIG_CMA
> > @@ -543,6 +545,7 @@ enum zone_flags {
> > * many pages under writeback
> > */
> > ZONE_FAIR_DEPLETED, /* fair zone policy batch depleted */
> > + ZONE_COMPACTION_DEPLETED, /* compaction possiblity depleted */
> > };
> >
> > static inline unsigned long zone_end_pfn(const struct zone *zone)
> > diff --git a/mm/compaction.c b/mm/compaction.c
> > index c2d3d6a..de96e9d 100644
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -129,6 +129,23 @@ static struct page *pageblock_pfn_to_page(unsigned long start_pfn,
> >
> > /* Do not skip compaction more than 64 times */
> > #define COMPACT_MAX_DEFER_SHIFT 6
> > +#define COMPACT_MIN_DEPLETE_THRESHOLD 1UL
> > +
> > +static bool compaction_depleted(struct zone *zone)
> > +{
> > + unsigned long threshold;
> > + unsigned long success = zone->compact_success;
> > +
> > + /*
> > + * Now, to imitate current compaction deferring approach,
> > + * choose threshold to 1. It will be changed in the future.
> > + */
> > + threshold = COMPACT_MIN_DEPLETE_THRESHOLD;
> > + if (success >= threshold)
> > + return false;
> > +
> > + return true;
> > +}
> >
> > /*
> > * Compaction is deferred when compaction fails to result in a page
> > @@ -223,6 +240,16 @@ static void __reset_isolation_suitable(struct zone *zone)
> > zone->compact_cached_free_pfn = end_pfn;
> > zone->compact_blockskip_flush = false;
> >
> > + if (compaction_depleted(zone)) {
> > + if (test_bit(ZONE_COMPACTION_DEPLETED, &zone->flags))
> > + zone->compact_depletion_depth++;
> > + else {
> > + set_bit(ZONE_COMPACTION_DEPLETED, &zone->flags);
> > + zone->compact_depletion_depth = 0;
> > + }
> > + }
> > + zone->compact_success = 0;
>
> It's possible that the following comment is made moot by further patches, but:
>
> I assume doing this in __reset_isolation_suitable() is to react on the
> compaction_restarting() state. But __reset_isolation_suitable() is called also
> from manually invoked compaction, and from kswapd. What if compaction has
> succeeded S times, but threshold is T, S < T and T is larger than 1 (by a later
> patch). Then kswapd or manual compaction will reset S to zero, without giving it
> chance to reach T, even when compaction would succeed?

Okay. I will fix it.

Thanks.