Re: [PATCH 05/10] mm, compaction: remember position within pageblock in free pages scanner

[Vlastimil Babka]

On 06/11/2014 10:16 AM, Joonsoo Kim wrote:
> On Wed, Jun 11, 2014 at 11:12:13AM +0900, Minchan Kim wrote:
> > On Mon, Jun 09, 2014 at 11:26:17AM +0200, Vlastimil Babka wrote:
> > > Unlike the migration scanner, the free scanner remembers the beginning of the
> > > last scanned pageblock in cc->free_pfn. It might be therefore rescanning pages
> > > uselessly when called several times during single compaction. This might have
> > > been useful when pages were returned to the buddy allocator after a failed
> > > migration, but this is no longer the case.
> > >
> > > This patch changes the meaning of cc->free_pfn so that if it points to a
> > > middle of a pageblock, that pageblock is scanned only from cc->free_pfn to the
> > > end. isolate_freepages_block() will record the pfn of the last page it looked
> > > at, which is then used to update cc->free_pfn.
> > >
> > > In the mmtests stress-highalloc benchmark, this has resulted in lowering the
> > > ratio between pages scanned by both scanners, from 2.5 free pages per migrate
> > > page, to 2.25 free pages per migrate page, without affecting success rates.
> > >
> > > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> > Reviewed-by: Minchan Kim <minchan@xxxxxxxxxx>
> >
> > Below is a nitpick.
> >
> > > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > > Cc: Mel Gorman <mgorman@xxxxxxx>
> > > Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > > Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> > > Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> > > Cc: Christoph Lameter <cl@xxxxxxxxx>
> > > Cc: Rik van Riel <riel@xxxxxxxxxx>
> > > Cc: David Rientjes <rientjes@xxxxxxxxxx>
> > > ---
> > >   mm/compaction.c | 33 ++++++++++++++++++++++++++++-----
> > >   1 file changed, 28 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/mm/compaction.c b/mm/compaction.c
> > > index 83f72bd..58dfaaa 100644
> > > --- a/mm/compaction.c
> > > +++ b/mm/compaction.c
> > > @@ -297,7 +297,7 @@ static bool suitable_migration_target(struct page *page)
> > >    * (even though it may still end up isolating some pages).
> > >    */
> > >   static unsigned long isolate_freepages_block(struct compact_control *cc,
> > > -				unsigned long blockpfn,
> > > +				unsigned long *start_pfn,
> > >   				unsigned long end_pfn,
> > >   				struct list_head *freelist,
> > >   				bool strict)
> > > @@ -306,6 +306,7 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
> > >   	struct page *cursor, *valid_page = NULL;
> > >   	unsigned long flags;
> > >   	bool locked = false;
> > > +	unsigned long blockpfn = *start_pfn;
> > >
> > >   	cursor = pfn_to_page(blockpfn);
> > >
> > > @@ -314,6 +315,9 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
> > >   		int isolated, i;
> > >   		struct page *page = cursor;
> > >
> > > +		/* Record how far we have got within the block */
> > > +		*start_pfn = blockpfn;
> > > +
> >
> > Couldn't we move this out of the loop for just one store?

Ah, I get it now. Ignore my previous reply.

> Hello, Vlastimil.
>
> Moreover, start_pfn can't be updated to end pfn with this approach.
> Is it okay?

That's intentional, as end_pfn means the scanner would restart at the 
beginning of next pageblock. So I want to record last pfn *inside* the 
pageblock that was fully scanned. Note that there's a high change that 
fully scanning pageblock means that I haven't isolated enough and 
isolate_freepages() will advance to the previous pageblock anyway, and 
the recorded value will be overwritten. But still it's better to prevent 
this corner case.

So outside the loop, I would need to do:

*start_pfn = max(blockpfn, end_pfn - 1);

It looks a bit tricky but probably better than multiple assignments.

Thanks.

> Thanks.

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