Re: [PATCH 11/25] mm, compaction: Use free lists to quickly locate a migration source

[Mel Gorman]

On Wed, Jan 16, 2019 at 04:00:22PM +0100, Vlastimil Babka wrote:
> On 1/16/19 3:33 PM, Mel Gorman wrote:
> >>> +				break;
> >>> +			}
> >>> +
> >>> +			/*
> >>> +			 * If low PFNs are being found and discarded then
> >>> +			 * limit the scan as fast searching is finding
> >>> +			 * poor candidates.
> >>> +			 */
> >>
> >> I wonder about the "low PFNs are being found and discarded" part. Maybe
> >> I'm missing it, but I don't see them being discarded above, this seems
> >> to be the first check against cc->migrate_pfn. With the min() part in
> >> update_fast_start_pfn(), does it mean we can actually go back and rescan
> >> (or skip thanks to skip bits, anyway) again pageblocks that we already
> >> scanned?
> >>
> > 
> > Extremely poor phrasing. My mind was thinking in terms of discarding
> > unsuitable candidates as they were below the migration scanner and it
> > did not translate properly.
> > 
> > Based on your feedback, how does the following untested diff look?
> 
> IMHO better. Meanwhile I noticed that the next patch removes the
> set_pageblock_skip() so maybe it's needless churn to introduce the
> fast_find_block, but I'll check more closely.
> 

Indeed but the patches should standalone and preserve bisection as best
as possible so while it's weird looking, I'll add the logic and just take
it back out again in the next patch. Merging the patches together would
be lead to a tricky review!

> The new comment about pfns below cc->migrate_pfn is better but I still
> wonder if it would be better to really skip over those candidates (they
> are still called unsuitable) and not go backwards with cc->migrate_pfn.
> But if you think the pageblock skip bits and halving of limit minimizes
> pointless rescan sufficiently, then fine.

I'll check if it works out better to ensure they are really skipped.

-- 
Mel Gorman
SUSE Labs