Re: [PATCH 5/5] mm: have order > 0 compaction start near a pageblock with free pages

[Minchan Kim <minchan@kernel.org>]

On Thu, Aug 09, 2012 at 09:23:28AM +0100, Mel Gorman wrote:
> On Thu, Aug 09, 2012 at 09:12:12AM +0900, Minchan Kim wrote:
> > > <SNIP>
> > > 
> > > Second, it updates compact_cached_free_pfn in a more limited set of
> > > circumstances.
> > > 
> > > If a scanner has wrapped, it updates compact_cached_free_pfn to the end
> > > 	of the zone. When a wrapped scanner isolates a page, it updates
> > > 	compact_cached_free_pfn to point to the highest pageblock it
> > > 	can isolate pages from.
> > 
> > Okay until here.
> > 
> 
> Great.
> 
> > > 
> > > If a scanner has not wrapped when it has finished isolated pages it
> > > 	checks if compact_cached_free_pfn is pointing to the end of the
> > > 	zone. If so, the value is updated to point to the highest
> > > 	pageblock that pages were isolated from. This value will not
> > > 	be updated again until a free page scanner wraps and resets
> > > 	compact_cached_free_pfn.
> > 
> > I tried to understand your intention of this part but unfortunately failed.
> > By this part, the problem you mentioned could happen again?
> > 
> 
> Potentially yes, I did say it still races in the changelog.
> 
> >  				    			C
> >  Process A		M     S     			F
> >  		|---------------------------------------|
> >  Process B		M 	FS
> >  
> >  C is zone->compact_cached_free_pfn
> >  S is cc->start_pfree_pfn
> >  M is cc->migrate_pfn
> >  F is cc->free_pfn
> > 
> > In this diagram, Process A has just reached its migrate scanner, wrapped
> > around and updated compact_cached_free_pfn to end of the zone accordingly.
> > 
> 
> Yes. Now that it has wrapped it updates the compact_cached_free_pfn
> every loop of isolate_freepages here.
> 
>                 if (isolated) {
>                         high_pfn = max(high_pfn, pfn);
> 
>                         /*
>                          * If the free scanner has wrapped, update
>                          * compact_cached_free_pfn to point to the highest
>                          * pageblock with free pages. This reduces excessive
>                          * scanning of full pageblocks near the end of the
>                          * zone
>                          */
>                         if (cc->order > 0 && cc->wrapped)
>                                 zone->compact_cached_free_pfn = high_pfn;
>                 }
> 
> 
> 
> > Simultaneously, Process B finishes isolating in a block and peek 
> > compact_cached_free_pfn position and know it's end of the zone so
> > update compact_cached_free_pfn to highest pageblock that pages were
> > isolated from.
> > 
> 
> Yes, they race at this point. One of two things happen here and I agree
> that this is racy
> 
> 1. Process A does another iteration of its loop and sets it back
> 2. Process A does not do another iteration of the loop, the cached_pfn
>    is further along that it should. The next compacting process will
>    wrap early and reset cached_pfn again but continue to scan the zone.
> 
> Either option is relatively harmless because in both cases the zone gets
> scanned. In patch 4 it was possible that large portions of the zone were
> frequently missed.
> 
> > Process A updates compact_cached_free_pfn to the highest pageblock which
> > was set by process B because process A has wrapped. It ends up big jump
> > without any scanning in process A.
> > 
> 
> It recovers quickly and is nowhere near as severe as what patch 4
> suffers from.

Agreed.
Thanks, Mel.

-- 
Kind regards,
Minchan Kim

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