Re: [RFC v4 PATCH 2/5] mm/__free_one_page: skip merge for order-0 page unless compaction failed

[Aaron Lu <aaron.lu@intel.com>]

On Fri, Oct 19, 2018 at 08:00:53AM -0700, Daniel Jordan wrote:
> On Fri, Oct 19, 2018 at 09:54:35AM +0100, Mel Gorman wrote:
> > On Fri, Oct 19, 2018 at 01:57:03PM +0800, Aaron Lu wrote:
> > > > 
> > > > I don't think this is the right way of thinking about it because it's
> > > > possible to have the system split in such a way so that the migration
> > > > scanner only encounters unmovable pages before it meets the free scanner
> > > > where unmerged buddies were in the higher portion of the address space.
> > > 
> > > Yes it is possible unmerged pages are in the higher portion.
> > > 
> > > My understanding is, when the two scanners meet, all unmerged pages will
> > > be either used by the free scanner as migrate targets or sent to merge
> > > by the migration scanner.
> > > 
> > 
> > It's not guaranteed if the lower portion of the address space consisted
> > entirely of pages that cannot migrate (because they are unmovable or because
> > migration failed due to pins). It's actually a fundamental limitation
> > of compaction that it can miss migration and compaction opportunities
> > due to how the scanners are implemented. It was designed that way to
> > avoid pageblocks being migrated unnecessarily back and forth but the
> > downside is missed opportunities.
> > 
> > > > You either need to keep unmerged buddies on a separate list or search
> > > > the order-0 free list for merge candidates prior to compaction.
> > > > 
> > > > > > It's needed to form them efficiently but excessive reclaim or writing 3
> > > > > > to drop_caches can also do it. Be careful of tying lazy buddy too
> > > > > > closely to compaction.
> > > > > 
> > > > > That's the current design of this patchset, do you see any immediate
> > > > > problem of this? Is it that you are worried about high-order allocation
> > > > > success rate using this design?
> > > > 
> > > > I've pointed out what I see are the design flaws but yes, in general, I'm
> > > > worried about the high order allocation success rate using this design,
> > > > the reliance on compaction and the fact that the primary motivation is
> > > > when THP is disabled.
> > > 
> > > When THP is in use, zone lock contention is pretty much nowhere :-)
> > > 
> > > I'll see what I can get with 'address space range' lock first and will
> > > come back to 'lazy buddy' if it doesn't work out.
> 
> With the address space range idea, wouldn't the zone free_area require changes
> too?  I can't see how locking by address range could synchronize it as it
> exists now otherwise, with per order/mt list heads.
> 
> One idea is to further subdivide the free area according to how the locking
> works and find some reasonable way to handle having to search for pages of a
> given order/mt in multiple places.

I plan to create one free_are per 'address space range'. The challenge
will be how to quickly locate a free_area that has the required free
page on allocation path. Other details like how big the address space
range should be etc. will need to be explored with testing.

I think this approach is worth a try because it wouldn't cause
fragmentation.