Re: [PATCH V4 14/15] mm, compaction: try to capture the just-created high-order freepage

[Vlastimil Babka]

On 07/25/2014 02:56 PM, Mel Gorman wrote:
> On Wed, Jul 16, 2014 at 03:48:22PM +0200, Vlastimil Babka wrote:
> > Compaction uses watermark checking to determine if it succeeded in creating
> > a high-order free page. My testing has shown that this is quite racy and it
> > can happen that watermark checking in compaction succeeds, and moments later
> > the watermark checking in page allocation fails, even though the number of
> > free pages has increased meanwhile.
> >
> > It should be more reliable if direct compaction captured the high-order free
> > page as soon as it detects it, and pass it back to allocation. This would
> > also reduce the window for somebody else to allocate the free page.
> >
> > Capture has been implemented before by 1fb3f8ca0e92 ("mm: compaction: capture
> > a suitable high-order page immediately when it is made available"), but later
> > reverted by 8fb74b9f ("mm: compaction: partially revert capture of suitable
> > high-order page") due to a bug.
> >
> > This patch differs from the previous attempt in two aspects:
> >
> > 1) The previous patch scanned free lists to capture the page. In this patch,
> >     only the cc->order aligned block that the migration scanner just finished
> >     is considered, but only if pages were actually isolated for migration in
> >     that block. Tracking cc->order aligned blocks also has benefits for the
> >     following patch that skips blocks where non-migratable pages were found.
> >
> > 2) The operations done in buffered_rmqueue() and get_page_from_freelist() are
> >     closely followed so that page capture mimics normal page allocation as much
> >     as possible. This includes operations such as prep_new_page() and
> >     page->pfmemalloc setting (that was missing in the previous attempt), zone
> >     statistics are updated etc. Due to subtleties with IRQ disabling and
> >     enabling this cannot be simply factored out from the normal allocation
> >     functions without affecting the fastpath.
> >
> > This patch has tripled compaction success rates (as recorded in vmstat) in
> > stress-highalloc mmtests benchmark, although allocation success rates increased
> > only by a few percent. Closer inspection shows that due to the racy watermark
> > checking and lack of lru_add_drain(), the allocations that resulted in direct
> > compactions were often failing, but later allocations succeeeded in the fast
> > path. So the benefit of the patch to allocation success rates may be limited,
> > but it improves the fairness in the sense that whoever spent the time
> > compacting has a higher change of benefitting from it, and also can stop
> > compacting sooner, as page availability is detected immediately. With better
> > success detection, the contribution of compaction to high-order allocation
> > success success rates is also no longer understated by the vmstats.
> >
> > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> > 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>
> > <SNIP>
> > @@ -2279,14 +2307,43 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
> >   	 */
> >   	count_vm_event(COMPACTSTALL);
> >
> > -	/* Page migration frees to the PCP lists but we want merging */
> > -	drain_pages(get_cpu());
> > -	put_cpu();
> > +	/* Did we capture a page? */
> > +	if (page) {
> > +		struct zone *zone;
> > +		unsigned long flags;
> > +		/*
> > +		 * Mimic what buffered_rmqueue() does and capture_new_page()
> > +		 * has not yet done.
> > +		 */
> > +		zone = page_zone(page);
> > +
> > +		local_irq_save(flags);
> > +		zone_statistics(preferred_zone, zone, gfp_mask);
> > +		local_irq_restore(flags);
> >
> > -	page = get_page_from_freelist(gfp_mask, nodemask,
> > -			order, zonelist, high_zoneidx,
> > -			alloc_flags & ~ALLOC_NO_WATERMARKS,
> > -			preferred_zone, classzone_idx, migratetype);
> > +		VM_BUG_ON_PAGE(bad_range(zone, page), page);
> > +		if (!prep_new_page(page, order, gfp_mask))
> > +			/* This is normally done in get_page_from_freelist() */
> > +			page->pfmemalloc = !!(alloc_flags &
> > +					ALLOC_NO_WATERMARKS);
> > +		else
> > +			page = NULL;
> > +	}
> > +
> > +	/* No capture but let's try allocating anyway */
> > +	if (!page) {
> > +		/*
> > +		 * Page migration frees to the PCP lists but we want
> > +		 * merging
> > +		 */
> > +		drain_pages(get_cpu());
> > +		put_cpu();
> > +
>
> Would the attempted capture not drained already? Not a big deal so

Well, the compaction might have returned without capture, just because 
e.g. the watermarks became OK in the meanwhile. Although that should 
mean that drain is not needed. So it might be redundant, but really not 
hurt.

I've also realized that this pcp draining (also in capture) could be 
done for a single zone and it's useless to flush all zones. Memory 
isolation could perhaps also benefit from that. But I'll add that to a 
future series.

> Acked-by: Mel Gorman <mgorman@xxxxxxx>

Thanks for all the acks. I personally believe 1-14 are ready. Patch 15 
could be deferred, the series can do without it just fine. Patch 15 does 
work in most cases and the RFC was only to make sure it's an improvement 
without subtle regressions. But it seems to affect fragmentation and 
needs more testing and perhaps tuning. I also hoped for some external 
testing, e.g. by DavidR who originally sent his version of the patch. So 
Patch 15 could be fine in -mm but not in mainline yet.

Vlastimil

>

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