Re: [Qemu-devel] [PATCH 8/9] mm: compaction: Cache if a pageblock was scanned and no pages were isolated

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 24 Sep 2012 10:39:38 +0100
Mel Gorman <mgorman@suse.de> wrote:

> On Fri, Sep 21, 2012 at 02:36:56PM -0700, Andrew Morton wrote:
> 
> > Also, what has to be done to avoid the polling altogether?  eg/ie, zap
> > a pageblock's PB_migrate_skip synchronously, when something was done to
> > that pageblock which justifies repolling it?
> > 
> 
> The "something" event you are looking for is pages being freed or
> allocated in the page allocator. A movable page being allocated in block
> or a page being freed should clear the PB_migrate_skip bit if it's set.
> Unfortunately this would impact the fast path of the alloc and free paths
> of the page allocator. I felt that that was too high a price to pay.

We already do a similar thing in the page allocator: clearing of
->all_unreclaimable and ->pages_scanned.  But that isn't on the "fast
path" really - it happens once per pcp unload.  Can we do something
like that?  Drop some hint into the zone without having to visit each
page?

> > >
> > > ...
> > >
> > > +static void reset_isolation_suitable(struct zone *zone)
> > > +{
> > > +	unsigned long start_pfn = zone->zone_start_pfn;
> > > +	unsigned long end_pfn = zone->zone_start_pfn + zone->spanned_pages;
> > > +	unsigned long pfn;
> > > +
> > > +	/*
> > > +	 * Do not reset more than once every five seconds. If allocations are
> > > +	 * failing sufficiently quickly to allow this to happen then continually
> > > +	 * scanning for compaction is not going to help. The choice of five
> > > +	 * seconds is arbitrary but will mitigate excessive scanning.
> > > +	 */
> > > +	if (time_before(jiffies, zone->compact_blockskip_expire))
> > > +		return;
> > > +	zone->compact_blockskip_expire = jiffies + (HZ * 5);
> > > +
> > > +	/* Walk the zone and mark every pageblock as suitable for isolation */
> > > +	for (pfn = start_pfn; pfn < end_pfn; pfn += pageblock_nr_pages) {
> > > +		struct page *page;
> > > +		if (!pfn_valid(pfn))
> > > +			continue;
> > > +
> > > +		page = pfn_to_page(pfn);
> > > +		if (zone != page_zone(page))
> > > +			continue;
> > > +
> > > +		clear_pageblock_skip(page);
> > > +	}
> > 
> > What's the worst-case loop count here?
> > 
> 
> zone->spanned_pages >> pageblock_order

What's the worst-case value of (zone->spanned_pages >> pageblock_order) :)