Re: [PATCH 2/5] mm/page_alloc: Track range of active PCP lists during bulk free

[Mel Gorman <mgorman@techsingularity.net>]

On Wed, Feb 16, 2022 at 01:02:01PM +0100, Vlastimil Babka wrote:
> On 2/15/22 15:51, Mel Gorman wrote:
> > free_pcppages_bulk() frees pages in a round-robin fashion. Originally,
> > this was dealing only with migratetypes but storing high-order pages
> > means that there can be many more empty lists that are uselessly
> > checked. Track the minimum and maximum active pindex to reduce the
> > search space.
> > 
> > Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> > ---
> >  mm/page_alloc.c | 13 +++++++++++--
> >  1 file changed, 11 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 08de32cfd9bb..c5110fdeb115 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1450,6 +1450,8 @@ static void free_pcppages_bulk(struct zone *zone, int count,
> >  					struct per_cpu_pages *pcp)
> >  {
> >  	int pindex = 0;
> > +	int min_pindex = 0;
> > +	int max_pindex = NR_PCP_LISTS - 1;
> >  	int batch_free = 0;
> >  	int nr_freed = 0;
> >  	unsigned int order;
> > @@ -1478,10 +1480,17 @@ static void free_pcppages_bulk(struct zone *zone, int count,
> >  			if (++pindex == NR_PCP_LISTS)
> 
> Hmm, so in the very first iteration at this point pindex is already 1. This
> looks odd even before the patch, as order 0 MIGRATE_UNMOVABLE list is only
> processed after all the higher orders?
> 

Yes and this was the behaviour before and after. I don't recall why. It
might have been to preserve UNMOVABLE pages but after the series is
finished, the reasoning is weak. I'll add a specific check.

> >  				pindex = 0;
> 
> Also shouldn't this wrap-around check also use min_index/max_index instead
> of NR_PCP_LISTS and 0?
> 

Yes, it should and it's a rebasing error from an earlier prototype that
I missed. I'll fix it.

> >  			list = &pcp->lists[pindex];
> > -		} while (list_empty(list));
> > +			if (!list_empty(list))
> > +				break;
> > +
> > +			if (pindex == max_pindex)
> > +				max_pindex--;
> > +			if (pindex == min_pindex)
> 
> So with pindex 1 and min_pindex == 0 this will not trigger until
> (eventually) the first pindex wrap around, which seems suboptimal. But I can
> see the later patches change things substantially anyway so it may be moot...
> 

It could potentially be more optimal but at the cost of complexity which
I wanted to avoid in this path as much as possible. Initialising
min_pindex == pindex could result in an infinite loop if the lower lists
need to be cleared.

-- 
Mel Gorman
SUSE Labs