Re: [RFC PATCH] mm: page_alloc: High-order per-cpu page allocator

[Vlastimil Babka]

On 11/23/2016 05:33 PM, Mel Gorman wrote:
> > > +
> > > +static inline unsigned int pindex_to_order(unsigned int pindex)
> > > +{
> > > +	return pindex < MIGRATE_PCPTYPES ? 0 : pindex - MIGRATE_PCPTYPES + 1;
> > > +}
> > > +
> > > +static inline unsigned int order_to_pindex(int migratetype, unsigned int order)
> > > +{
> > > +	return (order == 0) ? migratetype : MIGRATE_PCPTYPES - 1 + order;
> >
> > Here I think that "MIGRATE_PCPTYPES + order - 1" would be easier to
> > understand as the array is for all migratetypes, but the order is shifted?
>
> As in migratetypes * costly_order ? That would be excessively large.

No, I just meant that instead of "MIGRATE_PCPTYPES - 1 + order" it could 
be "MIGRATE_PCPTYPES + order - 1" as we are subtracting from order, not 
migratetypes. Just made me confused a bit when seeing the code for the 
first time.

> > > @@ -1083,10 +1083,12 @@ static bool bulkfree_pcp_prepare(struct page *page)
> > >   * pinned" detection logic.
> > >   */
> > >  static void free_pcppages_bulk(struct zone *zone, int count,
> > > -					struct per_cpu_pages *pcp)
> > > +					struct per_cpu_pages *pcp,
> > > +					int migratetype)
> > >  {
> > > -	int migratetype = 0;
> > > -	int batch_free = 0;
> > > +	unsigned int pindex = 0;
> >
> > Should pindex be initialized to migratetype to match the list below?
>
> Functionally it doesn't matter. It affects which list is tried first if
> the preferred list is empty. Arguably it would make more sense to init
> it to NR_PCP_LISTS - 1 so all order-0 lists are always drained before the
> high-order pages but there is not much justification for that.

OK

> I'll take your suggestion until there is data supporting that high-order
> caches should be preserved.
>
> Thanks.