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

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Message-ID: <20161124074318.a72wbn6lx5skxuxf@techsingularity.net>
Date:   Thu, 24 Nov 2016 07:43:18 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Linux-MM <linux-mm@...ck.org>, Christoph Lameter <cl@...ux.com>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] mm: page_alloc: High-order per-cpu page allocator

On Thu, Nov 24, 2016 at 08:26:39AM +0100, Vlastimil Babka wrote:
> 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.
> 

Oh ok. At the time I was thinking in terms of the starting offset for
the high-order and this seemed more natural but I'm ok with it either
way.

As an aside, the sizing of the array was still wrong but I corrected
it yesterday shortly after sending the mail. I also realised that the
free_pcppages_bulk was not interleaving properly and it should be fixed
now. More tests are in progress.

Thanks.

-- 
Mel Gorman
SUSE Labs