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

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Message-ID: <a1f8d311-1f69-b672-1dad-9867c212147f@suse.cz>
Date:   Thu, 24 Nov 2016 08:26:39 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>
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 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.
>