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Subject: Re: [PATCH v2 4/9] dmapool: improve scalability of dma_pool_alloc
References: <1dbe6204-17fc-efd9-2381-48186cae2b94@cybernetics.com>
 <CAHp75Vdj_jcv3j2pNf4EnzasN9zCJ1f+2aWwT2f5GKG=yFAm4Q@mail.gmail.com>
From: Tony Battersby <tonyb@cybernetics.com>
Message-ID: <b304d118-8cca-594c-1321-bb65e39700e4@cybernetics.com>
Date: Fri, 3 Aug 2018 09:55:17 -0400
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To: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>, Christoph Hellwig <hch@lst.de>, Marek Szyprowski <m.szyprowski@samsung.com>, Sathya Prakash <sathya.prakash@broadcom.com>, Chaitra P B <chaitra.basappa@broadcom.com>, Suganath Prabu Subramani <suganath-prabu.subramani@broadcom.com>, iommu@lists.linux-foundation.org, linux-mm <linux-mm@kvack.org>, linux-scsi <linux-scsi@vger.kernel.org>, MPT-FusionLinux.pdl@broadcom.com

On 08/03/2018 05:02 AM, Andy Shevchenko wrote:
> On Thu, Aug 2, 2018 at 10:58 PM, Tony Battersby <tonyb@cybernetics.com> wrote:
>> dma_pool_alloc() scales poorly when allocating a large number of pages
>> because it does a linear scan of all previously-allocated pages before
>> allocating a new one.  Improve its scalability by maintaining a separate
>> list of pages that have free blocks ready to (re)allocate.  In big O
>> notation, this improves the algorithm from O(n^2) to O(n).
>>  struct dma_pool {              /* the pool */
>> +#define POOL_FULL_IDX   0
>> +#define POOL_AVAIL_IDX  1
>> +#define POOL_N_LISTS    2
>> +       struct list_head page_list[POOL_N_LISTS];
> To be consistent with naming scheme and common practice I would rather
> name the last one as
>
> POOL_MAX_IDX 2
OK.
>
>> +       INIT_LIST_HEAD(&retval->page_list[0]);
>> +       INIT_LIST_HEAD(&retval->page_list[1]);
> You introduced defines and don't use them.
>
Just a matter of style.A  In this context, it only matters that both
index 0 and 1 get initialized, not which index corresponds to which
list.A  But I suppose using the defines would improve keyword search, so
I'll change it.