linux-kernel - Re: [PATCH 04/10] dmapool: improve accuracy of debug statistics

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Message-ID: <417bd608-0eeb-b3a0-31e3-8e241ab75e59@arm.com>
Date:   Tue, 31 May 2022 22:55:57 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Tony Battersby <tonyb@...ernetics.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     Tony Lindgren <tony@...mide.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        iommu@...ts.linux-foundation.org, Keith Busch <kbusch@...nel.org>,
        kernel-team@...com
Subject: Re: [PATCH 04/10] dmapool: improve accuracy of debug statistics

On 2022-05-31 20:52, Tony Battersby wrote:
> On 5/31/22 15:48, Robin Murphy wrote:
>> On 2022-05-31 19:17, Tony Battersby wrote:
>>
>>>    				 pool->name, blocks,
>>> -				 (size_t) pages *
>>> -				 (pool->allocation / pool->size),
>>> +				 (size_t) pages * pool->blks_per_alloc,
>>>    				 pool->size, pages);
>>>    		size -= temp;
>>>    		next += temp;
>>> @@ -168,6 +168,9 @@ struct dma_pool *dma_pool_create(const char *name, struct device *dev,
>>>    	retval->size = size;
>>>    	retval->boundary = boundary;
>>>    	retval->allocation = allocation;
>>> +	retval->blks_per_alloc =
>>> +		(allocation / boundary) * (boundary / size) +
>>> +		(allocation % boundary) / size;
>> Do we really need to store this? Sure, 4 divisions (which could possibly
>> be fewer given the constraints on boundary) isn't the absolute cheapest
>> calculation, but I still can't imagine anyone would be polling sysfs
>> stats hard enough to even notice.
>>
> The stored value is also used in patch #5, in more performance-critical
> code, although only when debug is enabled.

Ah, fair enough. On second look I think 64-bit systems could effectively 
store this for free anyway, if patch #2 moved "size" down past "dev" in 
struct dma_pool, such that blks_per_alloc then ends up padding out the 
hole again.

FWIW the mathematician in me also now can't help seeing the algebraic 
reduction to at least "(allocation + (allocation % boundary)) / size", 
but is now too tired to reason about the power-of-two constraints and 
whether the intermediate integer truncations matter...

Cheers,
Robin.