linux-kernel - Re: [PATCH] dma-debug: dynamic allocation of hash table

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Date:   Fri, 31 Jan 2020 17:43:45 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Christoph Hellwig <hch@....de>, Joerg Roedel <jroedel@...e.de>,
        iommu@...ts.linux-foundation.org,
        Eric Dumazet <eric.dumazet@...il.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dma-debug: dynamic allocation of hash table

On 31/01/2020 2:42 pm, Eric Dumazet wrote:
> On Fri, Jan 31, 2020 at 4:30 AM Robin Murphy <robin.murphy@....com> wrote:
>>
>> ...and when that represents ~5% of the total system RAM it is a *lot*
>> less reasonable than even 12KB. As I said, it's great to make a debug
>> option more efficient such that what it observes is more representative
>> of the non-debug behaviour, but it mustn't come at the cost of making
>> the entire option unworkable for other users.
>>
> 
> Then I suggest you send a patch to reduce PREALLOC_DMA_DEBUG_ENTRIES
> because having 65536 preallocated entries consume 4 MB of memory.

...unless it's overridden on the command-line ;)

> Actually this whole attempt to re-implement slab allocations in this
> file is suspect.

Again that's a matter of expected usage patterns - typically the 
"reasonable default" or user-defined preallocation should still be 
enough (as it *had* to be before), and the kind of systems that can 
sustain so many live mappings as to regularly hit the dynamic expansion 
path are also likely to have enough memory not to care that later-unused 
entries never get 'properly' freed back to the kernel (plus as you've 
observed, many workloads tend to reach a steady state where entries are 
mostly only transiently free anyway). The reasoning for the exact 
implementation details is there in the commit logs.

> Do not get me wrong, but if you really want to run linux on a 16MB host,
> I guess you need to add CONFIG_BASE_SMALL all over the places,
> not only in this kernel/dma/debug.c file.

Right, nobody's suggesting that defconfig should "just work" on your 
router/watch/IoT shoelaces/whatever, I'm just saying that tuning any 
piece of common code for datacenter behemoths, for quality-of-life 
rather than functional necessity, and leaving no way to adjust it other 
than hacking the source, would represent an unnecessary degree of 
short-sightedness that we can and should avoid.

Taking a closer look at the code, it appears fairly straightforward to 
make the hash size variable, and in fact making it self-adjusting 
doesn't seem too big a jump from there. I'm happy to have a go at that 
myself if you like.

Robin.