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

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Date:   Fri, 31 Jan 2020 09:46:06 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Robin Murphy <robin.murphy@....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 Fri, Jan 31, 2020 at 9:43 AM Robin Murphy <robin.murphy@....com> wrote:
>
> 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.


Sure, go ahead, I have no plan implementing changes for 20 years old platforms.

Thanks.