linux-kernel - Re: [RFC PATCH] iommu/iova: Add a best-fit algorithm

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Date:   Wed, 19 Feb 2020 12:28:52 +0000
From:   Will Deacon <will@...nel.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     "Isaac J. Manjarres" <isaacm@...eaurora.org>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com, pratikp@...eaurora.org,
        Liam Mark <lmark@...eaurora.org>
Subject: Re: [RFC PATCH] iommu/iova: Add a best-fit algorithm

On Mon, Feb 17, 2020 at 04:03:54PM +0000, Robin Murphy wrote:
> On 14/02/2020 11:06 pm, Isaac J. Manjarres wrote:
> > From: Liam Mark <lmark@...eaurora.org>
> > 
> > Using the best-fit algorithm, instead of the first-fit
> > algorithm, may reduce fragmentation when allocating
> > IOVAs.
> 
> What kind of pathological allocation patterns make that a serious problem?
> Is there any scope for simply changing the order of things in the callers?
> Do these drivers also run under other DMA API backends (e.g. 32-bit Arm)?
> 
> More generally, if a driver knows enough to want to second-guess a generic
> DMA API allocator, that's a reasonable argument that it should perhaps be
> properly IOMMU-aware and managing its own address space anyway. Perhaps this
> effort might be better spent finishing off the DMA ops bypass stuff to make
> that approach more robust and welcoming.

Anecdotally, it appears to be a fairly common problem for 32-bit capable
DMA masters to hit fragmentation problems with the current IOVA allocator
but yes, some numbers to show how that is improved using best-fit (as
opposed to e.g. worst-fit) are definitely required here.

It might be that we can simply swizzle the algorithm to focus on reduced
fragmentation for smaller (i.e. 32-bit) address spaces, but leave larger
domains with the current approach to avoid increasing the allocation
overhead.

Will