Re: [PATCH v4] iommu: Optimise PCI SAC address trick

[Robin Murphy]

On 2023-04-14 12:45, Joerg Roedel wrote:
> Hi Robin,
>
> On Thu, Apr 13, 2023 at 02:40:25PM +0100, Robin Murphy wrote:
> > Per the reasoning in commit 4bf7fda4dce2 ("iommu/dma: Add config for
> > PCI SAC address trick") and its subsequent revert, this mechanism no
> > longer serves its original purpose, but now only works around broken
> > hardware/drivers in a way that is unfortunately too impactful to remove.
> >
> > This does not, however, prevent us from solving the performance impact
> > which that workaround has on large-scale systems that don't need it.
> > Once the 32-bit IOVA space fills up and a workload starts allocating and
> > freeing on both sides of the boundary, the opportunistic SAC allocation
> > can then end up spending significant time hunting down scattered
> > fragments of free 32-bit space, or just reestablishing max32_alloc_size.
> > This can easily be exacerbated by a change in allocation pattern, such
> > as by changing the network MTU, which can increase pressure on the
> > 32-bit space by leaving a large quantity of cached IOVAs which are now
> > the wrong size to be recycled, but also won't be freed since the
> > non-opportunistic allocations can still be satisfied from the whole
> > 64-bit space without triggering the reclaim path.
> >
> > However, in the context of a workaround where smaller DMA addresses
> > aren't simply a preference but a necessity, if we get to that point at
> > all then in fact it's already the endgame. The nature of the allocator
> > is currently such that the first IOVA we give to a device after the
> > 32-bit space runs out will be the highest possible address for that
> > device, ever. If that works, then great, we know we can optimise for
> > speed by always allocating from the full range. And if it doesn't, then
> > the worst has already happened and any brokenness is now showing, so
> > there's little point in continuing to try to hide it.
> >
> > To that end, implement a flag to refine the SAC business into a
> > per-device policy that can automatically get itself out of the way if
> > and when it stops being useful.
>
> Thanks for working on this, I think this is good to go. But given the
> issues we had with last attempt I'd like to have this in linux-next for
> a few weeks before sending it upstream. Therefore I will defer this
> patch and merge it early in the next cycle.

Sounds good - I'm considerably more confident in this approach, but 
although it should not be able to break any scenario which wasn't 
already broken, it could potentially still make such a breakage more 
noticeable. Thus in all honesty I'd feel happiest giving it a full cycle 
of -next coverage as well.

Cheers,
Robin.