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

[Robin Murphy <robin.murphy@arm.com>]

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.