Messages in this thread | | | Date | Fri, 14 Apr 2023 18:45:57 +0100 | Subject | Re: [PATCH v4] iommu: Optimise PCI SAC address trick | From | Robin Murphy <> |
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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.
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