Re: [PATCH 1/1] Revert "iommu/iova: Retry from last rb tree node if iova search fails"

From: Robin Murphy
Date: Mon Mar 01 2021 - 08:21:29 EST

On 2021-02-25 13:54, John Garry wrote:
On 29/01/2021 12:03, Robin Murphy wrote:
On 2021-01-29 09:48, Leizhen (ThunderTown) wrote:

Currently, we are thinking about the solution to the problem. However, because the end time of v5.11 is approaching, this patch is sent first.

However, that commit was made for a reason - how do we justify that one thing being slow is more important than another thing being completely broken? It's not practical to just keep doing the patch hokey-cokey based on whoever shouts loudest :(

On 2021/1/29 17:21, Zhen Lei wrote:
This reverts commit 4e89dce725213d3d0b0475211b500eda4ef4bf2f.

We find that this patch has a great impact on performance. According to
our test: the iops decreases from 1655.6K to 893.5K, about half.

Hardware: 1 SAS expander with 12 SAS SSD
Command:  Only the main parameters are listed.
           fio bs=4k rw=read iodepth=128 cpus_allowed=0-127

FWIW, I'm 99% sure that what you really want is [1], but then you get to battle against an unknown quantity of dodgy firmware instead.


Something which has not been said before is that this only happens for strict mode.

I think that makes sense - once you *have* actually failed to allocate from the 32-bit space, max32_alloc_size will make subsequent attempts fail immediately. In non-strict mode you're most likely freeing 32-bit IOVAs back to the tree - and thus reset max32_alloc_size - much less often, and you'll make more total space available each time, both of which will amortise the cost of getting back into that failed state again. Conversely, the worst case in strict mode is to have multiple threads getting into this pathological cycle:

1: allocate, get last available IOVA
2: allocate, fail and set max32_alloc_size
3: free one IOVA, reset max32_alloc_size, goto 1

Now, given the broken behaviour where the cached PFN can get stuck near the bottom of the address space, step 2 might well have been faster and more premature than it should have, but I hope you can appreciate that relying on an allocator being broken at its fundamental purpose of allocating is not a good or sustainable thing to do.

While max32_alloc_size indirectly tracks the largest *contiguous* available space, one of the ideas from which it grew was to simply keep count of the total number of free PFNs. If you're really spending significant time determining that the tree is full, as opposed to just taking longer to eventually succeed, then it might be relatively innocuous to tack on that semi-redundant extra accounting as a self-contained quick fix for that worst case.

Anyway, we see ~50% throughput regression, which is intolerable. As seen in [0], I put this down to the fact that we have so many IOVA requests which exceed the rcache size limit, which means many RB tree accesses for non-cacheble IOVAs, which are now slower.

On another point, as for longterm IOVA aging issue, it seems that there is no conclusion there. However I did mention the issue of IOVA sizes exceeding rcache size for that issue, so maybe we can find a common solution. Similar to a fixed rcache depot size, it seems that having a fixed rcache max size range value (at 6) doesn't scale either.

Well, I'd say that's more of a workload tuning thing than a scalability one - a massive system with hundreds of CPUs that spends all day flinging 1500-byte network packets around as fast as it can might be happy with an even smaller value and using the saved memory for something else. IIRC the value of 6 is a fairly arbitrary choice for a tradeoff between expected utility and memory consumption, so making it a Kconfig or command-line tuneable does seem like a sensible thing to explore.

As for 4e89dce72521, so even if it's proper to retry for a failed alloc, it is not always necessary. I mean, if we're limiting ourselves to 32b subspace for this SAC trick and we fail the alloc, then we can try the space above 32b first (if usable). If that fails, then retry there. I don't see a need to retry the 32b subspace if we're not limited to it. How about it? We tried that idea and it looks to just about restore performance.

The thing is, if you do have an actual PCI device where DAC might mean a 33% throughput loss and you're mapping a long-lived buffer, or you're on one of these systems where firmware fails to document address limits and using the full IOMMU address width quietly breaks things, then you almost certainly *do* want the allocator to actually do a proper job of trying to satisfy the given request.

Furthermore, what you propose is still fragile for your own use-case anyway. If someone makes internal changes to the allocator - converts it to a different tree structure, implements split locking for concurrency, that sort of thing - and it fundamentally loses the dodgy cached32_node behaviour which makes the initial failure unintentionally fast for your workload's allocation pattern, that extra complexity will suddenly just be dead weight and you'll probably be complaining of a performance regression again.

We're talking about an allocation that you know you don't need to make, and that you even expect to fail, so I still maintain that it's absurd to focus on optimising for failure; focus on *not even doing it at all*. It just needs an approach that's not going to mess up the unknown but apparently nonzero number of systems inadvertently relying on 32-bit IOVAs for correctness.

Robin.