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

[John Garry <john.garry@huawei.com>]

On 01/03/2021 13:20, Robin Murphy wrote:
>>> 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.

I figure that you're talking about 4e89dce72521 now. I would have liked 
to know which real-life problem it solved in practice.

> 
> 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.

I will attempt to prove this by increasing RCACHE RANGE, such that all 
IOVA sizes may be cached.

>>
>> 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 -

ok

> 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.

Even if it is were configurable, wouldn't it make sense to have it 
configurable per IOVA domain?

Furthermore, as mentioned above, I still want to solve this IOVA aging 
issue, and this fixed RCACHE RANGE size seems to be the at the center of 
that problem.

> 
>> 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.

If those conditions were true, then it seems quite a tenuous position, 
so trying to help that scenario in general terms will have limited efficacy.

> 
> 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; 

Of course, but....

> 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.

We are seeing a ~50% throughput performance hit, and it's quite 
reasonable to request a short-term fix, rather than accepting that this 
problem is something which we need to solve medium/long-term and we 
don't know how yet.

Going forward, we should try to fix/workaround any broken platforms, 
rather than hide them all. Indeed, the current approach will just give 
rise to more broken platforms - people only fix generally what they see 
is broken. I do wonder how many there really are.

So how about stick the change to avoid the SAC trick for PCIe devices 
behind a kconfig option, and handle issues on a case-by-case basis, as 
they arise? I think that this is what Joerg suggested earlier.

In addition to that, revisit IOVA aging issue and related topic of fixed 
RCACHE RANGE. Hopefully we can solve our short-term performance issue there.

Thanks,
John
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