Re: [PATCH v2] iommu/iova: Optimize alloc_iova with rbtree_augmented

[Peng Zhang]

> If you really have a performance issue with alloc_iova_fast() -> 
> alloc_iova() then I suggest that you consider trying to use 
> dma_opt_mapping_size() to teach the DMA engine driver to not create 
> requests whose overall size exceeds to the rcache limit.

Yes. But I don't think it essentially solves the problem.
A library for users should run stably ant it shouldn't hold
the spinlock for a long time in some cases. It can even be
said to be a bug.

Like this:
[Wed May 25 05:27:59 2022] watchdog: BUG: soft lockup - CPU#58
stuck for 23s!
[Wed May 25 05:27:59 2022] Call Trace:
[Wed May 25 05:27:59 2022]  alloc_iova+0xf2/0x140
[Wed May 25 05:27:59 2022]  alloc_iova_fast+0x56/0x251

Now we avoid this problem with iommu=pt, but it didn't solve the problem.

Thanks,
Peng