[Intel-wired-lan] Regression 5.12.0-rc4 net: ice: significant throughput drop

[Jussi Maki]

On Wed, Jun 2, 2021 at 2:49 PM Robin Murphy <robin.murphy at arm.com> wrote:
> >> Thanks for the quick response & patch. I tried it out and indeed it
> >> does solve the issue:
>
> Cool, thanks Jussi. May I infer a Tested-by tag from that?

Of course!

> Given that the race looks to have been pretty theoretical until now, I'm
> not convinced it's worth the bother of digging through the long history
> of default domain and DMA ops movement to figure where it started, much
> less attempt invasive backports. The flush queue change which made it
> apparent only landed in 5.13-rc1, so as long as we can get this in as a
> fix in the current cycle we should be golden - in the meantime, note
> that booting with "iommu.strict=0" should also restore the expected
> behaviour.
>
> FWIW I do still plan to resend the patch "properly" soon (in all honesty
> it wasn't even compile-tested!)

BTW, even with the patch there's quite a bit of spin lock contention
coming from ice_xmit_xdp_ring->dma_map_page_attrs->...->alloc_iova.
CPU load drops from 85% to 20% (~80Mpps, 64b UDP) when iommu is
disabled. Is this type of overhead to be expected?