On Sat, May 30, 2020 at 09:15:05PM +1200, Barry Song wrote:

As tests show the latency of dma_unmap can increase dramatically while
calling them cross NUMA nodes, especially cross CPU packages, eg.
300ns vs 800ns while waiting for the completion of CMD_SYNC in an
empty command queue. The large latency causing by remote node will
in turn make contention of the command queue more serious, and enlarge
the latency of DMA users within local NUMA nodes.

Users might intend to enforce NUMA locality with the consideration of
the position of SMMU. The patch provides minor benefit by presenting
this information to users directly, as they might want to know it without
checking hardware spec at all.

I don't think that's a very good reason to expose things to userspace.
I know sysfs shouldn't be treated as ABI, but the grim reality is that
once somebody relies on this stuff then we can't change it, so I'd
rather avoid exposing it unless it's absolutely necessary.

Thanks,

Will

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