Le 06/07/2020 à 10:26, Jonathan Cameron a écrit :

On Sun, 5 Jul 2020 09:53:58 +0000
"Song Bao Hua (Barry Song)" [off-list ref] wrote:

quoted

quoted

-----Original Message-----
From: Will Deacon [mailto:will@kernel.org]
Sent: Saturday, July 4, 2020 4:22 AM
To: Song Bao Hua (Barry Song) <redacted>
Cc: robin.murphy@arm.com; hch@lst.de; m.szyprowski@samsung.com;
iommu@lists.linux-foundation.org; linux-arm-kernel@lists.infradead.org;
Linuxarm [off-list ref]
Subject: Re: [PATCH] iommu/arm-smmu-v3: expose numa_node attribute to
users in sysfs

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

quoted

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.  

Will, thanks for taking a look!

I am not sure if it is absolutely necessary, but it is useful to users. The whole story started
from some users who wanted to know the hardware topology very clear by reading some
sysfs node just like they are able to do that for pci devices. The intention is that users can
know hardware topology of various devices easily from linux since they maybe don't know
all the hardware details.

For pci devices, kernel has done that. And there are some other drivers out of pci
exposing numa_node as well. It seems it is hard to say it is absolutely necessary
for them too since sysfs shouldn't be treated as ABI. 

Brice,

Given hwloc is probably the most demanding user of topology information
currently...

How useful would this info be for hwloc and hwloc users?
Sort of feels like it might be useful in some cases.

The very brief description of what we have here is exposing the numa node
of an IOMMU.  The discussion also diverted into whether it just makes sense
to expose this for all platform devices or even do it at the device level.

Hello

We don't have anything about IOMMU in hwloc so far, likely because its
locality never mattered in the past? I guess we'll get some user
requests for it once more platforms show this issue and some
performance-critical applications are not happy with it.

Can you clarify what the whole machine topology look like? Are we
talking about some PCI devices being attached to one socket but talking
to the IOMMU of the other socket?

Brice


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