Re: [bug report] iommu_dma_unmap_sg() is very slow then running IO from remote numa node

[Robin Murphy]

On 2021-07-28 16:17, Ming Lei wrote:
> On Wed, Jul 28, 2021 at 11:38:18AM +0100, John Garry wrote:
> > On 28/07/2021 02:32, Ming Lei wrote:
> > > On Mon, Jul 26, 2021 at 3:51 PM John Garry<john.garry@xxxxxxxxxx>  wrote:
> > > > On 23/07/2021 11:21, Ming Lei wrote:
> > > > > > Thanks, I was also going to suggest the latter, since it's what
> > > > > > arm_smmu_cmdq_issue_cmdlist() does with IRQs masked that should be most
> > > > > > indicative of where the slowness most likely stems from.
> > > > > The improvement from 'iommu.strict=0' is very small:
> > > > Have you tried turning off the IOMMU to ensure that this is really just
> > > > an IOMMU problem?
> > > >
> > > > You can try setting CONFIG_ARM_SMMU_V3=n in the defconfig or passing
> > > > cmdline param iommu.passthrough=1 to bypass the the SMMU (equivalent to
> > > > disabling for kernel drivers).
> > > Bypassing SMMU via iommu.passthrough=1 basically doesn't make a difference
> > > on this issue.
> >
> > A ~90% throughput drop still seems to me to be too high to be a software
> > issue. More so since I don't see similar on my system. And that throughput
> > drop does not lead to a total CPU usage drop, from the fio log.

Indeed, it now sounds like $SUBJECT has been a complete red herring, and 
although the SMMU may be reflecting the underlying slowness it is not in 
fact a significant contributor to it. Presumably perf shows any 
difference in CPU time moving elsewhere once iommu_dma_unmap_sg() is out 
of the picture?

> > Do you know if anyone has run memory benchmark tests on this board to find
> > out NUMA effect? I think lmbench or stream could be used for this.
>
> https://lore.kernel.org/lkml/YOhbc5C47IzC893B@T590/

Hmm, a ~4x discrepancy in CPU<->memory bandwidth is pretty significant, 
but it's still not the ~10x discrepancy in NVMe throughput. Possibly 
CPU<->PCIe and/or PCIe<->memory bandwidth is even further impacted 
between sockets, or perhaps all the individual latencies just add up - 
that level of detailed performance analysis is beyond my expertise. 
Either way I guess it's probably time to take it up with the system 
vendor to see if there's anything which can be tuned in hardware/firmware.

Robin.