Re: [RFC PATCH v2 00/19] Try to reduce lock contention on the SMMUv3 command queue

From: Will Deacon <will@kernel.org>
To: John Garry <john.garry@huawei.com>
Cc: Vijay Kilary <vkilari@codeaurora.org>,
	Jean-Philippe Brucker <jean-philippe.brucker@arm.com>,
	Jon Masters <jcm@redhat.com>, Jan Glauber <jglauber@marvell.com>,
	Alex Williamson <alex.williamson@redhat.com>,
	iommu@lists.linux-foundation.org,
	Jayachandran Chandrasekharan Nair <jnair@marvell.com>,
	Robin Murphy <robin.murphy@arm.com>
Subject: Re: [RFC PATCH v2 00/19] Try to reduce lock contention on the SMMUv3 command queue
Date: Wed, 24 Jul 2019 13:20:19 +0100	[thread overview]
Message-ID: <20190724122019.tzllwvqzy5ptfz6c@willie-the-truck> (raw)
In-Reply-To: <c8dcc53f-8afa-0966-dcfd-ca79b099893f@huawei.com>

On Wed, Jul 24, 2019 at 10:58:26AM +0100, John Garry wrote:
> On 11/07/2019 18:19, Will Deacon wrote:
> > This is a significant rework of the RFC I previously posted here:
> > 
> >   https://lkml.kernel.org/r/20190611134603.4253-1-will.deacon@arm.com
> > 
> > But this time, it looks like it might actually be worthwhile according
> > to my perf profiles, where __iommu_unmap() falls a long way down the
> > profile for a multi-threaded netperf run. I'm still relying on others to
> > confirm this is useful, however.
> > 
> > Some of the changes since last time are:
> > 
> >   * Support for constructing and submitting a list of commands in the
> >     driver
> > 
> >   * Numerous changes to the IOMMU and io-pgtable APIs so that we can
> >     submit commands in batches
> > 
> >   * Removal of cmpxchg() from cmdq_shared_lock() fast-path
> > 
> >   * Code restructuring and cleanups
> > 
> > This current applies against my iommu/devel branch that Joerg has pulled
> > for 5.3. If you want to test it out, I've put everything here:
> > 
> >   https://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git/log/?h=iommu/cmdq
> > 
> > Feedback welcome. I appreciate that we're in the merge window, but I
> > wanted to get this on the list for people to look at as an RFC.
> > 
> 
> I tested storage performance on this series, which I think is a better
> scenario to test than network performance, that being generally limited by
> the network link speed.

Interesting, thanks for sharing. Do you also see a similar drop in CPU time
to the one reported by Ganapat?

> Baseline performance (will/iommu/devel, commit 9e6ea59f3)
> 8x SAS disks D05	839K IOPS
> 1x NVMe D05		454K IOPS
> 1x NVMe D06		442k IOPS
> 
> Patchset performance (will/iommu/cmdq)
> 8x SAS disk D05		835K IOPS
> 1x NVMe D05		472K IOPS
> 1x NVMe D06		459k IOPS
> 
> So we see a bit of an NVMe boost, but about the same for 8x disks. No iommu
> performance is about 918K IOPs for 8x disks, so it is not limited by the
> medium.

It would be nice to know if this performance gap is because of Linux, or
simply because of the translation overhead in the SMMU hardware. Are you
able to get a perf profile to see where we're spending time?

Thanks,

Will
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