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

From: Will Deacon <will@kernel.org>
To: Ganapatrao Kulkarni <gklkml16@gmail.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 13:28:47 +0100	[thread overview]
Message-ID: <20190724122846.5mynuxkm3rgipxhp@willie-the-truck> (raw)
In-Reply-To: <CAKTKpr58zHi0Nw=Fb8d4xHUenW1d76V2pkQ_0+BqWQ0OfBmtCQ@mail.gmail.com>

On Fri, Jul 19, 2019 at 09:55:39AM +0530, Ganapatrao Kulkarni wrote:
> On Thu, Jul 11, 2019 at 10:58 PM Will Deacon <will@kernel.org> 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 have tried branch iommu/cmdq on ThunderX2. I do see there is drastic
> reduction in CPU bandwidth consumption(from 15 to 20% to 1 to 2% in
> perf top) from SMMU CMDQ helper functions, when I run iperf with more
> than 64 clients(-P 64). However I have not noticed any measurable
> performance improvement in iperf results. IMO, this might/should help
> in performance improvement of IO intensive workloads.
> 
> FWIW, you can add,
> Tested-by: Ganapatrao Kulkarni  <gkulkarni@marvell.com>

Brilliant, thanks. Your measurements reflect mine in that I can saturate
the NICs I have access to regardless of these changes, but the CPU time
is drastically reduced.

Will
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