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Message-ID: <20200220043835.GB30338@joy-OptiPlex-7040>
Date: Wed, 19 Feb 2020 23:38:35 -0500
From: Yan Zhao <yan.y.zhao@...el.com>
To: Alex Williamson <alex.williamson@...hat.com>
Cc: "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 0/3] vfio/type1: Reduce vfio_iommu.lock contention
On Thu, Feb 20, 2020 at 04:52:47AM +0800, Alex Williamson wrote:
> On Wed, 19 Feb 2020 04:04:17 -0500
> Yan Zhao <yan.y.zhao@...el.com> wrote:
>
> > On Fri, Jan 17, 2020 at 09:10:51AM +0800, Yan Zhao wrote:
> > > Thank you, Alex!
> > > I'll try it and let you know the result soon. :)
> > >
> > > On Fri, Jan 17, 2020 at 02:17:49AM +0800, Alex Williamson wrote:
> > > > Hi Yan,
> > > >
> > > > I wonder if this might reduce the lock contention you're seeing in the
> > > > vfio_dma_rw series. These are only compile tested on my end, so I hope
> > > > they're not too broken to test. Thanks,
> > > >
> > > > Alex
> > > >
> > > > ---
> > > >
> > > > Alex Williamson (3):
> > > > vfio/type1: Convert vfio_iommu.lock from mutex to rwsem
> > > > vfio/type1: Replace obvious read lock instances
> > > > vfio/type1: Introduce pfn_list mutex
> > > >
> > > >
> > > > drivers/vfio/vfio_iommu_type1.c | 67 ++++++++++++++++++++++++---------------
> > > > 1 file changed, 41 insertions(+), 26 deletions(-)
> > > >
> >
> > hi Alex
> > I have finished testing of this series.
> > It's quite stable and passed our MTBF testing :)
> >
> > However, after comparing the performance data obtained from several
> > benchmarks in guests (see below),
> > it seems that this series does not bring in obvious benefit.
> > (at least to cases we have tested, and though I cannot fully explain it yet).
> > So, do you think it's good for me not to include this series into my next
> > version of "use vfio_dma_rw to read/write IOVAs from CPU side"?
>
> Yes, I would not include it in your series. No reason to bloat your
> series for a feature that doesn't clearly show an improvement. Thanks
> for the additional testing, we can revive them if this lock ever
> resurfaces. I'm was actually more hopeful that holding an external
> group reference might provide a better performance improvement, the
> lookup on every vfio_dma_rw is not very efficient. Thanks,
>
got it. thanks~
Yan
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