From: Alex Williamson <alex.williamson@redhat.com>
To: Yan Zhao <yan.y.zhao@intel.com>
Cc: "zhenyuw@linux.intel.com" <zhenyuw@linux.intel.com>,
"intel-gvt-dev@lists.freedesktop.org"
<intel-gvt-dev@lists.freedesktop.org>,
"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"pbonzini@redhat.com" <pbonzini@redhat.com>,
"Tian, Kevin" <kevin.tian@intel.com>,
"peterx@redhat.com" <peterx@redhat.com>
Subject: Re: [PATCH v2 2/2] drm/i915/gvt: subsitute kvm_read/write_guest with vfio_dma_rw
Date: Mon, 20 Jan 2020 13:01:57 -0700 [thread overview]
Message-ID: <20200120130157.0ee7042d@w520.home> (raw)
In-Reply-To: <20200119100637.GD1759@joy-OptiPlex-7040>
On Sun, 19 Jan 2020 05:06:37 -0500
Yan Zhao <yan.y.zhao@intel.com> wrote:
> On Thu, Jan 16, 2020 at 11:37:29PM +0800, Alex Williamson wrote:
> > On Thu, 16 Jan 2020 00:49:41 -0500
> > Yan Zhao <yan.y.zhao@intel.com> wrote:
> >
> > > On Thu, Jan 16, 2020 at 04:06:51AM +0800, Alex Williamson wrote:
> > > > On Tue, 14 Jan 2020 22:54:55 -0500
> > > > Yan Zhao <yan.y.zhao@intel.com> wrote:
> > > >
> > > > > As a device model, it is better to read/write guest memory using vfio
> > > > > interface, so that vfio is able to maintain dirty info of device IOVAs.
> > > > >
> > > > > Compared to kvm interfaces kvm_read/write_guest(), vfio_dma_rw() has ~600
> > > > > cycles more overhead on average.
> > > > >
> > > > > -------------------------------------
> > > > > | interface | avg cpu cycles |
> > > > > |-----------------------------------|
> > > > > | kvm_write_guest | 1554 |
> > > > > | ----------------------------------|
> > > > > | kvm_read_guest | 707 |
> > > > > |-----------------------------------|
> > > > > | vfio_dma_rw(w) | 2274 |
> > > > > |-----------------------------------|
> > > > > | vfio_dma_rw(r) | 1378 |
> > > > > -------------------------------------
> > > >
> > > > In v1 you had:
> > > >
> > > > -------------------------------------
> > > > | interface | avg cpu cycles |
> > > > |-----------------------------------|
> > > > | kvm_write_guest | 1546 |
> > > > | ----------------------------------|
> > > > | kvm_read_guest | 686 |
> > > > |-----------------------------------|
> > > > | vfio_iova_rw(w) | 2233 |
> > > > |-----------------------------------|
> > > > | vfio_iova_rw(r) | 1262 |
> > > > -------------------------------------
> > > >
> > > > So the kvm numbers remained within +0.5-3% while the vfio numbers are
> > > > now +1.8-9.2%. I would have expected the algorithm change to at least
> > > > not be worse for small accesses and be better for accesses crossing
> > > > page boundaries. Do you know what happened?
> > > >
> > > I only tested the 4 interfaces in GVT's environment, where most of the
> > > guest memory accesses are less than one page.
> > > And the different fluctuations should be caused by the locks.
> > > vfio_dma_rw contends locks with other vfio accesses which are assumed to
> > > be abundant in the case of GVT.
> >
> > Hmm, so maybe it's time to convert vfio_iommu.lock from a mutex to a
> > rwsem? Thanks,
> >
>
> hi Alex
> I tested your rwsem patches at (https://lkml.org/lkml/2020/1/16/1869).
> They works without any runtime error at my side. :)
> However, I found out that the previous fluctuation may be because I didn't
> take read/write counts in to account.
> For example. though the two tests have different avg read/write cycles,
> their average cycles are almost the same.
> ______________________________________________________________________
> | | avg read | | avg write | | |
> | | cycles | read cnt | cycles | write cnt | avg cycles |
> |----------------------------------------------------------------------|
> | test 1 | 1339 | 29,587,120 | 2258 | 17,098,364 | 1676 |
> | test 2 | 1340 | 28,454,262 | 2238 | 16,501,788 | 1670 |
> ----------------------------------------------------------------------
>
> After measuring the exact read/write cnt and cycles of a specific workload,
> I get below findings:
>
> (1) with single VM running glmark2 inside.
> glmark2: 40M+ read+write cnt, among which 63% is read.
> among reads, 48% is of PAGE_SIZE, the rest is less than a page.
> among writes, 100% is less than a page.
>
> __________________________________________________
> | cycles | read | write | avg | inc |
> |--------------------------------------------------|
> | kvm_read/write_page | 694 | 1506 | 993 | / |
> |--------------------------------------------------|
> | vfio_dma_rw(mutex) | 1340 | 2248 | 1673 | 680 |
> |--------------------------------------------------|
> | vfio_dma_rw(rwsem r) | 1323 | 2198 | 1645 | 653 |
> ---------------------------------------------------
>
> so vfio_dma_rw generally has 650+ more cycles per each read/write.
> While kvm->srcu is of 160 cycles on average with one vm is running, the
> cycles spending on locks for vfio_dma_rw spread like this:
> ___________________________
> | cycles | avg |
> |---------------------------|
> | iommu->lock | 117 |
> |---------------------------|
> | vfio.group_lock | 108 |
> |---------------------------|
> | group->unbound_lock | 114 |
> |---------------------------|
> | group->device_lock | 115 |
> |---------------------------|
> | group->mutex | 113 |
> ---------------------------
>
> I measured the cycles for a mutex without any contention is 104 cycles
> on average (including time for get_cycles() and measured in the same way
> as other locks). So the contention of a single lock in a single vm
> environment is light. probably because there's a vgpu lock hold in GVT already.
>
> (2) with two VMs each running glmark2 inside.
> The contention increases a little.
>
> ___________________________________________________
> | cycles | read | write | avg | inc |
> |---------------------------------------------------|
> | kvm_read/write_page | 1035 | 1832 | 1325 | / |
> |---------------------------------------------------|
> | vfio_dma_rw(mutex) | 2104 | 2886 | 2390 | 1065 |
> |---------------------------------------------------|
> | vfio_dma_rw(rwsem r) | 1965 | 2778 | 2260 | 935 |
> ---------------------------------------------------
>
>
> -----------------------------------------------
> | avg cycles | one VM | two VMs |
> |-----------------------------------------------|
> | iommu lock (mutex) | 117 | 150 |
> |-----------------------------------|-----------|
> | iommu lock (rwsem r) | 117 | 156 |
> |-----------------------------------|-----------|
> | kvm->srcu | 160 | 213 |
> -----------------------------------------------
>
> In the kvm case, avg cycles increased 332 cycles, while kvm->srcu only costed
> 213 cycles. The rest 109 cycles may be spent on atomic operations.
> But I didn't measure them, as get_cycles() operation itself would influence final
> cycles by ~20 cycles.
It seems like we need to extend the vfio external user interface so
that GVT-g can hold the group and container user references across
multiple calls. For instance if we had a
vfio_group_get_external_user_from_dev() (based on
vfio_group_get_external_user()) then i915 could get an opaque
vfio_group pointer which it could use to call vfio_group_dma_rw() which
would leave us with only the iommu rw_sem locking. i915 would release
the reference with vfio_group_put_external_user() when the device is
released. The same could be done with the pin pages interface to
streamline that as well. Thoughts? Thanks,
Alex
next prev parent reply other threads:[~2020-01-20 20:02 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-15 3:41 [PATCH v2 0/2] use vfio_dma_rw to read/write IOVAs from CPU side Yan Zhao
2020-01-15 3:53 ` [PATCH v2 1/2] vfio: introduce vfio_dma_rw to read/write a range of IOVAs Yan Zhao
2020-01-15 20:06 ` Alex Williamson
2020-01-16 2:30 ` Mika Penttilä
2020-01-16 2:59 ` Alex Williamson
2020-01-16 3:15 ` Mika Penttilä
2020-01-16 3:58 ` Alex Williamson
2020-01-16 5:32 ` Yan Zhao
2020-01-15 3:54 ` [PATCH v2 2/2] drm/i915/gvt: subsitute kvm_read/write_guest with vfio_dma_rw Yan Zhao
2020-01-15 20:06 ` Alex Williamson
2020-01-16 5:49 ` Yan Zhao
2020-01-16 15:37 ` Alex Williamson
2020-01-19 10:06 ` Yan Zhao
2020-01-20 20:01 ` Alex Williamson [this message]
2020-01-21 8:12 ` Yan Zhao
2020-01-21 16:51 ` Alex Williamson
2020-01-21 22:10 ` Yan Zhao
2020-01-22 3:07 ` Yan Zhao
2020-01-23 10:02 ` Yan Zhao
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