Re: [PATCH v2 2/2] drm/i915/gvt: subsitute kvm_read/write_guest with vfio_dma_rw

Alex Williamson
2020-01-21T16:51:28Z

On Tue, 21 Jan 2020 03:12:07 -0500
Yan Zhao <yan.y.zhao@intel.com> wrote:

> On Tue, Jan 21, 2020 at 04:01:57AM +0800, Alex Williamson wrote:
> > 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,
> >
> hi Alex,
> it works!

Hurrah!

> now the average vfio_dma_rw cycles can reduced to 1198.
> one thing I want to propose is that, in sight of dma->task is always user
> space process, instead of calling get_task_mm(dma->task), can we just use
> "mmget_not_zero(dma->task->mm)"? in this way, the avg cycles can
> further reduce to 1051.

I'm not an expert there. As noted in the type1 code we hold a
reference to the task because it's not advised to hold a long term
reference to the mm, so do we know we can look at task->mm without
acquiring task_lock()? It's possible this is safe, but it's not
abundantly obvious to me. Please research further and provide
justification if you think it's correct. Thanks,

Alex