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

Yan Zhao
2020-01-19T10:15:53Z

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.

Thanks
Yan

>
> > > > Comparison of benchmarks scores are as blow:
> > > > ------------------------------------------------------
> > > > | avg score | kvm_read/write_guest | vfio_dma_rw |
> > > > |----------------------------------------------------|
> > > > | Glmark2 | 1284 | 1296 |
> > > > |----------------------------------------------------|
> > > > | Lightsmark | 61.24 | 61.27 |
> > > > |----------------------------------------------------|
> > > > | OpenArena | 140.9 | 137.4 |
> > > > |----------------------------------------------------|
> > > > | Heaven | 671 | 670 |
> > > > ------------------------------------------------------
> > > > No obvious performance downgrade found.
> > > >
> > > > Cc: Kevin Tian <kevin.tian@intel.com>
> > > > Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
> > > > ---
> > > > drivers/gpu/drm/i915/gvt/kvmgt.c | 26 +++++++-------------------
> > > > 1 file changed, 7 insertions(+), 19 deletions(-)
> > > >
> > > > diff --git a/drivers/gpu/drm/i915/gvt/kvmgt.c b/drivers/gpu/drm/i915/gvt/kvmgt.c
> > > > index bd79a9718cc7..17edc9a7ff05 100644
> > > > --- a/drivers/gpu/drm/i915/gvt/kvmgt.c
> > > > +++ b/drivers/gpu/drm/i915/gvt/kvmgt.c
> > > > @@ -1966,31 +1966,19 @@ static int kvmgt_rw_gpa(unsigned long handle, unsigned long gpa,
> > > > void *buf, unsigned long len, bool write)
> > > > {
> > > > struct kvmgt_guest_info *info;
> > > > - struct kvm *kvm;
> > > > - int idx, ret;
> > > > - bool kthread = current->mm == NULL;
> > > > + int ret;
> > > > + struct intel_vgpu *vgpu;
> > > > + struct device *dev;
> > > >
> > > > if (!handle_valid(handle))
> > > > return -ESRCH;
> > > >
> > > > info = (struct kvmgt_guest_info *)handle;
> > > > - kvm = info->kvm;
> > > > -
> > > > - if (kthread) {
> > > > - if (!mmget_not_zero(kvm->mm))
> > > > - return -EFAULT;
> > > > - use_mm(kvm->mm);
> > > > - }
> > > > -
> > > > - idx = srcu_read_lock(&kvm->srcu);
> > > > - ret = write ? kvm_write_guest(kvm, gpa, buf, len) :
> > > > - kvm_read_guest(kvm, gpa, buf, len);
> > > > - srcu_read_unlock(&kvm->srcu, idx);
> > > > + vgpu = info->vgpu;
> > > > + dev = mdev_dev(vgpu->vdev.mdev);
> > > >
> > > > - if (kthread) {
> > > > - unuse_mm(kvm->mm);
> > > > - mmput(kvm->mm);
> > > > - }
> > > > + ret = write ? vfio_dma_rw(dev, gpa, buf, len, true) :
> > > > + vfio_dma_rw(dev, gpa, buf, len, false);
> > >
> > > As Paolo suggested previously, this can be simplified:
> > >
> > > ret = vfio_dma_rw(dev, gpa, buf, len, write);
> > >
> > > >
> > > > return ret;
> > >
> > > Or even more simple, remove the ret variable:
> > >
> > > return vfio_dma_rw(dev, gpa, buf, len, write);
> > >
> > oh, it seems that I missed Paolo's mail. will change it. thank you!
> >
> > Thanks
> > Yan
> > >
> > > > }
> > >
> >
>