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