[dpdk-dev] [PATCH] vhost_user: protect active rings from async ring changes

[Tan, Jianfeng]

> -----Original Message-----
> From: Maxime Coquelin [mailto:maxime.coquelin at redhat.com]
> Sent: Friday, December 8, 2017 6:12 PM
> To: Tan, Jianfeng; Victor Kaplansky; dev at dpdk.org; yliu at fridaylinux.org; Bie,
> Tiwei
> Cc: stable at dpdk.org; jfreiman at redhat.com
> Subject: Re: [PATCH] vhost_user: protect active rings from async ring
> changes
> 
> Hi Jianfeng,
> 
> On 12/08/2017 09:51 AM, Tan, Jianfeng wrote:
> >
> >
> >> -----Original Message-----
> >> From: Maxime Coquelin [mailto:maxime.coquelin at redhat.com]
> >> Sent: Friday, December 8, 2017 4:36 PM
> >> To: Tan, Jianfeng; Victor Kaplansky; dev at dpdk.org; yliu at fridaylinux.org;
> Bie,
> >> Tiwei
> >> Cc: stable at dpdk.org; jfreiman at redhat.com
> >> Subject: Re: [PATCH] vhost_user: protect active rings from async ring
> >> changes
> >>
> >>
> >>
> >> On 12/08/2017 03:14 AM, Tan, Jianfeng wrote:
> >>>
> >>>
> >>>> -----Original Message-----
> >>>> From: Maxime Coquelin [mailto:maxime.coquelin at redhat.com]
> >>>> Sent: Thursday, December 7, 2017 6:02 PM
> >>>> To: Tan, Jianfeng; Victor Kaplansky; dev at dpdk.org;
> yliu at fridaylinux.org;
> >> Bie,
> >>>> Tiwei
> >>>> Cc: stable at dpdk.org; jfreiman at redhat.com
> >>>> Subject: Re: [PATCH] vhost_user: protect active rings from async ring
> >>>> changes
> >>>>
> >>>>
> >>>>
> >>>> On 12/07/2017 10:33 AM, Tan, Jianfeng wrote:
> >>>>>
> >>>>>
> >>>>>> -----Original Message-----
> >>>>>> From: Victor Kaplansky [mailto:vkaplans at redhat.com]
> >>>>>> Sent: Wednesday, December 6, 2017 9:56 PM
> >>>>>> To: dev at dpdk.org; yliu at fridaylinux.org; Bie, Tiwei; Tan, Jianfeng;
> >>>>>> vkaplans at redhat.com
> >>>>>> Cc: stable at dpdk.org; jfreiman at redhat.com; Maxime Coquelin
> >>>>>> Subject: [PATCH] vhost_user: protect active rings from async ring
> >> changes
> >>>>>>
> >>>>>> When performing live migration or memory hot-plugging,
> >>>>>> the changes to the device and vrings made by message handler
> >>>>>> done independently from vring usage by PMD threads.
> >>>>>>
> >>>>>> This causes for example segfauls during live-migration
> >>>>>
> >>>>> segfauls ->segfaults?
> >>>>>
> >>>>>> with MQ enable, but in general virtually any request
> >>>>>> sent by qemu changing the state of device can cause
> >>>>>> problems.
> >>>>>>
> >>>>>> These patches fixes all above issues by adding a spinlock
> >>>>>> to every vring and requiring message handler to start operation
> >>>>>> only after ensuring that all PMD threads related to the divece
> >>>>>
> >>>>> Another typo: divece.
> >>>>>
> >>>>>> are out of critical section accessing the vring data.
> >>>>>>
> >>>>>> Each vring has its own lock in order to not create contention
> >>>>>> between PMD threads of different vrings and to prevent
> >>>>>> performance degradation by scaling queue pair number.
> >>>>>
> >>>>> Also wonder how much overhead it brings.
> >>>>>
> >>>>> Instead of locking each vring, can we just, waiting a while (10us for
> >> example)
> >>>> after call destroy_device() callback so that every PMD thread has
> enough
> >>>> time to skip out the criterial area?
> >>>>
> >>>> No, because we are not destroying the device when it is needed.
> >>>> Actually, once destroy_device() is called, it is likely that the
> >>>> application has taken care the ring aren't being processed anymore
> >>>> before returning from the callback (This is at least the case with Vhost
> >>>> PMD).
> >>>
> >>> OK, I did not put it right way as there are multiple cases above: migration
> >> and memory hot plug. Let me try again:
> >>>
> >>> Whenever a vhost thread handles a message affecting PMD threads,
> (like
> >> SET_MEM_TABLE, GET_VRING_BASE, etc) we can remove the dev flag -
> >> VIRTIO_DEV_RUNNING, and wait for a while so that PMD threads skip out
> of
> >> those criterial area. After message handling, reset the flag -
> >> VIRTIO_DEV_RUNNING.
> >>
> >> I think you mean clearing vq's enabled flag, because PMD threads never
> >> check the VIRTIO_DEV_RUNNING flag.
> >
> > Ah, yes.
> >
> >>
> >>> I suppose it can work, basing on an assumption that PMD threads work
> in
> >> polling mode and can skip criterial area quickly and inevitably.
> >>
> >> That sounds very fragile, because if the CPU aren't perfectly isolated,
> >> your PMD thread can be preempted for interrupt handling for example.
> >>
> >> Or what if for some reason the PMD thread CPU stalls for a short while?
> >>
> >> The later is unlikely, but if it happens, it will be hard to debug.
> >>
> >> Let's see first the performance impact of using the spinlock. It might
> >> not be that important because 99.9999% of the times, it will not even
> >> spin.
> >
> > Fair enough.
> 
> I did some benchmarks on my Broadwell test bench (see patch below), and
> it seems that depending on the benchmark, perfs are on par, or better
> with the spinlock! I guess it explains because with the spinlock there
> is a better batching and less concurrent accesses on the rings, but I'm
> not sure.
> 
> Please find my results below (CPU E5-2667 v4 @ 3.20GHz):
> 
>        Bench         v17.11     v17.11 + spinlock
>   ---------------- ----------- -------------------
>    PVP Bidir run1   19.29Mpps   19.26Mpps
>    PVP Bidir run2   19.26Mpps   19.28Mpps
>    TxOnly           18.47Mpps   18.83Mpps
>    RxOnly           13.83Mpps   13.83Mpps
>    IO Loopback      7.94Mpps    7.97Mpps
> 

This number seems really good for throughput.

FYI, we are recently doing a test to measure how long it takes for a noop (means there is no packets on the virtqueue) vhost dequeue operation. It's like:

    start_tsc = rte_rdtsc();
    for (j = 0; j < 10000000; ++j) 
            nb_rx = rte_eth_rx_burst(port_id, queue_id, pkts_burst, nb_pkt_per_burst);
    end_tsc = rte_rdtsc();
    printf("%"PRIu64"\n", (end_tsc - start_tsc) / 10000000);

Turns out that this patch will make it from 50 cycles -> 80 cycles for each noop rte_eth_rx_burst() operation on vhost port. My server is Haswell 2.3GHz.

Note that I'm not against this patch. Just that if such operation takes so many cycles, we might consider to introduce interrupt mode for vhost ports.

Thanks,
Jianfeng


> 
> 
> Patch:
> ---------------------------------------------------------
> 
>  From 7e18cefce682235558fed66a1fb87ab937ec9297 Mon Sep 17 00:00:00
> 2001
> From: Maxime Coquelin <maxime.coquelin at redhat.com>
> Date: Fri, 8 Dec 2017 04:21:25 -0500
> Subject: [PATCH] vhost: spinlock benchmarking
> 
> Signed-off-by: Maxime Coquelin <maxime.coquelin at redhat.com>
> ---
>   lib/librte_vhost/vhost.c      |  2 ++
>   lib/librte_vhost/vhost.h      |  2 ++
>   lib/librte_vhost/virtio_net.c | 12 ++++++++++++
>   3 files changed, 16 insertions(+)
> 
> diff --git a/lib/librte_vhost/vhost.c b/lib/librte_vhost/vhost.c
> index 4f8b73a..d9752cf 100644
> --- a/lib/librte_vhost/vhost.c
> +++ b/lib/librte_vhost/vhost.c
> @@ -219,6 +219,8 @@ struct virtio_net *
>   	vq->kickfd = VIRTIO_UNINITIALIZED_EVENTFD;
>   	vq->callfd = VIRTIO_UNINITIALIZED_EVENTFD;
> 
> +	rte_spinlock_init(&vq->access_lock);
> +
>   	vhost_user_iotlb_init(dev, vring_idx);
>   	/* Backends are set to -1 indicating an inactive device. */
>   	vq->backend = -1;
> diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
> index 1cc81c1..56242a8 100644
> --- a/lib/librte_vhost/vhost.h
> +++ b/lib/librte_vhost/vhost.h
> @@ -131,6 +131,8 @@ struct vhost_virtqueue {
>   	struct batch_copy_elem	*batch_copy_elems;
>   	uint16_t		batch_copy_nb_elems;
> 
> +	rte_spinlock_t access_lock;
> +
>   	rte_rwlock_t	iotlb_lock;
>   	rte_rwlock_t	iotlb_pending_lock;
>   	struct rte_mempool *iotlb_pool;
> diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
> index 6fee16e..50cc3de 100644
> --- a/lib/librte_vhost/virtio_net.c
> +++ b/lib/librte_vhost/virtio_net.c
> @@ -329,6 +329,8 @@
>   	if (unlikely(vq->enabled == 0))
>   		return 0;
> 
> +	rte_spinlock_lock(&vq->access_lock);
> +
>   	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
>   		vhost_user_iotlb_rd_lock(vq);
> 
> @@ -419,6 +421,8 @@
>   	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
>   		vhost_user_iotlb_rd_unlock(vq);
> 
> +	rte_spinlock_unlock(&vq->access_lock);
> +
>   	return count;
>   }
> 
> @@ -654,6 +658,8 @@
>   	if (unlikely(vq->enabled == 0))
>   		return 0;
> 
> +	rte_spinlock_lock(&vq->access_lock);
> +
>   	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
>   		vhost_user_iotlb_rd_lock(vq);
> 
> @@ -715,6 +721,8 @@
>   	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
>   		vhost_user_iotlb_rd_unlock(vq);
> 
> +	rte_spinlock_unlock(&vq->access_lock);
> +
>   	return pkt_idx;
>   }
> 
> @@ -1183,6 +1191,8 @@
>   	if (unlikely(vq->enabled == 0))
>   		return 0;
> 
> +	rte_spinlock_lock(&vq->access_lock);
> +
>   	vq->batch_copy_nb_elems = 0;
> 
>   	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
> @@ -1356,6 +1366,8 @@
>   	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
>   		vhost_user_iotlb_rd_unlock(vq);
> 
> +	rte_spinlock_unlock(&vq->access_lock);
> +
>   	if (unlikely(rarp_mbuf != NULL)) {
>   		/*
>   		 * Inject it to the head of "pkts" array, so that switch's mac
> --
> 1.8.3.1