Re: [dpdk-stable] [PATCH v3] vhost_user: protect active rings from async ring changes

[Maxime Coquelin <maxime.coquelin@redhat.com>]

Hi Victor,

On 12/14/2017 12:35 PM, Victor Kaplansky wrote:
> 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 segfaults during live-migration
> 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 device
> 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.
> 
> See https://bugzilla.redhat.com/show_bug.cgi?id=1450680
> 
> Signed-off-by: Victor Kaplansky <victork@redhat.com>
> Tested-by: Maxime Coquelin <maxime.coquelin@redhat.com>

Sorry, but I didn't tested this patch. I just benchmarked the fact to
add a lock in the hot paths.

> ---
> 
> v3:
>     o Added locking to enqueue flow.
>     o Enqueue path guarded as well as dequeue path.
>     o Changed name of active_lock.
>     o Added initialization of guarding spinlock.
>     o Reworked functions skimming over all virt-queues.
>     o Performance measurements done by Maxime Coquelin shows
>       no degradation in bandwidth and throughput.
>     o Spelling.
>     o Taking lock only on set operations.
>     o IOMMU messages are not guarded by access lock.
> 
> v2:
>     o Fixed checkpatch complains.
>     o Added Signed-off-by.
>     o Refined placement of guard to exclude IOMMU messages.
>     o TODO: performance degradation measurement.
> 
> 
>   lib/librte_vhost/vhost.h      | 15 ++++++++++
>   lib/librte_vhost/vhost.c      |  1 +
>   lib/librte_vhost/vhost_user.c | 65 +++++++++++++++++++++++++++++++++++++++++++
>   lib/librte_vhost/virtio_net.c | 20 +++++++++++--
>   4 files changed, 99 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
> index 1cc81c17..26e2c571 100644
> --- a/lib/librte_vhost/vhost.h
> +++ b/lib/librte_vhost/vhost.h
> @@ -137,6 +137,8 @@ struct vhost_virtqueue {
>   	TAILQ_HEAD(, vhost_iotlb_entry) iotlb_list;
>   	int				iotlb_cache_nr;
>   	TAILQ_HEAD(, vhost_iotlb_entry) iotlb_pending_list;
> +
> +	rte_spinlock_t	access_lock;

On previous revision of the patch, Jianfeng mentioned taking the lock 
has an impact when nothing to enqueue/dequeue (80 cycles vs. 50 cycles
IIRC). I wonder whether moving the lock closer to enabled field would
make it be in same cache line and so might improve performance.

>   } __rte_cache_aligned;
>   
>   /* Old kernels have no such macros defined */
> @@ -302,6 +304,19 @@ vhost_log_used_vring(struct virtio_net *dev, struct vhost_virtqueue *vq,
>   	vhost_log_write(dev, vq->log_guest_addr + offset, len);
>   }
>   
> +static __rte_always_inline void
> +vhost_user_access_lock(struct vhost_virtqueue *vq)
> +{
> +	rte_spinlock_lock(&vq->access_lock);
> +}
> +
> +static __rte_always_inline void
> +vhost_user_access_unlock(struct vhost_virtqueue *vq)
> +{
> +	rte_spinlock_unlock(&vq->access_lock);
> +}
> +
> +
>   /* Macros for printing using RTE_LOG */
>   #define RTE_LOGTYPE_VHOST_CONFIG RTE_LOGTYPE_USER1
>   #define RTE_LOGTYPE_VHOST_DATA   RTE_LOGTYPE_USER1
> diff --git a/lib/librte_vhost/vhost.c b/lib/librte_vhost/vhost.c
> index 4f8b73a0..dcc42fc7 100644
> --- a/lib/librte_vhost/vhost.c
> +++ b/lib/librte_vhost/vhost.c
> @@ -259,6 +259,7 @@ alloc_vring_queue(struct virtio_net *dev, uint32_t vring_idx)
>   
>   	dev->virtqueue[vring_idx] = vq;
>   	init_vring_queue(dev, vring_idx);
> +	rte_spinlock_init(&vq->access_lock);
>   
>   	dev->nr_vring += 1;
>   
> diff --git a/lib/librte_vhost/vhost_user.c b/lib/librte_vhost/vhost_user.c
> index f4c7ce46..a5b5e4be 100644
> --- a/lib/librte_vhost/vhost_user.c
> +++ b/lib/librte_vhost/vhost_user.c
> @@ -1190,12 +1190,48 @@ vhost_user_check_and_alloc_queue_pair(struct virtio_net *dev, VhostUserMsg *msg)
>   	return alloc_vring_queue(dev, vring_idx);
>   }
>   
> +static void
> +vhost_user_lock_all_queue_pairs(struct virtio_net *dev)
> +{
> +	unsigned int i = 0;
> +	unsigned int vq_num = 0;
> +
> +	while (vq_num < dev->nr_vring) {
> +		struct vhost_virtqueue *vq = dev->virtqueue[i];
> +
> +		if (vq) {
> +			vhost_user_access_lock(vq);
> +			vq_num++;
> +		}
> +		i++;
> +	}
> +}
> +
> +static void
> +vhost_user_unlock_all_queue_pairs(struct virtio_net *dev)
> +{
> +	unsigned int i = 0;
> +	unsigned int vq_num = 0;
> +
> +	while (vq_num < dev->nr_vring) {
> +		struct vhost_virtqueue *vq = dev->virtqueue[i];
> +
> +		if (vq) {
> +			vhost_user_access_unlock(vq);
> +			vq_num++;
> +		}
> +		i++;
> +	}
> +}
> +
>   int
>   vhost_user_msg_handler(int vid, int fd)
>   {
>   	struct virtio_net *dev;
>   	struct VhostUserMsg msg;
>   	int ret;
> +	int unlock_required = 0;
> +
>   
>   	dev = get_device(vid);
>   	if (dev == NULL)
> @@ -1241,6 +1277,32 @@ vhost_user_msg_handler(int vid, int fd)
>   		return -1;
>   	}
>   
> +	switch (msg.request.master) {
> +	case VHOST_USER_SET_FEATURES:
> +	case VHOST_USER_SET_PROTOCOL_FEATURES:
> +	case VHOST_USER_SET_OWNER:
> +	case VHOST_USER_RESET_OWNER:
> +	case VHOST_USER_SET_MEM_TABLE:
> +	case VHOST_USER_SET_LOG_BASE:
> +	case VHOST_USER_SET_LOG_FD:
> +	case VHOST_USER_SET_VRING_NUM:
> +	case VHOST_USER_SET_VRING_ADDR:
> +	case VHOST_USER_SET_VRING_BASE:
> +	case VHOST_USER_SET_VRING_KICK:
> +	case VHOST_USER_SET_VRING_CALL:
> +	case VHOST_USER_SET_VRING_ERR:
> +	case VHOST_USER_SET_VRING_ENABLE:

For the VRING specific requests, I think we could only take the
corresponding VQ lock instead of blocking all queues.

There might be some exceptions, like GET_VRING_BASE.

Maybe only protecting critical sections would be better?

> +	case VHOST_USER_SEND_RARP:
> +	case VHOST_USER_NET_SET_MTU:
> +	case VHOST_USER_SET_SLAVE_REQ_FD:
> +		vhost_user_lock_all_queue_pairs(dev
> +		unlock_required = 1;
> +		break;
> +	default:
> +		break;
> +
> +	}
> +
>   	switch (msg.request.master) {
>   	case VHOST_USER_GET_FEATURES:
>   		msg.payload.u64 = vhost_user_get_features(dev);
> @@ -1342,6 +1404,9 @@ vhost_user_msg_handler(int vid, int fd)
>   
>   	}
>   
> +	if (unlock_required)
> +		vhost_user_unlock_all_queue_pairs(dev);
> +
>   	if (msg.flags & VHOST_USER_NEED_REPLY) {
>   		msg.payload.u64 = !!ret;
>   		msg.size = sizeof(msg.payload.u64);
> diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
> index 6fee16e5..b584e380 100644
> --- a/lib/librte_vhost/virtio_net.c
> +++ b/lib/librte_vhost/virtio_net.c
> @@ -44,6 +44,7 @@
>   #include <rte_udp.h>
>   #include <rte_sctp.h>
>   #include <rte_arp.h>
> +#include <rte_spinlock.h>
>   
>   #include "iotlb.h"
>   #include "vhost.h"
> @@ -326,8 +327,11 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t queue_id,
>   	}
>   
>   	vq = dev->virtqueue[queue_id];
> +
> +	vhost_user_access_lock(vq);
I suggested to use rte_spinlock_trylock() when reviewing previous
revision. It would have the advantage to not block other devices being
handled on the same CPU whlie handling the message.

Any thoughts?

> +
>   	if (unlikely(vq->enabled == 0))
> -		return 0;
> +		goto out;
>   
>   	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
>   		vhost_user_iotlb_rd_lock(vq);
> @@ -416,6 +420,7 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t queue_id,
>   			&& (vq->callfd >= 0))
>   		eventfd_write(vq->callfd, (eventfd_t)1);
>   out:
> +	vhost_user_access_unlock(vq);

I think it should be moved after the iotlb lock unlock:
LOCK A
LOCK B
UNLOCK B
UNLOCK A

>   	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
>   		vhost_user_iotlb_rd_unlock(vq);
>   
> @@ -651,8 +656,11 @@ virtio_dev_merge_rx(struct virtio_net *dev, uint16_t queue_id,
>   	}
>   
>   	vq = dev->virtqueue[queue_id];
> +
> +	vhost_user_access_lock(vq);
> +
>   	if (unlikely(vq->enabled == 0))
> -		return 0;
> +		goto out;
>   
>   	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
>   		vhost_user_iotlb_rd_lock(vq);
> @@ -712,6 +720,7 @@ virtio_dev_merge_rx(struct virtio_net *dev, uint16_t queue_id,
>   	}
>   
>   out:
> +	vhost_user_access_unlock(vq);

Ditto

>   	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
>   		vhost_user_iotlb_rd_unlock(vq);
>   
> @@ -1180,9 +1189,12 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
>   	}
>   
>   	vq = dev->virtqueue[queue_id];
> +
>   	if (unlikely(vq->enabled == 0))
>   		return 0;
>   
> +	vhost_user_access_lock(vq);
> +

To be consistent with other paths, I guess you should lock before 
checking the vq is enabled.

>   	vq->batch_copy_nb_elems = 0;
>   
>   	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
> @@ -1240,6 +1252,7 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
>   		if (rarp_mbuf == NULL) {
>   			RTE_LOG(ERR, VHOST_DATA,
>   				"Failed to allocate memory for mbuf.\n");
> +			vhost_user_access_unlock(vq);
>   			return 0;
>   		}
>   
> @@ -1356,6 +1369,8 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
>   	if (dev->features & (1ULL << VIRTIO_F_IOMMU_PLATFORM))
>   		vhost_user_iotlb_rd_unlock(vq);
>   
> +	vhost_user_access_unlock(vq);
> +
>   	if (unlikely(rarp_mbuf != NULL)) {
>   		/*
>   		 * Inject it to the head of "pkts" array, so that switch's mac
> @@ -1366,5 +1381,6 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
>   		i += 1;
>   	}
>   
> +

Remove trailing line.

>   	return i;
>   }
> 

Thanks,
Maxime