Re: [dpdk-dev] [PATCH v4 2/2] virtio: one way barrier for packed vring desc used flags

["Gavin Hu (Arm Technology China)" <Gavin.Hu@arm.com>]

Hi Yinan,

Thanks for verification and reporting this, I will dump the assembly code and make an analysis after I am back from travel.
From my understanding the assembly code for x86 should be same, but I am not sure if anything we were missing. 

Is this <1 perf drop coming from run to run variances, or system noise? 

/Gavin

> -----Original Message-----
> From: Wang, Yinan <yinan.wang@intel.com>
> Sent: Wednesday, September 18, 2019 7:21 AM
> To: Joyce Kong (Arm Technology China) <Joyce.Kong@arm.com>;
> dev@dpdk.org
> Cc: nd <nd@arm.com>; maxime.coquelin@redhat.com; Richardson, Bruce
> <bruce.richardson@intel.com>; Bie, Tiwei <tiwei.bie@intel.com>; Wang,
> Zhihong <zhihong.wang@intel.com>; amorenoz@redhat.com; Wang, Xiao W
> <xiao.w.wang@intel.com>; Liu, Yong <yong.liu@intel.com>;
> jfreimann@redhat.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; Gavin Hu (Arm Technology China)
> <Gavin.Hu@arm.com>
> Subject: RE: [PATCH v4 2/2] virtio: one way barrier for packed vring desc used
> flags
> 
> 
> Hi Joyce,
> 
> I test performance impact of your patch set with code base commit id:
> d03d8622db48918d14bfe805641b1766ecc40088, after applying your v4 patch
> set , packed ring shows small performance drop as below:
> 
> PVP vhost/virtio 1c1q test	       commit:d03d8622db48918	apply v4
> patch set
> 
> pvp_virtio11_mergeable	                   7.218	           7.147
> pvp_virtio11_normal	                   7.217	           7.182
> 
> 
> Thanks,
> Yinan
> 
> 
> 
> 
> > -----Original Message-----
> > From: Joyce Kong [mailto:joyce.kong@arm.com]
> > Sent: 2019年9月17日 13:28
> > To: dev@dpdk.org
> > Cc: nd@arm.com; maxime.coquelin@redhat.com; Wang, Yinan
> > <yinan.wang@intel.com>; Richardson, Bruce <bruce.richardson@intel.com>;
> Bie,
> > Tiwei <tiwei.bie@intel.com>; Wang, Zhihong <zhihong.wang@intel.com>;
> > amorenoz@redhat.com; Wang, Xiao W <xiao.w.wang@intel.com>; Liu, Yong
> > <yong.liu@intel.com>; jfreimann@redhat.com;
> honnappa.nagarahalli@arm.com;
> > gavin.hu@arm.com
> > Subject: [PATCH v4 2/2] virtio: one way barrier for packed vring desc used
> flags
> >
> > In case VIRTIO_F_ORDER_PLATFORM(36) is not negotiated, then the
> frontend
> > and backend are assumed to be implemented in software, that is they can
> run on
> > identical CPUs in an SMP configuration.
> > Thus a weak form of memory barriers like rte_smp_r/wmb, other than
> > rte_cio_r/wmb, is sufficient for this case(vq->hw->weak_barriers == 1) and
> > yields better performance.
> > For the above case, this patch helps yielding even better performance by
> > replacing the two-way barriers with C11 one-way barriers for used flags in
> packed
> > ring.
> >
> > Signed-off-by: Joyce Kong <joyce.kong@arm.com>
> > Reviewed-by: Gavin Hu <gavin.hu@arm.com>
> > Reviewed-by: Phil Yang <phil.yang@arm.com>
> > Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> > ---
> >  drivers/net/virtio/virtio_rxtx.c                 | 12 +++++++---
> >  drivers/net/virtio/virtio_user/virtio_user_dev.c |  4 ++--
> >  drivers/net/virtio/virtqueue.h                   | 28
> > +++++++++++++++++++++++-
> >  lib/librte_vhost/virtio_net.c                    |  5 ++---
> >  4 files changed, 40 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
> > index a87ffe1..2f0879c 100644
> > --- a/drivers/net/virtio/virtio_rxtx.c
> > +++ b/drivers/net/virtio/virtio_rxtx.c
> > @@ -122,9 +122,11 @@ virtqueue_dequeue_burst_rx_packed(struct
> virtqueue
> > *vq,
> >
> >  	for (i = 0; i < num; i++) {
> >  		used_idx = vq->vq_used_cons_idx;
> > +		/* desc_is_used has a load-acquire or rte_cio_rmb inside
> > +		 * and wait for used desc in virtqueue.
> > +		 */
> >  		if (!desc_is_used(&desc[used_idx], vq))
> >  			return i;
> > -		virtio_rmb(vq->hw->weak_barriers);
> >  		len[i] = desc[used_idx].len;
> >  		id = desc[used_idx].id;
> >  		cookie = (struct rte_mbuf *)vq->vq_descx[id].cookie; @@ -
> 233,8
> > +235,10 @@ virtio_xmit_cleanup_inorder_packed(struct virtqueue *vq, int
> num)
> >  	struct vq_desc_extra *dxp;
> >
> >  	used_idx = vq->vq_used_cons_idx;
> > +	/* desc_is_used has a load-acquire or rte_cio_rmb inside
> > +	 * and wait for used desc in virtqueue.
> > +	 */
> >  	while (num > 0 && desc_is_used(&desc[used_idx], vq)) {
> > -		virtio_rmb(vq->hw->weak_barriers);
> >  		id = desc[used_idx].id;
> >  		do {
> >  			curr_id = used_idx;
> > @@ -265,8 +269,10 @@ virtio_xmit_cleanup_normal_packed(struct
> virtqueue
> > *vq, int num)
> >  	struct vq_desc_extra *dxp;
> >
> >  	used_idx = vq->vq_used_cons_idx;
> > +	/* desc_is_used has a load-acquire or rte_cio_rmb inside
> > +	 * and wait for used desc in virtqueue.
> > +	 */
> >  	while (num-- && desc_is_used(&desc[used_idx], vq)) {
> > -		virtio_rmb(vq->hw->weak_barriers);
> >  		id = desc[used_idx].id;
> >  		dxp = &vq->vq_descx[id];
> >  		vq->vq_used_cons_idx += dxp->ndescs;
> > diff --git a/drivers/net/virtio/virtio_user/virtio_user_dev.c
> > b/drivers/net/virtio/virtio_user/virtio_user_dev.c
> > index 7911c39..1c575d0 100644
> > --- a/drivers/net/virtio/virtio_user/virtio_user_dev.c
> > +++ b/drivers/net/virtio/virtio_user/virtio_user_dev.c
> > @@ -698,8 +698,8 @@ virtio_user_handle_cq_packed(struct
> virtio_user_dev
> > *dev, uint16_t queue_idx)
> >  		if (vq->used_wrap_counter)
> >  			flags |= VRING_PACKED_DESC_F_AVAIL_USED;
> >
> > -		rte_smp_wmb();
> > -		vring->desc[vq->used_idx].flags = flags;
> > +		__atomic_store_n(&vring->desc[vq->used_idx].flags, flags,
> > +				 __ATOMIC_RELEASE);
> >
> >  		vq->used_idx += n_descs;
> >  		if (vq->used_idx >= dev->queue_size) { diff --git
> > a/drivers/net/virtio/virtqueue.h b/drivers/net/virtio/virtqueue.h index
> > b728ff8..8d7f197 100644
> > --- a/drivers/net/virtio/virtqueue.h
> > +++ b/drivers/net/virtio/virtqueue.h
> > @@ -54,6 +54,32 @@ virtio_wmb(uint8_t weak_barriers)
> >  		rte_cio_wmb();
> >  }
> >
> > +static inline uint16_t
> > +virtqueue_fetch_flags_packed(struct vring_packed_desc *dp,
> > +			      uint8_t weak_barriers)
> > +{
> > +	uint16_t flags;
> > +
> > +	if (weak_barriers) {
> > +/* x86 prefers to using rte_smp_rmb over __atomic_load_n as it reports
> > + * a better perf(~1.5%), which comes from the saved branch by the
> compiler.
> > + * The if and else branch are identical with the smp and cio barriers
> > +both
> > + * defined as compiler barriers on x86.
> > + */
> > +#ifdef RTE_ARCH_X86_64
> > +		flags = dp->flags;
> > +		rte_smp_rmb();
> > +#else
> > +		flags = __atomic_load_n(&dp->flags, __ATOMIC_ACQUIRE);
> #endif
> > +	} else {
> > +		flags = dp->flags;
> > +		rte_cio_rmb();
> > +	}
> > +
> > +	return flags;
> > +}
> > +
> >  static inline void
> >  virtqueue_store_flags_packed(struct vring_packed_desc *dp,
> >  			      uint16_t flags, uint8_t weak_barriers) @@ -307,7
> +333,7
> > @@ desc_is_used(struct vring_packed_desc *desc, struct virtqueue *vq)  {
> >  	uint16_t used, avail, flags;
> >
> > -	flags = desc->flags;
> > +	flags = virtqueue_fetch_flags_packed(desc, vq->hw->weak_barriers);
> >  	used = !!(flags & VRING_PACKED_DESC_F_USED);
> >  	avail = !!(flags & VRING_PACKED_DESC_F_AVAIL);
> >
> > diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c index
> > e7463ff..241d467 100644
> > --- a/lib/librte_vhost/virtio_net.c
> > +++ b/lib/librte_vhost/virtio_net.c
> > @@ -110,8 +110,6 @@ flush_shadow_used_ring_packed(struct virtio_net
> *dev,
> >  			used_idx -= vq->size;
> >  	}
> >
> > -	rte_smp_wmb();
> > -
> >  	for (i = 0; i < vq->shadow_used_idx; i++) {
> >  		uint16_t flags;
> >
> > @@ -147,7 +145,8 @@ flush_shadow_used_ring_packed(struct virtio_net
> *dev,
> >  		}
> >  	}
> >
> > -	vq->desc_packed[head_idx].flags = head_flags;
> > +	__atomic_store_n(&vq->desc_packed[head_idx].flags, head_flags,
> > +			 __ATOMIC_RELEASE);
> >
> >  	vhost_log_cache_used_vring(dev, vq,
> >  				head_idx *
> > --
> > 2.7.4