[dpdk-dev] [PATCH] virtio: fix rx ring descriptor starvation
Bruce Richardson
bruce.richardson at intel.com
Wed Feb 10 16:07:01 CET 2016
On Tue, Jan 05, 2016 at 07:13:04AM +0000, Xie, Huawei wrote:
> On 12/17/2015 7:18 PM, Tom Kiely wrote:
> >
> >
> > On 11/25/2015 05:32 PM, Xie, Huawei wrote:
> >> On 11/13/2015 5:33 PM, Tom Kiely wrote:
> >>> If all rx descriptors are processed while transient
> >>> mbuf exhaustion is present, the rx ring ends up with
> >>> no available descriptors. Thus no packets are received
> >>> on that ring. Since descriptor refill is performed post
> >>> rx descriptor processing, in this case no refill is
> >>> ever subsequently performed resulting in permanent rx
> >>> traffic drop.
> >>>
> >>> Signed-off-by: Tom Kiely <tkiely at brocade.com>
> >>> ---
> >>> drivers/net/virtio/virtio_rxtx.c | 6 ++++--
> >>> 1 file changed, 4 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/drivers/net/virtio/virtio_rxtx.c
> >>> b/drivers/net/virtio/virtio_rxtx.c
> >>> index 5770fa2..a95e234 100644
> >>> --- a/drivers/net/virtio/virtio_rxtx.c
> >>> +++ b/drivers/net/virtio/virtio_rxtx.c
> >>> @@ -586,7 +586,8 @@ virtio_recv_pkts(void *rx_queue, struct rte_mbuf
> >>> **rx_pkts, uint16_t nb_pkts)
> >>> if (likely(num > DESC_PER_CACHELINE))
> >>> num = num - ((rxvq->vq_used_cons_idx + num) %
> >>> DESC_PER_CACHELINE);
> >>> - if (num == 0)
> >>> + /* Refill free descriptors even if no pkts recvd */
> >>> + if (num == 0 && virtqueue_full(rxvq))
> >> Should the return condition be that no used buffers and we have avail
> >> descs in avail ring, i.e,
> >> num == 0 && rxvq->vq_free_cnt != rxvq->vq_nentries
> >>
> >> rather than
> >> num == 0 && rxvq->vq_free_cnt == 0
> > Yes we could do that but I don't see a good reason to wait until the
> > vq_free_cnt == vq_nentries
> > before attempting the refill. The existing code will attempt refill
> > even if only 1 packet was received
> > and the free count is small. To me it seems safer to extend that to
> > try refill even if no packet was received
> > but the free count is non-zero.
> The existing code attempt to refill only if 1 packet was received.
>
> If we want to refill even no packet was received, then the strict
> condition should be
> num == 0 && rxvq->vq_free_cnt != rxvq->vq_nentries
>
> The safer condition, what you want to use, should be
> num == 0 && !virtqueue_full(...)
> rather than
> num == 0 && virtqueue_full(...)
>
> We could simplify things a bit, just remove this check, if the following
> receiving code already takes care of the "num == 0" condition.
>
> I find virtqueue_full is confusing, maybe we could change it to some
> other meaningful name.
>
> >
> > Tom
> >
Ping.
Tom and Huawei, what is the status of this patch? Will there be a V2?
/Bruce
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