From: "Xie, Huawei" <huawei.xie@intel.com>
To: Tom Kiely <tkiely@brocade.com>, Kyle Larose <eomereadig@gmail.com>
Cc: "dev@dpdk.org" <dev@dpdk.org>
Subject: Re: [dpdk-dev] [PATCH] virtio: fix rx ring descriptor starvation
Date: Tue, 23 Feb 2016 08:26:28 +0000 [thread overview]
Message-ID: <C37D651A908B024F974696C65296B57B4C5F0BCD@SHSMSX101.ccr.corp.intel.com> (raw)
In-Reply-To: <56CB35E9.50309@brocade.com>
On 2/23/2016 12:23 AM, Tom Kiely wrote:
> Hi,
> Sorry I missed the last few messages until now. I'm happy with
> just removing the "if". Kyle, when you say you fixed it, do you mean
> that you will push the patch or have already done so ?
> Thanks,
> Tom
>
> On 02/18/2016 02:03 PM, Kyle Larose wrote:
>> On Tue, Jan 5, 2016 at 2:13 AM, Xie, Huawei <huawei.xie@intel.com>
>> 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@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.
>>>
>> FWIW, I fixed this issue myself by just removing the if(num == 0)
>> checks entirely. I didn't see any benefit in short-circuiting a loop
>> which pretty much does nothing anyway when num == 0. Further, we only
>> hit this case when there's no packets to receive, which means there's
>> probably a few cycles to spare. This is even simpler.
Yes, as i said, that is the simplest fix.
>>
>>> I find virtqueue_full is confusing, maybe we could change it to some
>>> other meaningful name.
>>>
>>>> Tom
>>>>
>>>>>> return 0;
>>>>>> num = virtqueue_dequeue_burst_rx(rxvq, rcv_pkts, len, num);
>>>>>> @@ -683,7 +684,8 @@ virtio_recv_mergeable_pkts(void *rx_queue,
>>>>>> virtio_rmb();
>>>>>> - if (nb_used == 0)
>>>>>> + /* Refill free descriptors even if no pkts recvd */
>>>>>> + if (nb_used == 0 && virtqueue_full(rxvq))
>>>>>> return 0;
>>>>>> PMD_RX_LOG(DEBUG, "used:%d\n", nb_used);
>>>>
>
>
next prev parent reply other threads:[~2016-02-23 8:26 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-11-13 9:30 Tom Kiely
2015-11-24 21:20 ` Thomas Monjalon
2015-11-25 1:50 ` Yuanhan Liu
2015-11-25 2:51 ` Xie, Huawei
2015-11-25 17:32 ` Xie, Huawei
[not found] ` <C37D651A908B024F974696C65296B57B4C545F75@SHSMSX101.ccr.corp.intel.com>
2015-12-17 9:22 ` Tom Kiely
2015-12-17 11:18 ` Tom Kiely
2016-01-05 7:13 ` Xie, Huawei
2016-02-10 15:07 ` Bruce Richardson
2016-02-18 14:03 ` Kyle Larose
2016-02-22 16:23 ` Tom Kiely
2016-02-23 8:26 ` Xie, Huawei [this message]
2016-03-04 6:16 ` Xie, Huawei
2016-03-04 8:11 ` Tom Kiely
2016-03-04 13:25 ` Kyle Larose
2016-03-09 21:37 ` Bruce Richardson
2016-03-10 14:46 ` Kyle Larose
2016-02-26 18:58 Kyle Larose
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