[PATCH v3] kni: fix possible alloc_q starvation when mbufs are exhausted

[Thomas Monjalon]

04/01/2023 15:34, Ferruh Yigit:
> On 1/4/2023 11:57 AM, Matt wrote:
> > Hi Ferruh,
> > 
> > In my case, the traffic is not large, so I can't see the impact.
> > I also tested under high load(>2Mpps with 2 DPDK cores and 2 kernel threads)
> > and found no significant difference in performance either.
> > I think the reason should be that it will not
> > run to 'kni_fifo_count(kni->alloc_q) == 0' under high load.
> > 
> 
> I agree, additional check most likely hit on the low bandwidth,
> thanks for checking for performance impact.
> 
> > On Tue, Jan 3, 2023 at 8:47 PM Ferruh Yigit <ferruh.yigit at amd.com
> > <mailto:ferruh.yigit at amd.com>> wrote:
> > 
> >     On 12/30/2022 4:23 AM, Yangchao Zhou wrote:
> >     > In some scenarios, mbufs returned by rte_kni_rx_burst are not freed
> >     > immediately. So kni_allocate_mbufs may be failed, but we don't know.
> >     >
> >     > Even worse, when alloc_q is completely exhausted, kni_net_tx in
> >     > rte_kni.ko will drop all tx packets. kni_allocate_mbufs is never
> >     > called again, even if the mbufs are eventually freed.
> >     >
> >     > In this patch, we try to allocate mbufs for alloc_q when it is empty.
> >     >
> >     > According to historical experience, the performance bottleneck of KNI
> >     > is offen the usleep_range of kni thread in rte_kni.ko.
> >     > The check of kni_fifo_count is trivial and the cost should be
> >     acceptable.
> >     >
> > 
> >     Hi Yangchao,
> > 
> >     Are you observing any performance impact with this change in you use
> >     case?
> > 
> > 
> >     > Fixes: 3e12a98fe397 ("kni: optimize Rx burst")
> >     > Cc: stable at dpdk.org <mailto:stable at dpdk.org>
> >     >
> >     > Signed-off-by: Yangchao Zhou <zhouyates at gmail.com
> >     <mailto:zhouyates at gmail.com>>
> 
> Acked-by: Ferruh Yigit <ferruh.yigit at amd.com>

Applied, thanks.