netdev - RE: [PATCH net v2 2/2] vhost_net: fix high cpu load when sendmsg fails

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Date:   Wed, 23 Dec 2020 13:21:11 +0000
From:   wangyunjian <wangyunjian@...wei.com>
To:     Jason Wang <jasowang@...hat.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>
CC:     Network Development <netdev@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "Lilijun (Jerry)" <jerry.lilijun@...wei.com>,
        chenchanghu <chenchanghu@...wei.com>,
        xudingke <xudingke@...wei.com>,
        "huangbin (J)" <brian.huangbin@...wei.com>
Subject: RE: [PATCH net v2 2/2] vhost_net: fix high cpu load when sendmsg
 fails

> -----Original Message-----
> From: Jason Wang [mailto:jasowang@...hat.com]
> Sent: Wednesday, December 23, 2020 10:54 AM
> To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
> Cc: wangyunjian <wangyunjian@...wei.com>; Network Development
> <netdev@...r.kernel.org>; Michael S. Tsirkin <mst@...hat.com>;
> virtualization@...ts.linux-foundation.org; Lilijun (Jerry)
> <jerry.lilijun@...wei.com>; chenchanghu <chenchanghu@...wei.com>;
> xudingke <xudingke@...wei.com>; huangbin (J)
> <brian.huangbin@...wei.com>
> Subject: Re: [PATCH net v2 2/2] vhost_net: fix high cpu load when sendmsg fails
> 
> 
> On 2020/12/22 下午10:24, Willem de Bruijn wrote:
> > On Mon, Dec 21, 2020 at 11:41 PM Jason Wang <jasowang@...hat.com>
> wrote:
> >>
> >> On 2020/12/22 上午7:07, Willem de Bruijn wrote:
> >>> On Wed, Dec 16, 2020 at 3:20 AM wangyunjian<wangyunjian@...wei.com>
> wrote:
> >>>> From: Yunjian Wang<wangyunjian@...wei.com>
> >>>>
> >>>> Currently we break the loop and wake up the vhost_worker when
> >>>> sendmsg fails. When the worker wakes up again, we'll meet the same
> >>>> error.
> >>> The patch is based on the assumption that such error cases always
> >>> return EAGAIN. Can it not also be ENOMEM, such as from tun_build_skb?
> >>>
> >>>> This will cause high CPU load. To fix this issue, we can skip this
> >>>> description by ignoring the error. When we exceeds sndbuf, the
> >>>> return value of sendmsg is -EAGAIN. In the case we don't skip the
> >>>> description and don't drop packet.
> >>> the -> that
> >>>
> >>> here and above: description -> descriptor
> >>>
> >>> Perhaps slightly revise to more explicitly state that
> >>>
> >>> 1. in the case of persistent failure (i.e., bad packet), the driver
> >>> drops the packet 2. in the case of transient failure (e.g,. memory
> >>> pressure) the driver schedules the worker to try again later
> >>
> >> If we want to go with this way, we need a better time to wakeup the
> >> worker. Otherwise it just produces more stress on the cpu that is
> >> what this patch tries to avoid.
> > Perhaps I misunderstood the purpose of the patch: is it to drop
> > everything, regardless of transient or persistent failure, until the
> > ring runs out of descriptors?
> 
> 
> My understanding is that the main motivation is to avoid high cpu utilization
> when sendmsg() fail due to guest reason (e.g bad packet).
> 

My main motivation is to avoid the tx queue stuck.

Should I describe it like this:
Currently the driver don't drop a packet which can't be send by tun
(e.g bad packet). In this case, the driver will always process the
same packet lead to the tx queue stuck.

To fix this issue:
1. in the case of persistent failure (e.g bad packet), the driver can skip
this descriptior by ignoring the error.
2. in the case of transient failure (e.g -EAGAIN and -ENOMEM), the driver
schedules the worker to try again.

Thanks

> 
> >
> > I can understand both a blocking and drop strategy during memory
> > pressure. But partial drop strategy until exceeding ring capacity
> > seems like a peculiar hybrid?
> 
> 
> Yes. So I wonder if we want to be do better when we are in the memory
> pressure. E.g can we let socket wake up us instead of rescheduling the
> workers here? At least in this case we know some memory might be freed?
> 
> Thanks
> 
> 
> >