-----Original Message-----
From: Jason Wang [mailto:jasowang@redhat.com]
Sent: Tuesday, December 22, 2020 12:41 PM
To: Willem de Bruijn <willemdebruijn.kernel@gmail.com>; wangyunjian
[off-list ref]
Cc: Network Development <redacted>; Michael S. Tsirkin
[off-list ref]; virtualization@lists.linux-foundation.org; Lilijun (Jerry)
[off-list ref]; chenchanghu [off-list ref];
xudingke [off-list ref]; huangbin (J)
[off-list ref]
Subject: Re: [PATCH net v2 2/2] vhost_net: fix high cpu load when sendmsg fails


On 2020/12/22 上午7:07, Willem de Bruijn wrote:

quoted

On Wed, Dec 16, 2020 at 3:20 AM wangyunjian[off-list ref]

wrote:

quoted

quoted

From: Yunjian Wang<redacted>

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?

quoted

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.

The problem was initially discovered when a VM sent an abnormal packet,
which causing the VM can't send packets anymore. After this patch
"feb8892cb441c7 vhost_net: conditionally enable tx polling", there have
also been high CPU consumption issues. 

It is the first problem that I am actually more concerned with and want
to solve.

Thanks

Thanks

quoted