linux-kernel - Re: [PATCH 6/6] vhost_net: don't poll on -EFAULT

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <4F8D05AF.4000309@redhat.com>
Date:	Tue, 17 Apr 2012 13:54:55 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	netdev@...r.kernel.org, xma@...ibm.com, davem@...emloft.net,
	linux-kernel@...r.kernel.org, ebiederm@...ssion.com
Subject: Re: [PATCH 6/6] vhost_net: don't poll on -EFAULT

On 04/17/2012 12:57 PM, Michael S. Tsirkin wrote:
> On Tue, Apr 17, 2012 at 11:27:01AM +0800, Jason Wang wrote:
>> On 04/16/2012 09:39 PM, Michael S. Tsirkin wrote:
>>> On Mon, Apr 16, 2012 at 04:28:10PM +0800, Jason Wang wrote:
>>>>>   On 04/16/2012 03:16 PM, Michael S. Tsirkin wrote:
>>>>>>   >On Mon, Apr 16, 2012 at 02:08:33PM +0800, Jason Wang wrote:
>>>>>>>   >>Currently, we restart tx polling unconditionally when sendmsg()
>>>>>>>   >>fails. This would cause unnecessary wakeups of vhost wokers as it's
>>>>>>>   >>only needed when the socket send buffer were exceeded.
>>>>>>   >Why is this a problem?
>>>>>   >   This issue is when guest driver is able to hit the
>>>> -EFAULT, vhost
>>>>>   discard the the descriptor and restart the polling. This would wake
>>>>>   vhost thread and repeat the loop again which waste cpu.
>>> Does same thing happen if we get an error from copy from user?
>>>
>> Right, so do you think it makes sense that we only restart polling
>> on -EAGAIN or -ENOBUFS?
> Sounds OK. BTW how do you test this?
>

Not very hard, w/o this patch, we can see almost 100% cpu utilization 
for vhost thread if guest hit EFAULT or EINVAL. With this patch, the cpu 
utilization should be very low I think.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/