Subject: Re: [PATCH] net-tun: restructure tun_do_read for better sleep/wakeup efficiency

From: Jason Wang <[email-masked]>
Date: Tue, 13 May 2014 16:46:59 +0800

On 05/13/2014 04:20 PM, Michael S. Tsirkin wrote:
> On Tue, May 13, 2014 at 02:15:25PM +0800, Jason Wang wrote:
>> On 05/12/2014 02:15 PM, Michael S. Tsirkin wrote:
>>> On Fri, May 09, 2014 at 11:10:43AM +0800, Jason Wang wrote:
>>>>> On 05/09/2014 02:22 AM, Xi Wang wrote:
>>>>>>> On Tue, May 6, 2014 at 8:40 PM, Jason Wang <[email-masked]> wrote:
>>>>>>>>> On 05/07/2014 08:24 AM, Xi Wang wrote:
>>>>>>>>>>> tun_do_read always adds current thread to wait queue, even if a packet
>>>>>>>>>>> is ready to read. This is inefficient because both sleeper and waker
>>>>>>>>>>> want to acquire the wait queue spin lock when packet rate is high.
>>>>>>>>> After commit 61a5ff15ebdab87887861a6b128b108404e4706d, this will only
>>>>>>>>> help for blocking read. Looks like for performance critical userspaces,
>>>>>>>>> they will use non blocking reads.
>>>>>>>>>>> We restructure the read function and use common kernel networking
>>>>>>>>>>> routines to handle receive, sleep and wakeup. With the change
>>>>>>>>>>> available packets are checked first before the reading thread is added
>>>>>>>>>>> to the wait queue.
>>>>>>>>> This is interesting, since it may help if we want to add rx busy loop
>>>>>>>>> for tun. (In fact I worked a similar patch like this).
>>>>>>> Yes this should be a good side effect and I am also interested in trying.
>>>>>>> Busy polling in user space is not ideal as it doesn't give the lowest latency.
>>>>>>> Besides differences in interrupt latency etc., there is a bad case for
>>>>>>> non blocking mode: When a packet arrives right before the polling thread
>>>>>>> returns to userspace. The control flow has to cross kernel/userspace
>>>>>>> boundary 3 times before the packet can be processed, while kernel
>>>>>>> blocking or busy polling only needs 1 boundary crossing.
>>>>> So if we want to implement this, we need a feature bit to turn it on.
>>>>> Then vhost may benefit from this.
>>> IFF_TUN_POLL_BUSY_LOOP ? I'm not sure it has to be
>>> a flag. Maybe an ioctl is better, if userspace
>>> misconfigures this it is only hurting itself, right?
>> Flag has the same effect. But adding new ioctls means userspace needs to
>> be modified. This is different with current rx busy polling for tcp/udp
>> socket which is transparent to userspace application.
> OTOH risk is much lower though.
>
>>> Maybe add a module parameter to control polling timeout,
>>> or reuse low_latency_poll.
>>>
>> If we don't need a global parameter, we can just implement it without
>> generic helper like __skb_recv_datagram().
> not sure I get the meaning here.
>

I mean current __skb_recv_datagram() does not accept a polling timeout
as parameter. So we need either extend it or just call
sk_can_busy_loop()/sk_busy_loop() in tun_do_read().