On 16-11-10 07:31 PM, Michael S. Tsirkin wrote:
> On Fri, Nov 11, 2016 at 10:07:44AM +0800, Jason Wang wrote:
>>
>>
>> On 2016年11月10日 00:38, Michael S. Tsirkin wrote:
>>> On Wed, Nov 09, 2016 at 03:38:31PM +0800, Jason Wang wrote:
>>>> Backlog were used for tuntap rx, but it can only process 1 packet at
>>>> one time since it was scheduled during sendmsg() synchronously in
>>>> process context. This lead bad cache utilization so this patch tries
>>>> to do some batching before call rx NAPI. This is done through:
>>>>
>>>> - accept MSG_MORE as a hint from sendmsg() caller, if it was set,
>>>> batch the packet temporarily in a linked list and submit them all
>>>> once MSG_MORE were cleared.
>>>> - implement a tuntap specific NAPI handler for processing this kind of
>>>> possible batching. (This could be done by extending backlog to
>>>> support skb like, but using a tun specific one looks cleaner and
>>>> easier for future extension).
>>>>
>>>> Signed-off-by: Jason Wang <jasow...@redhat.com>
>>> So why do we need an extra queue?
>>
>> The idea was borrowed from backlog to allow some kind of bulking and avoid
>> spinlock on each dequeuing.
>>
>>> This is not what hardware devices do.
>>> How about adding the packet to queue unconditionally, deferring
>>> signalling until we get sendmsg without MSG_MORE?
>>
>> Then you need touch spinlock when dequeuing each packet.
>
Random thought, I have a cmpxchg ring I am using for the qdisc work that
could possibly replace the spinlock implementation. I haven't figured
out the resizing API yet because I did not need it but I assume it could
help here and let you dequeue multiple skbs in one operation.
I can post the latest version if useful or an older version is
somewhere on patchworks as well.
.John
> It runs on the same CPU, right? Otherwise we should use skb_array...
>
>>>
>>>
>>>> ---
>>>> drivers/net/tun.c | 71
>>>> ++++++++++++++++++++++++++++++++++++++++++++++++++-----
>>>> 1 file changed, 65 insertions(+), 6 deletions(-)
>>>>
>>
>> [...]
>>
>>>> rxhash = skb_get_hash(skb);
>>>> - netif_rx_ni(skb);
>>>> + skb_queue_tail(&tfile->socket.sk->sk_write_queue, skb);
>>>> +
>>>> + if (!more) {
>>>> + local_bh_disable();
>>>> + napi_schedule(&tfile->napi);
>>>> + local_bh_enable();
>>> Why do we need to disable bh here? I thought napi_schedule can
>>> be called from any context.
>>
>> Yes, it's unnecessary. Will remove.
>>
>> Thanks
