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SubjectRe: [PATCH 1/3] tuntap: rx batching
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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 <jasowang@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.

>
>
>> ---
>> 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

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