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Date: Thu, 10 Nov 2016 20:17:24 -0800 From: John Fastabend <john.fastabend@...il.com> To: "Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com> Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH 1/3] tuntap: rx batching 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 <jasowang@...hat.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
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