From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Wang Subject: Re: [PATCH 1/3] tuntap: rx batching Date: Fri, 11 Nov 2016 10:07:44 +0800 Message-ID: References: <1478677113-13126-1-git-send-email-jasowang@redhat.com> <20161109183259-mutt-send-email-mst@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org To: "Michael S. Tsirkin" Return-path: Received: from mx1.redhat.com ([209.132.183.28]:51824 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S964913AbcKKCHs (ORCPT ); Thu, 10 Nov 2016 21:07:48 -0500 In-Reply-To: <20161109183259-mutt-send-email-mst@kernel.org> Sender: netdev-owner@vger.kernel.org List-ID: 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 > 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