From mboxrd@z Thu Jan 1 00:00:00 1970 From: Zoltan Kiss Subject: Re: [PATCH net-next] xen-netback: Rework rx_work_todo Date: Wed, 15 Jan 2014 15:08:12 +0000 Message-ID: <52D6A45C.1060705@citrix.com> References: <1389727719-21439-1-git-send-email-zoltan.kiss@citrix.com> <20140115103707.GI5698@zion.uk.xensource.com> <52D67536.4030106@citrix.com> <20140115144519.GO5698@zion.uk.xensource.com> <52D6A0B9.2030204@citrix.com> <20140115145951.GP5698@zion.uk.xensource.com> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1"; format=flowed Content-Transfer-Encoding: 7bit Cc: , , , , To: Wei Liu Return-path: In-Reply-To: <20140115145951.GP5698@zion.uk.xensource.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 15/01/14 14:59, Wei Liu wrote: > On Wed, Jan 15, 2014 at 02:52:41PM +0000, Zoltan Kiss wrote: >> On 15/01/14 14:45, Wei Liu wrote: >>>>>> The recent patch to fix receive side flow control (11b57f) solved the spinning >>>>>>>>> thread problem, however caused an another one. The receive side can stall, if: >>>>>>>>> - xenvif_rx_action sets rx_queue_stopped to false >>>>>>>>> - interrupt happens, and sets rx_event to true >>>>>>>>> - then xenvif_kthread sets rx_event to false >>>>>>>>> >>>>>>> >>>>>>> If you mean "rx_work_todo" returns false. >>>>>>> >>>>>>> In this case >>>>>>> >>>>>>> (!skb_queue_empty(&vif->rx_queue) && !vif->rx_queue_stopped) || vif->rx_event; >>>>>>> >>>>>>> can still be true, can't it? >>>>> Sorry, I should wrote rx_queue_stopped to true >>>>> >>> In this case, if rx_queue_stopped is true, then we're expecting frontend >>> to notify us, right? >>> >>> rx_queue_stopped is set to true if we cannot make any progress to queue >>> packet into the ring. In that situation we can expect frontend will send >>> notification to backend after it goes through the backlog in the ring. >>> That means rx_event is set to true, and rx_work_todo is true again. So >>> the ring is actually not stalled in this case as well. Did I miss >>> something? >>> >> >> Yes, we expect the guest to notify us, and it does, and we set >> rx_event to true (see second point), but then the thread set it to >> false (see third point). Talking with Paul, another solution could >> be to set rx_event false before calling xenvif_rx_action. But using >> rx_last_skb_slots makes it quicker for the thread to see if it >> doesn't have to do anything. >> > > OK, this is a better explaination. So actually there's no bug in the > original implementation and your patch is sort of an improvement. > > Could you send a new version of this patch with relevant information in > commit message? Talking to people offline is faster, but I would like to > have public discussion and relevant information archived in a searchable > form. Thanks. No, there is a bug in the original implementation: - [THREAD] xenvif_rx_action sets rx_queue_stopped to true - [INTERRUPT] interrupt happens, and sets rx_event to true - [THREAD] then xenvif_kthread sets rx_event to false - [THREAD] rx_work_todo never returns true anymore I will update the explanation and send in the patch again. Zoli