From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas Monjalon Subject: Re: [PATCH] cast used->idx to volatile Date: Wed, 01 Apr 2015 09:50:26 +0200 Message-ID: <13730553.gtSSln9O6Y@xps13> References: <1426925237-8312-1-git-send-email-haifeng.lin@huawei.com> <55191575.5020805@huawei.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7Bit Cc: dev-VfR2kkLFssw@public.gmane.org To: "Xie, Huawei" Return-path: In-Reply-To: List-Id: patches and discussions about DPDK List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces-VfR2kkLFssw@public.gmane.org Sender: "dev" 2015-03-30 15:56, Xie, Huawei: > On 3/30/2015 5:21 PM, Linhaifeng wrote: > > On 2015/3/24 18:06, Xie, Huawei wrote: > >> On 3/24/2015 3:44 PM, Linhaifeng wrote: > >>> On 2015/3/24 9:53, Xie, Huawei wrote: > >>>> On 3/24/2015 9:00 AM, Linhaifeng wrote: > >>>>> On 2015/3/23 20:54, Xie, Huawei wrote: > >>>>>> We have compiler barrier before and an external function call behind, so we don't need volatile here. > >>>>>> Do you meet issue? > >>>>>> > >>>>> Tx_q is sometimes stopped when we use virtio_net. Because vhost thought there are no buffers in tx_q and virtio_net > >>>>> though vhost haven't handle all packets so we have to restart VM to restore work. > >>>>> > >>>>> The status in VM is: > >>>>> Mar 18 17:11:10 linux-b2ij kernel: [46337.246687] net eth7: virtnet_poll > >>>>> Mar 18 17:11:10 linux-b2ij kernel: [46337.246690] net eth7: receive_buf > >>>>> Mar 18 17:11:10 linux-b2ij kernel: [46337.246693] net eth7: vi->num=239 > >>>>> Mar 18 17:11:10 linux-b2ij kernel: [46337.246695] net eth7: svq:avail->idx=52939 used->idx=52939 num_free=18 num_added=0 svq->last_used_idx=52820 > >>>>> Mar 18 17:11:10 linux-b2ij kernel: [46337.246699] net eth7: rvq:avail->idx=36215 used->idx=35977 num_free=18 num_added=0 rvq->last_used_idx=35977 > >>>>> Mar 18 17:11:11 linux-b2ij kernel: [46337.901038] net eth7: dev_queue_xmit, qdisc->flags=4, qdisc->state deactiveed=0 > >>>>> Mar 18 17:11:12 linux-b2ij kernel: [46337.901042] net eth7: dev_queue_xmit, txq->state=1, stopped=1 > >>>>> > >>>>> Why compiler barrier not take effect in our case? Is compiler barrier depended on -O3 option? We use -O2 option. > >>>> compiler barrier always works regardless of the optimization option. > >>>> I don't get your story, but the key thing is, do you check the asm code? > >>>> If called from outside as an API, how is it possible it is optimized? > >>>> there is only one update to used->idx in that function. > >>> Do you mean rte_vhost_enqueue_burst also not need cast used->idx to volatile ? Why not remove it? > >> I checked the code. Seems we can remove. That is another issue. > >> For your issue, you meet problem, and submit this this patch, but i am a > >> bit confused it is the root cause. Do you check the asm code that > >> volatile is optimized? > >> > > I had wrote a demo try to find out the different between rte_compiler_barrier and volatile. > > It seems no any effect on rte_compiler_barrier(). > > Haifeng: > > I think it doesn't make too much sense to use volatile for local variables. > > In our rte_vhost_dequeue_burst, there is one memory write to the > used->idx, and there is compiler barrier to keep the order. > Besides, as an API, how could that memory write to be optimized as > register access? > > Even if you call rte_vhost_dequeue_burst in the same src file, which > means in the same translation unit, there is function call after which > has side effect, it still couldn't be optimized. > > Anyway, could we directly check the asm code of rte_vhost_dequeue_burst > to see whether it is optimized? Conclusion is not clear. Is this patch rejected?