Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S936553AbcKXJRF (ORCPT ); Thu, 24 Nov 2016 04:17:05 -0500 Received: from mx0b-0016f401.pphosted.com ([67.231.156.173]:43912 "EHLO mx0b-0016f401.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933657AbcKXJRD (ORCPT ); Thu, 24 Nov 2016 04:17:03 -0500 Date: Thu, 24 Nov 2016 17:11:59 +0800 From: Jisheng Zhang To: Arnd Bergmann CC: , Marcin Wojtas , Gregory CLEMENT , Thomas Petazzoni , Andrew Lunn , "Jason Cooper" , , , "David S. Miller" , Sebastian Hesselbarth Subject: Re: [PATCH net-next 1/4] net: mvneta: Convert to be 64 bits compatible Message-ID: <20161124171159.2a82da4f@xhacker> In-Reply-To: <21520380.oWTKcrq8DS@wuerfel> References: <20161122164844.19566-1-gregory.clement@free-electrons.com> <20161124163327.1cc261ab@xhacker> <21520380.oWTKcrq8DS@wuerfel> X-Mailer: Claws Mail 3.14.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2016-11-24_04:,, signatures=0 X-Proofpoint-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1609300000 definitions=main-1611240162 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2333 Lines: 54 On Thu, 24 Nov 2016 10:00:36 +0100 Arnd Bergmann wrote: > On Thursday, November 24, 2016 4:37:36 PM CET Jisheng Zhang wrote: > > solB (a SW shadow cookie) perhaps gives a better performance: in hot path, > > such as mvneta_rx(), the driver accesses buf_cookie and buf_phys_addr of > > rx_desc which is allocated by dma_alloc_coherent, it's noncacheable if the > > device isn't cache-coherent. I didn't measure the performance difference, > > because in fact we take solA as well internally. From your experience, > > can the performance gain deserve the complex code? > > Yes, a read from uncached memory is fairly slow, so if you have a chance > to avoid that it will probably help. When adding complexity to the code, > it probably makes sense to take a runtime profile anyway quantify how > much it gains. > > On machines that have cache-coherent DMA, accessing the descriptor > should be fine, as you already have to load the entire cache line > to read the status field. > > Looking at this snippet: > > rx_status = rx_desc->status; > rx_bytes = rx_desc->data_size - (ETH_FCS_LEN + MVNETA_MH_SIZE); > data = (unsigned char *)rx_desc->buf_cookie; > phys_addr = rx_desc->buf_phys_addr; > pool_id = MVNETA_RX_GET_BM_POOL_ID(rx_desc); > bm_pool = &pp->bm_priv->bm_pools[pool_id]; > > if (!mvneta_rxq_desc_is_first_last(rx_status) || > (rx_status & MVNETA_RXD_ERR_SUMMARY)) { > err_drop_frame_ret_pool: > /* Return the buffer to the pool */ > mvneta_bm_pool_put_bp(pp->bm_priv, bm_pool, > rx_desc->buf_phys_addr); > err_drop_frame: > > > I think there is more room for optimizing if you start: you read > the status field twice (the second one in MVNETA_RX_GET_BM_POOL_ID) > and you can cache the buf_phys_addr along with the virtual address > once you add that. oh, yeah! buf_phy_addr could be included too. > > Generally speaking, I'd recommend using READ_ONCE()/WRITE_ONCE() > to access the descriptor fields, to ensure the compiler doesn't > add extra references as well as to annotate the expensive > operations. > > Arnd Got it. Thanks so much for the detailed guide.