From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-17.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A0D24C433ED for ; Fri, 2 Apr 2021 02:52:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 632406108B for ; Fri, 2 Apr 2021 02:52:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234133AbhDBCwa (ORCPT ); Thu, 1 Apr 2021 22:52:30 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:48141 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233870AbhDBCw3 (ORCPT ); Thu, 1 Apr 2021 22:52:29 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1617331948; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=sME/iN6vwvwo4Ih5YpRMgvNm1XNzNI9BHCvyMM7imLE=; b=TgNsfj/UCpJvw3lRAPs6RbLX/c36XXQYvKIS9F4aQPn6icpa3LmbMUw6AP/mDslPNjTgHC slewaaj+xUa9NiElJoaIs1FuFDDLc0se8QK8HdGuvDM0OGu1R6Q+tY8z83UBYcMKZIk2p9 JcNKhQ6tpxnsyMSzTOPZ0OiyC2re+gU= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-535-qDlrB3dKMvSQlv1kFgRIkg-1; Thu, 01 Apr 2021 22:52:26 -0400 X-MC-Unique: qDlrB3dKMvSQlv1kFgRIkg-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id E7975654; Fri, 2 Apr 2021 02:52:24 +0000 (UTC) Received: from wangxiaodeMacBook-Air.local (ovpn-12-24.pek2.redhat.com [10.72.12.24]) by smtp.corp.redhat.com (Postfix) with ESMTP id 4466F19C59; Fri, 2 Apr 2021 02:52:16 +0000 (UTC) Subject: Re: [PATCH net] net: avoid 32 x truesize under-estimation for tiny skbs To: Eric Dumazet , Xuan Zhuo Cc: Eric Dumazet , netdev , Alexander Duyck , Paolo Abeni , Greg Thelen , "David S . Miller" , Jakub Kicinski , su-lifan@linux.alibaba.com, "dust.li" , "Michael S. Tsirkin" References: <1617267183.5697193-1-xuanzhuo@linux.alibaba.com> From: Jason Wang Message-ID: <4a29cb99-749d-e697-34a6-db50361cedff@redhat.com> Date: Fri, 2 Apr 2021 10:52:12 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.9.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org 在 2021/4/1 下午5:58, Eric Dumazet 写道: > On Thu, Apr 1, 2021 at 11:04 AM Xuan Zhuo wrote: >> On Thu, 1 Apr 2021 15:14:18 +0800, Jason Wang wrote: >>> 在 2021/3/31 下午4:11, Michael S. Tsirkin 写道: >>>> On Mon, Mar 29, 2021 at 11:06:09AM +0200, Eric Dumazet wrote: >>>>> On Mon, Mar 29, 2021 at 10:52 AM Xuan Zhuo wrote: >>>>>> On Wed, 13 Jan 2021 08:18:19 -0800, Eric Dumazet wrote: >>>>>>> From: Eric Dumazet >>>>>>> >>>>>>> Both virtio net and napi_get_frags() allocate skbs >>>>>>> with a very small skb->head >>>>>>> >>>>>>> While using page fragments instead of a kmalloc backed skb->head might give >>>>>>> a small performance improvement in some cases, there is a huge risk of >>>>>>> under estimating memory usage. >>>>>>> >>>>>>> For both GOOD_COPY_LEN and GRO_MAX_HEAD, we can fit at least 32 allocations >>>>>>> per page (order-3 page in x86), or even 64 on PowerPC >>>>>>> >>>>>>> We have been tracking OOM issues on GKE hosts hitting tcp_mem limits >>>>>>> but consuming far more memory for TCP buffers than instructed in tcp_mem[2] >>>>>>> >>>>>>> Even if we force napi_alloc_skb() to only use order-0 pages, the issue >>>>>>> would still be there on arches with PAGE_SIZE >= 32768 >>>>>>> >>>>>>> This patch makes sure that small skb head are kmalloc backed, so that >>>>>>> other objects in the slab page can be reused instead of being held as long >>>>>>> as skbs are sitting in socket queues. >>>>>>> >>>>>>> Note that we might in the future use the sk_buff napi cache, >>>>>>> instead of going through a more expensive __alloc_skb() >>>>>>> >>>>>>> Another idea would be to use separate page sizes depending >>>>>>> on the allocated length (to never have more than 4 frags per page) >>>>>>> >>>>>>> I would like to thank Greg Thelen for his precious help on this matter, >>>>>>> analysing crash dumps is always a time consuming task. >>>>>> This patch causes a performance degradation of about 10% in the scenario of >>>>>> virtio-net + GRO. >>>>>> >>>>>> For GRO, there is no way to merge skbs based on frags with this patch, only >>>>>> frag_list can be used to link skbs. The problem that this cause are that compared >>>>>> to the GRO package merged into the frags way, the current skb needs to call >>>>>> kfree_skb_list to release each skb, resulting in performance degradation. >>>>>> >>>>>> virtio-net will store some data onto the linear space after receiving it. In >>>>>> addition to the header, there are also some payloads, so "headlen <= offset" >>>>>> fails. And skb->head_frag is failing when use kmalloc() for skb->head allocation. >>>>>> >>>>> Thanks for the report. >>>>> >>>>> There is no way we can make things both fast for existing strategies >>>>> used by _insert_your_driver >>>>> and malicious usages of data that can sit for seconds/minutes in socket queues. >>>>> >>>>> I think that if you want to gain this 10% back, you have to change >>>>> virtio_net to meet optimal behavior. >>>>> >>>>> Normal drivers make sure to not pull payload in skb->head, only headers. >>>> Hmm we do have hdr_len field, but seem to ignore it on RX. >>>> Jason do you see any issues with using it for the head len? >>> >>> This might work only if the device sets a correct hdr_len. I'm not sure >>> all of the devices can do this properly. E.g for tap, we use >>> skb_headlen() in virtio_net_hdr_from_skb() which depends highly on the >>> behaviour of the underlayer layers (device driver or GRO). And we only >>> set this hint for GSO packet but virtio-net may tries to do GRO for non >>> GSO packets. >>> >>> Thanks >> hi, Jason >> >> I personally prefer to use build_skb to create skb, so the problem here is >> actually gone. >> >> The premise of this is that the buffer added by add_recvbuf_mergeable must >> retain a skb_shared_info. Of course, then rx frags coalescing won't >> work. But I consider that suppose the size of the mrg_avg_pkt_len 1500, so we >> can still store 17 * 1500 = 24k packets in a skb. If the packet is really big, >> the mrg_avg_pkt_len will also increase, and the buffer allocated later will >> increase. When the mrg_avg_pkt_len is greater than PAGE_SIZE/2, rx frags >> coalesce is no longer needed. Because we can't allocate two bufs with a value of >> mrg_avg_pkt_len on the same page. >> > For the record I implemented build_skb() 10 years ago, so you can > trust me when I > am saying this will not help. > > Using build_skb() will waste additional skb_shared_info per MSS. > That's an increase of 20% of memory, for nothing at all. So I wonder something like the following like this help. We know the frag size, that means, if we know there's sufficient tailroom we can use build_skb() without reserving dedicated room for skb_shared_info. Thanks > > Also there are cases when this won't be possible, say if you use an MTU of 4000 > > > > > >> Thanks. >> >>> >>>> >>>>> Optimal GRO packets are when payload is in page fragments. >>>>> >>>>> (I am speaking not only for raw performance, but ability for systems >>>>> to cope with network outages and sudden increase of memory usage in >>>>> out of order queues) >>>>> >>>>> This has been quite clearly stated in my changelog. >>>>> >>>>> Thanks. >>>>> >>>>> >>>>>> int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb) >>>>>> { >>>>>> struct skb_shared_info *pinfo, *skbinfo = skb_shinfo(skb); >>>>>> unsigned int offset = skb_gro_offset(skb); >>>>>> unsigned int headlen = skb_headlen(skb); >>>>>> >>>>>> ....... >>>>>> >>>>>> if (headlen <= offset) { // virtio-net will fail >>>>>> ........ // merge by frags >>>>>> goto done; >>>>>> } else if (skb->head_frag) { // skb->head_frag is fail when use kmalloc() for skb->head allocation >>>>>> ........ // merge by frags >>>>>> goto done; >>>>>> } >>>>>> >>>>>> merge: >>>>>> ...... >>>>>> >>>>>> if (NAPI_GRO_CB(p)->last == p) >>>>>> skb_shinfo(p)->frag_list = skb; >>>>>> else >>>>>> NAPI_GRO_CB(p)->last->next = skb; >>>>>> >>>>>> ...... >>>>>> return 0; >>>>>> } >>>>>> >>>>>> >>>>>> test cmd: >>>>>> for i in $(seq 1 4) >>>>>> do >>>>>> redis-benchmark -r 10000000 -n 10000000 -t set -d 1024 -c 8 -P 32 -h -p 6379 2>&1 | grep 'per second' & >>>>>> done >>>>>> >>>>>> Reported-by: su-lifan@linux.alibaba.com >>>>>> >>>>>>> Fixes: fd11a83dd363 ("net: Pull out core bits of __netdev_alloc_skb and add __napi_alloc_skb") >>>>>>> Signed-off-by: Eric Dumazet >>>>>>> Cc: Alexander Duyck >>>>>>> Cc: Paolo Abeni >>>>>>> Cc: Michael S. Tsirkin >>>>>>> Cc: Greg Thelen >>>>>>> --- >>>>>>> net/core/skbuff.c | 9 +++++++-- >>>>>>> 1 file changed, 7 insertions(+), 2 deletions(-) >>>>>>> >>>>>>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c >>>>>>> index 7626a33cce590e530f36167bd096026916131897..3a8f55a43e6964344df464a27b9b1faa0eb804f3 100644 >>>>>>> --- a/net/core/skbuff.c >>>>>>> +++ b/net/core/skbuff.c >>>>>>> @@ -501,13 +501,17 @@ EXPORT_SYMBOL(__netdev_alloc_skb); >>>>>>> struct sk_buff *__napi_alloc_skb(struct napi_struct *napi, unsigned int len, >>>>>>> gfp_t gfp_mask) >>>>>>> { >>>>>>> - struct napi_alloc_cache *nc = this_cpu_ptr(&napi_alloc_cache); >>>>>>> + struct napi_alloc_cache *nc; >>>>>>> struct sk_buff *skb; >>>>>>> void *data; >>>>>>> >>>>>>> len += NET_SKB_PAD + NET_IP_ALIGN; >>>>>>> >>>>>>> - if ((len > SKB_WITH_OVERHEAD(PAGE_SIZE)) || >>>>>>> + /* If requested length is either too small or too big, >>>>>>> + * we use kmalloc() for skb->head allocation. >>>>>>> + */ >>>>>>> + if (len <= SKB_WITH_OVERHEAD(1024) || >>>>>>> + len > SKB_WITH_OVERHEAD(PAGE_SIZE) || >>>>>>> (gfp_mask & (__GFP_DIRECT_RECLAIM | GFP_DMA))) { >>>>>>> skb = __alloc_skb(len, gfp_mask, SKB_ALLOC_RX, NUMA_NO_NODE); >>>>>>> if (!skb) >>>>>>> @@ -515,6 +519,7 @@ struct sk_buff *__napi_alloc_skb(struct napi_struct *napi, unsigned int len, >>>>>>> goto skb_success; >>>>>>> } >>>>>>> >>>>>>> + nc = this_cpu_ptr(&napi_alloc_cache); >>>>>>> len += SKB_DATA_ALIGN(sizeof(struct skb_shared_info)); >>>>>>> len = SKB_DATA_ALIGN(len); >>>>>>> >>>>>>> -- >>>>>>> 2.30.0.284.gd98b1dd5eaa7-goog >>>>>>>