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=-11.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 6D31CC433E0 for ; Wed, 10 Feb 2021 17:27:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2298664E92 for ; Wed, 10 Feb 2021 17:27:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232637AbhBJR1b (ORCPT ); Wed, 10 Feb 2021 12:27:31 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:39235 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232193AbhBJR1U (ORCPT ); Wed, 10 Feb 2021 12:27:20 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1612977954; 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=/cSe7sL5Ep1uG0oJAcv3xu7xrZMjqjN2Tp23S5V2S98=; b=NtepvGNPxlp3RmtKukk4QPETwDN8TERWWbWhOmTsxOJ2/iuMpB/5PSnLMNaqC+qZ4H4utL dxpBqoj60GlQwp/Nn2FN6fV5zUWueRXFsFTeOYeR7ckA2Es5SijlgVwoOBHsoniN+JD9Xj AZAzw6He+7Fo8o2HNTJmKStcGwFukQo= 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-130-F5KEm5BxMWCru53xB6q2fA-1; Wed, 10 Feb 2021 12:25:49 -0500 X-MC-Unique: F5KEm5BxMWCru53xB6q2fA-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 4DAB4C7402; Wed, 10 Feb 2021 17:25:46 +0000 (UTC) Received: from carbon (unknown [10.36.110.45]) by smtp.corp.redhat.com (Postfix) with ESMTP id 600A962688; Wed, 10 Feb 2021 17:25:27 +0000 (UTC) Date: Wed, 10 Feb 2021 18:25:26 +0100 From: Jesper Dangaard Brouer To: Alexander Lobakin Cc: Paolo Abeni , "David S. Miller" , Jakub Kicinski , Jonathan Lemon , Eric Dumazet , Dmitry Vyukov , Willem de Bruijn , Randy Dunlap , Kevin Hao , Pablo Neira Ayuso , Jakub Sitnicki , Marco Elver , Dexuan Cui , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Taehee Yoo , Cong Wang , =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= , Miaohe Lin , Guillaume Nault , Yonghong Song , zhudi , Michal Kubecek , Marcelo Ricardo Leitner , Dmitry Safonov <0x7f454c46@gmail.com>, Yang Yingliang , linux-kernel@vger.kernel.org, netdev@vger.kernel.org, Edward Cree , brouer@redhat.com, Alexander Duyck Subject: Re: [v3 net-next 08/10] skbuff: reuse NAPI skb cache on allocation path (__build_skb()) Message-ID: <20210210182526.3fd3c0ba@carbon> In-Reply-To: <20210210122414.8064-1-alobakin@pm.me> References: <20210209204533.327360-1-alobakin@pm.me> <20210209204533.327360-9-alobakin@pm.me> <20210210122414.8064-1-alobakin@pm.me> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Wed, 10 Feb 2021 12:25:04 +0000 Alexander Lobakin wrote: > From: Paolo Abeni > Date: Wed, 10 Feb 2021 11:21:06 +0100 > > > I'm sorry for the late feedback, I could not step-in before. > > > > Also adding Jesper for awareness, as he introduced the bulk free > > infrastructure. Thanks (and Alexander Duyck also did part of the work while at Red Hat). In my initial versions of my patchsets I actually also had reuse of the SKBs that were defer freed during NAPI context. But I dropped that part because it was getting nitpicked and the merge window was getting close, so I ended up dropping that part. > > On Tue, 2021-02-09 at 20:48 +0000, Alexander Lobakin wrote: > > > @@ -231,7 +256,7 @@ struct sk_buff *__build_skb(void *data, unsigned int frag_size) > > > */ > > > struct sk_buff *build_skb(void *data, unsigned int frag_size) > > > { > > > - struct sk_buff *skb = __build_skb(data, frag_size); > > > + struct sk_buff *skb = __build_skb(data, frag_size, true); > > > > I must admit I'm a bit scared of this. There are several high speed > > device drivers that will move to bulk allocation, and we don't have any > > performance figure for them. > > > > In my experience with (low end) MIPS board, cache misses cost tend to > > be much less visible there compared to reasonably recent server H/W, > > because the CPU/memory access time difference is much lower. > > > > When moving to higher end H/W the performance gain you measured could > > be completely countered by less optimal cache usage. > > > > I fear also latency spikes - I'm unsure if a 32 skbs allocation vs a > > single skb would be visible e.g. in a round-robin test. Generally > > speaking bulk allocating 32 skbs looks a bit too much. IIRC, when > > Edward added listification to GRO, he did several measures with > > different list size and found 8 to be the optimal value (for the tested > > workload). Above such number the list become too big and the pressure > > on the cache outweighted the bulking benefits. > > I can change to logics the way so it would allocate the first 8. > I think I've already seen this batch value somewhere in XDP code, > so this might be a balanced one. (Speaking about SLUB code): Bulk ALLOC side disables interrupts, and can call slow path (___slab_alloc), which is bad for latency sensitive workloads. This I don't recommend large bulk ALLOCATIONS. > Regarding bulk-freeing: can the batch size make sense when freeing > or it's okay to wipe 32 (currently 64 in baseline) in a row? (Speaking about SLUB code): You can bulk FREE large amount of object without hurting latency sensitive workloads, because it doesn't disable interrupts (I'm quite proud that this was possible). > > Perhaps giving the device drivers the ability to opt-in on this infra > > via a new helper - as done back then with napi_consume_skb() - would > > make this change safer? > > That's actually a very nice idea. There's only a little in the code > to change to introduce an ability to take heads from the cache > optionally. This way developers could switch to it when needed. Well, I actually disagree that this should be hidden behind a switch for drivers to enable, as this will take forever to get proper enabled. > Thanks for the suggestions! I'll definitely absorb them into the code > and give it a test. > > > > @@ -838,31 +863,31 @@ void __consume_stateless_skb(struct sk_buff *skb) > > > kfree_skbmem(skb); > > > } > > > > > > -static inline void _kfree_skb_defer(struct sk_buff *skb) > > > +static void napi_skb_cache_put(struct sk_buff *skb) > > > { > > > struct napi_alloc_cache *nc = this_cpu_ptr(&napi_alloc_cache); > > > + u32 i; > > > > > > /* drop skb->head and call any destructors for packet */ > > > skb_release_all(skb); > > > > > > - /* record skb to CPU local list */ > > > + kasan_poison_object_data(skbuff_head_cache, skb); > > > nc->skb_cache[nc->skb_count++] = skb; > > > > > > -#ifdef CONFIG_SLUB > > > - /* SLUB writes into objects when freeing */ > > > - prefetchw(skb); > > > -#endif > > > > It looks like this chunk has been lost. Is that intentional? > > Yep. This prefetchw() assumed that skbuff_heads will be wiped > immediately or at the end of network softirq. Reusing this cache > means that heads can be reused later or may be kept in a cache for > some time, so prefetching makes no sense anymore. I agree with this statement, the prefetchw() is no-longer needed. -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer