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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id B1839C433EF for ; Thu, 6 Jan 2022 18:52:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243048AbiAFSwV (ORCPT ); Thu, 6 Jan 2022 13:52:21 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:32804 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242979AbiAFSwU (ORCPT ); Thu, 6 Jan 2022 13:52:20 -0500 Received: from mail-yb1-xb2d.google.com (mail-yb1-xb2d.google.com [IPv6:2607:f8b0:4864:20::b2d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 25FA1C061245 for ; Thu, 6 Jan 2022 10:52:20 -0800 (PST) Received: by mail-yb1-xb2d.google.com with SMTP id y130so10087539ybe.8 for ; Thu, 06 Jan 2022 10:52:20 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=IUMU++ei66mMLikIsKIpji8u8kZ4EgkMdb8bw4IJi7M=; b=tlN1qiT6Gf7LMzx8uZWwlCdeVUjgWHsn/rdMQ4OqiXOatO4j4I65aIsPQWrsbQTXMm VeyfzJwosM1D+4w/MrZOHodbr3H9qkQT/F8l/Pqc3NmqPJIKJ2uNSU+CntMOE5qTk2sx llCVdJv7PCutgMiWYji8xcelfaYiLS0PRX2GroQQIQtKmUyEmqQjQ8a6xaIRdpMuzFoB wa57qrmLP5ZD1AyqQrJyPbI6JW6KwyAANnuMwPbUivv/3GNF5g99VC7t+9PFavwPJgkk mgFD/Bj1spwxDllZuEIkeROgRSJj5kZAmViRYH27olKLFqsNwO3qp5uRrPPrfn3Y69W9 Cn3w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=IUMU++ei66mMLikIsKIpji8u8kZ4EgkMdb8bw4IJi7M=; b=2pJl8iUvq+WFPrMcVn4bw4LBMCXIQYwwW3U3Zudip3J1MWs76TYLRFU/w06/79Sp7F 3zybF3BjkMUDLZ2AjPTPiCdTec8EdnxctHcJFU4GRX0msVtwhFx8nGvY7+bJqcwTTrR+ SsGNtlyEI5ssSOJg6PVerflMYMiuG7AJ07ylsQAIwNftWFA55xryK64pB+2+yNkdIMCr C4YrshlV+/HyLjmJxavrGg+Kzczx51SfR5vtGDEu0n8yH7uY/DkjQIa3g0lwliXrEJ5+ 0jcq52awokMXYthqTfhbPSLrAPDWCaSCnElSyAnVZIPA63aD4h4gmTgsvyi5ukdmY8FK IqjA== X-Gm-Message-State: AOAM53000xyl3PIYKsgV1uSTuy/CfXjRcwXlMHdCv9xLkjAl4vDM/mnq 51h6Lc1JtibSUc8LumMBMipr6O9+6d74kpFrPu0P2A== X-Google-Smtp-Source: ABdhPJxS/EeBveDnUKjywJlxJMuNY1qYF6W0NzTjTkcduiGAwx5CMNjJFhQir1DyoVTfzpmGzz9X/HlHByvvv8gIUHU= X-Received: by 2002:a25:af14:: with SMTP id a20mr81702404ybh.753.1641495138938; Thu, 06 Jan 2022 10:52:18 -0800 (PST) MIME-Version: 1.0 References: In-Reply-To: From: Eric Dumazet Date: Thu, 6 Jan 2022 10:52:07 -0800 Message-ID: Subject: Re: Expensive tcp_collapse with high tcp_rmem limit To: Daniel Dao Cc: netdev , kernel-team , linux-kernel , David Miller , Jakub Kicinski , Hideaki YOSHIFUJI , Marek Majkowski Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 6, 2022 at 4:32 AM Daniel Dao wrote: > > On Wed, Jan 5, 2022 at 1:38 PM Eric Dumazet wrote: > > > > On Wed, Jan 5, 2022 at 4:15 AM Daniel Dao wrote: > > > > > > Hello, > > > > > > We are looking at increasing the maximum value of TCP receive buffer in order > > > to take better advantage of high BDP links. For historical reasons ( > > > https://blog.cloudflare.com/the-story-of-one-latency-spike/), this was set to > > > a lower than default value. > > > > > > We are still occasionally seeing long time spent in tcp_collapse, and the time > > > seems to be proportional with max rmem. For example, with net.ipv4.tcp_rmem = 8192 2097152 16777216, > > > we observe tcp_collapse latency with the following bpftrace command: > > > > > > > I suggest you add more traces, like the payload/truesize ratio when > > these events happen. > > and tp->rcv_ssthresh, sk->sk_rcvbuf > > > > TCP stack by default assumes a conservative [1] payload/truesize ratio of 50% > > I forgot to add that for this experiment we also set tcp_adv_win_scale > = -2 to see if it > reduces the chance of triggering tcp_collapse > > > > > Meaning that a 16MB sk->rcvbuf would translate to a TCP RWIN of 8MB. > > > > I suspect that you use XDP, and standard MTU=1500. > > Drivers in XDP mode use one page (4096 bytes on x86) per incoming frame. > > In this case, the ratio is ~1428/4096 = 35% > > > > This is one of the reason we switched to a 4K MTU at Google, because we > > have an effective ratio close to 100% (even if XDP was used) > > > > [1] The 50% ratio of TCP is defeated with small MSS, and malicious traffic. > > I updated the bpftrace script to get data on len/truesize on collapsed skb > > kprobe:tcp_collapse { > $sk = (struct sock *) arg0; > $tp = (struct tcp_sock *) arg0; > printf("tid %d: rmem_alloc=%ld sk_rcvbuf=%ld rcv_ssthresh=%ld\n", tid, > $sk->sk_backlog.rmem_alloc.counter, $sk->sk_rcvbuf, $tp->rcv_ssthresh); > printf("tid %d: advmss=%ld wclamp=%ld rcv_wnd=%ld\n", tid, $tp->advmss, > $tp->window_clamp, $tp->rcv_wnd); > @start[tid] = nsecs; > } > > kretprobe:tcp_collapse /@start[tid] != 0/ { > $us = (nsecs - @start[tid])/1000; > @us = hist($us); > printf("tid %d: %ld us\n", tid, $us); > delete(@start[tid]); > } > > kprobe:tcp_collapse_one { > $skb = (struct sk_buff *) arg1; > printf("tid %d: s=%ld len=%ld truesize=%ld\n", tid, sizeof(struct > sk_buff), $skb->len, $skb->truesize); > } > > interval:s:6000 { exit(); } > > Here is the output: > > tid 0: rmem_alloc=16780416 sk_rcvbuf=16777216 rcv_ssthresh=2920 > tid 0: advmss=1460 wclamp=4194304 rcv_wnd=450560 > tid 0: len=3316 truesize=15808 > tid 0: len=4106 truesize=16640 > tid 0: len=3967 truesize=16512 > tid 0: len=2988 truesize=15488 Ouch. What kind of NIC driver is used on your host ? > ... > tid 0: len=5279 truesize=17664 > tid 0: len=425 truesize=2048 > tid 0: 17176 us > > The skb looks indeed bloated (len=3316, truesize=15808), so collapsing > definitely > helps. It just took a long time to go through thousands of 16KB skb > > > > > > > > bpftrace -e 'kprobe:tcp_collapse { @start[tid] = nsecs; } kretprobe:tcp_collapse /@start[tid] != 0/ { $us = (nsecs - @start[tid])/1000; @us = hist($us); delete(@start[tid]); printf("%ld us\n", $us);} interval:s:6000 { exit(); }' > > > Attaching 3 probes... > > > 15496 us > > > 14301 us > > > 12248 us > > > @us: > > > [8K, 16K) 3 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| > > > > > > Spending up to 16ms with 16MiB maximum receive buffer seems high. Are there any > > > recommendations on possible approaches to reduce the tcp_collapse latency ? > > > Would clamping the duration of a tcp_collapse call be reasonable, since we only > > > need to spend enough time to free space to queue the required skb ? > > > > It depends if the incoming skb is queued in in-order queue or > > out-of-order queue. > > For out-of-orders, we have a strategy in tcp_prune_ofo_queue() which > > should work reasonably well after commit > > 72cd43ba64fc17 tcp: free batches of packets in tcp_prune_ofo_queue() > > > > Given the nature of tcp_collapse(), limiting it to even 1ms of processing time > > would still allow for malicious traffic to hurt you quite a lot. > > I don't yet understand why we have cases of bloated skbs. But it seems > like adapting the > batch prune strategy in tcp_prune_ofo_queue() to tcp_collapse makes sense to me. > Except that you would still have to parse the linear list. > I think every collapsed skb saves us truesize - len (?), and we can > set goal to free up 12.5% of sk_rcvbuf > same as tcp_prune_ofo_queue() I think that you should first look if you are under some kind of attack [1] Eventually you would still have to make room, involving expensive copies. 12% of 16MB is still a lot of memory to copy. [1] Detecting an attack signature could allow you to zap the socket and save ~16MB of memory per flow.