From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Dumazet Subject: Re: [RFC PATCH] net: don't keep lonely packets forever in the gro hash Date: Tue, 20 Nov 2018 08:28:02 -0800 Message-ID: <9773e2e6-a9b8-4663-932a-726e30b53b6c@gmail.com> References: <3c8b5aea0c812323d8e15b548789a1e240f499d7.1542709015.git.pabeni@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Cc: "David S. Miller" , Willem de Bruijn To: Paolo Abeni , netdev@vger.kernel.org Return-path: Received: from mail-pf1-f193.google.com ([209.85.210.193]:45577 "EHLO mail-pf1-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727257AbeKUC6C (ORCPT ); Tue, 20 Nov 2018 21:58:02 -0500 Received: by mail-pf1-f193.google.com with SMTP id g62so1223481pfd.12 for ; Tue, 20 Nov 2018 08:28:04 -0800 (PST) In-Reply-To: Content-Language: en-US Sender: netdev-owner@vger.kernel.org List-ID: On 11/20/2018 07:42 AM, Paolo Abeni wrote: > Hi, > > On Tue, 2018-11-20 at 05:49 -0800, Eric Dumazet wrote: >> >> On 11/20/2018 02:17 AM, Paolo Abeni wrote: >>> Eric noted that with UDP GRO and napi timeout, we could keep a single >>> UDP packet inside the GRO hash forever, if the related NAPI instance >>> calls napi_gro_complete() at an higher frequency than the napi timeout. >>> Willem noted that even TCP packets could be trapped there, till the >>> next retransmission. >>> This patch tries to address the issue, flushing the oldest packets before >>> scheduling the NAPI timeout. The rationale is that such a timeout should be >>> well below a jiffy and we are not flushing packets eligible for sane GRO. >>> >>> Reported-by: Eric Dumazet >>> Signed-off-by: Paolo Abeni >>> --- >>> Sending as RFC, as I fear I'm missing some relevant pieces. >>> Also I'm unsure if this should considered a fixes for "udp: implement >>> GRO for plain UDP sockets." or for "net: gro: add a per device gro flush timer" > > Thank you for your feedback! > >> Truth be told, relying on jiffies change is a bit fragile for HZ=100 or HZ=250 kernels. > > Yes, we have higher bound there. > >> See recent TCP commit that got rid of tcp_tso_should_defer() dependency on HZ/jiffies >> >> https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git/commit/?id=a682850a114aef947da5d603f7fd2cfe7eabbd72 > > I'm unsure I follow correctly. Are you suggesting to use ns precision > for skb aging in GRO? If so, could that be a separate change? (looks > more invasive) I am not suggesting adding ns in your patch. That can be done later if we care. I simply warn that some distros have low HZ value and thus the fix wont prevent packet sitting 4 or 10 ms in the queue.