From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753013AbbE0Okh (ORCPT ); Wed, 27 May 2015 10:40:37 -0400 Received: from mail-qk0-f170.google.com ([209.85.220.170]:36600 "EHLO mail-qk0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752743AbbE0Okd (ORCPT ); Wed, 27 May 2015 10:40:33 -0400 Date: Wed, 27 May 2015 10:40:29 -0400 From: Ido Yariv To: Eric Dumazet Cc: David Laight , "David S. Miller" , Alexey Kuznetsov , James Morris , Hideaki YOSHIFUJI , Patrick McHardy , Nandita Dukkipati , "netdev@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Ido Yariv Subject: Re: [PATCH] net: tcp: Fix a PTO timing granularity issue Message-ID: <20150527144029.GA558@WorkStation.home> References: <1432663992.4060.286.camel@edumazet-glaptop2.roam.corp.google.com> <1432671437-19140-1-git-send-email-ido@wizery.com> <063D6719AE5E284EB5DD2968C1650D6D1CB40A1A@AcuExch.aculab.com> <1432734077.4060.382.camel@edumazet-glaptop2.roam.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1432734077.4060.382.camel@edumazet-glaptop2.roam.corp.google.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Eric, On Wed, May 27, 2015 at 06:41:17AM -0700, Eric Dumazet wrote: > On Wed, 2015-05-27 at 11:36 +0000, David Laight wrote: > > From: Of Ido Yariv > > > Sent: 26 May 2015 21:17 > > > The Tail Loss Probe RFC specifies that the PTO value should be set to > > > max(2 * SRTT, 10ms), where SRTT is the smoothed round-trip time. > > > > > > The PTO value is converted to jiffies, so the timer may expire > > > prematurely. > > > > > > This is especially problematic on systems in which HZ <= 100, so work > > > around this by setting the timeout to at least 2 jiffies on such > > > systems. > > > > > > The 10ms figure was originally selected based on tests performed with > > > the current implementation and HZ = 1000. Thus, leave the behavior on > > > systems with HZ > 100 unchanged. > > > > > > Signed-off-by: Ido Yariv > > > --- > > > net/ipv4/tcp_output.c | 3 +++ > > > 1 file changed, 3 insertions(+) > > > > > > diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c > > > index 534e5fd..5321df8 100644 > > > --- a/net/ipv4/tcp_output.c > > > +++ b/net/ipv4/tcp_output.c > > > @@ -2208,6 +2208,9 @@ bool tcp_schedule_loss_probe(struct sock *sk) > > > timeout = max_t(u32, timeout, > > > (rtt + (rtt >> 1) + TCP_DELACK_MAX)); > > > timeout = max_t(u32, timeout, msecs_to_jiffies(10)); > > > +#if HZ <= 100 > > > + timeout = max_t(u32, timeout, 2); > > > +#endif > > > > Why not: > > timeout = max_t(u32, timeout, max_t(u32, 2, msecs_to_jiffies(10))); > > I think the RH max_t() is a compile time constant. > > > > You need 2 jiffies to guarantee a non-zero timeout. > > Even if HZ=199 with a 'rounding down' msecs_to_jiffies() you get 1 jiffy > > and a possible immediate timeout. > > > > Have you followed previous discussions ? > > I guess we can have a helper macro, but for the moment only one spot was > found. > > Its kind of depressing having to deal with HZ=100 issues, with modern > NO_HZ configurations. > > TCP rtts have now usec resolution, so HZ=100 is pushing TCP to very > imprecise behavior. HZ=100 is used on some embedded platforms, so it's still something we have to deal with unfortunately.. Since the '2' here is a lower bound, and msecs_to_jiffies(10) will return values greater than 2 for HZ>100 anyway, always ensuring the 2 jiffies lower bound shouldn't impact the behavior when HZ=1000. However, as far as I can tell, comparing msecs_to_jiffies(10) to 2, or comparing the whole timeout to 2 doesn't make much difference, since msecs_to_jiffies isn't inlined. In other words, keeping the #if shouldn't make much difference in behavior, but will save the small comparison. Cheers, Ido.