linux-kernel - Re: [net-next,v3] tcp: Improve setsockopt() TCP_USER

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Message-ID: <46861e12-1ae5-374f-e907-87ba6241de5f@gmail.com>
Date:   Tue, 10 Jul 2018 19:04:21 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jonathan Maxwell <jmaxwell37@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        David Laight <David.Laight@...lab.com>,
        kuznet <kuznet@....inr.ac.ru>,
        yoshfuji <yoshfuji@...ux-ipv6.org>,
        Netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jon Maxwell <jmaxwell@...hat.com>
Subject: Re: [net-next,v3] tcp: Improve setsockopt() TCP_USER_TIMEOUT accuracy



On 07/10/2018 05:33 PM, Jonathan Maxwell wrote:
> On Tue, Jul 10, 2018 at 10:48 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>>
>>
>> On 07/10/2018 05:38 AM, Eric Dumazet wrote:
>>
>>> Note that if we always do jiffies_to_msecs(icsk->icsk_user_timeout) in TCP,
>>> we also could change the convention and store msecs in this field instead of jiffies.
>>>
>>> That would eliminate the msecs_to_jiffies() and jiffies_to_msecs() dance.
>>>
>>> (That would be done in a patch of its own, of course)
>>
>> tcp_keepalive_timer() does use icsk->icsk_user_timeout directly in jiffies unit,
>> but considering keeapalive timers are rarely used, this point would have to
>> do the msecs_to_jiffies() conversion.
> 
> and also if icsk->icsk_user_timeout = 0, then timeout in retransmits_timed_out()
> is in jiffies and that would need to addressed.

Absolutely, this is what I was suggesting.

Pseudo code for this part, before your changes.

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 3b3611729928f77934e0298bb248e55c7a7c5def..cae7bbc956ed51e9d381650957f54550cc0967d9 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -183,8 +183,9 @@ static bool retransmits_timed_out(struct sock *sk,
                else
                        timeout = ((2 << linear_backoff_thresh) - 1) * rto_base +
                                (boundary - linear_backoff_thresh) * TCP_RTO_MAX;
+               timeout = jiffies_to_msecs(timeout)'
        }
-       return (tcp_time_stamp(tcp_sk(sk)) - start_ts) >= jiffies_to_msecs(timeout);
+       return (tcp_time_stamp(tcp_sk(sk)) - start_ts) >= timeout;
 }
 
 /* A write timeout has occurred. Process the after effects. */