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

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Date:   Wed, 11 Jul 2018 12:34:34 +1000
From:   Jonathan Maxwell <jmaxwell37@...il.com>
To:     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 Wed, Jul 11, 2018 at 12:04 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
> 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. */
>
>

That makes sense thanks. I'll look into this and get a patch together.