linux-kernel - RE: [net-next,v2] tcp: Improve setsockopt() TCP_USER

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Date:   Wed, 4 Jul 2018 14:57:06 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Neal Cardwell' <ncardwell@...gle.com>,
        Jonathan Maxwell <jmaxwell37@...il.com>
CC:     David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "jmaxwell@...hat.com" <jmaxwell@...hat.com>
Subject: RE: [net-next,v2] tcp: Improve setsockopt() TCP_USER_TIMEOUT accuracy

From: Neal Cardwell
> Sent: 04 July 2018 15:14
> 
> On Tue, Jul 3, 2018 at 8:06 PM Jon Maxwell <jmaxwell37@...il.com> wrote:
> >
> > Signed-off-by: Jon Maxwell <jmaxwell37@...il.com>
> > ---
> >  net/ipv4/tcp_timer.c | 48 +++++++++++++++++++++++++++++++++++++++---------
> >  1 file changed, 39 insertions(+), 9 deletions(-)
> >
> > diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> > index 3b3611729928..d129e670d02a 100644
> > --- a/net/ipv4/tcp_timer.c
> > +++ b/net/ipv4/tcp_timer.c
...
> > +static __u32 tcp_clamp_rto_to_user_timeout(struct sock *sk)
> > +{
> > +       struct inet_connection_sock *icsk = inet_csk(sk);
> > +       __u32 rto = icsk->icsk_rto;

Why cache rto past all the function calls until it is needed?

> > +       __u32 elapsed, user_timeout;
> > +       unsigned int start_ts;
> 
> I'd suggest u32 here for start_ts (per the rationale above).
> 
> > +
> > +       start_ts = tcp_retransmit_stamp(sk);
> > +       if (!icsk->icsk_user_timeout || !start_ts)
> > +               return rto;
> > +       elapsed = tcp_time_stamp(tcp_sk(sk)) - start_ts;
> > +       user_timeout = jiffies_to_msecs(icsk->icsk_user_timeout);
> > +       if (elapsed >= user_timeout)
> > +               rto = 1;  /* user timeout has passed; fire ASAP */
> > +       else
> > +               rto = min(rto, (__u32)msecs_to_jiffies(user_timeout - elapsed));
> 
> My sense is that min_t would be preferred here, e.g:
> 
>   rto = min_t(__u32, rto, msecs_to_jiffies(user_timeout - elapsed));

Think I'd just write the conditional...

> > +       return rto;
> > +}

Looks to me like it doesn't correctly allow for the conversions
between millisecond and jiffies either.
I suspect the msecs_to_jiffies() call can return zero (unless it rounds up).
Coding with a signed 'time until user timeout' might make it simpler.

	David