netdev - Re: [PATCH net-next] tcp: introduce rto_max

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Message-ID: <CAL+tcoA38fXgnJtdDz8NBm=F0-=oGp=oEySnWEhNB16dqzG9eQ@mail.gmail.com>
Date: Tue, 16 Jul 2024 14:41:17 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: kuba@...nel.org, pabeni@...hat.com, davem@...emloft.net, 
	dsahern@...nel.org, ncardwell@...gle.com, corbet@....net, 
	netdev@...r.kernel.org, Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next] tcp: introduce rto_max_us sysctl knob

Hello Eric,

On Mon, Jul 15, 2024 at 10:40 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Sun, Jul 14, 2024 at 8:31 PM Jason Xing <kerneljasonxing@...il.com> wrote:
> >
> > From: Jason Xing <kernelxing@...cent.com>
> >
> > As we all know, the algorithm of rto is exponential backoff as RFC
> > defined long time ago.
>
> This is weak sentence. Please provide RFC numbers instead.

RFC 6298. I will update it.

>
> > After several rounds of repeatedly transmitting
> > a lost skb, the expiry of rto timer could reach above 1 second within
> > the upper bound (120s).
>
> This is confusing. What do you mean exactly ?

I will rewrite this part. What I was trying to say is that waiting
more than 1 second is not very friendly to some applications,
especially the expiry time can reach 120 seconds which is too long.

>
> >
> > Waiting more than one second to retransmit for some latency-sensitive
> > application is a little bit unacceptable nowadays, so I decided to
> > introduce a sysctl knob to allow users to tune it. Still, the maximum
> > value is 120 seconds.
>
> I do not think this sysctl needs usec resolution.

Are you suggesting using jiffies is enough? But I have two reasons:
1) Keep the consistency with rto_min_us
2) If rto_min_us can be set to a very small value, why not rto_max?

What do you think?

>
> Also storing this sysctl once, and converting it millions of times per
> second to jiffies is not good.
> I suggest you use proc_dointvec_jiffies() instead in the sysctl handler.
>
> Minimal value of one jiffies makes also no sense. We can not predict
> if some distros/users
> might (ab)use this sysctl.

Okay. If the final solution is using jiffies, I will accordingly adjust it.

>
> You forgot to update
> Documentation/networking/net_cachelines/netns_ipv4_sysctl.rst

Oh sorry, I forgot.

> This means the location you chose for the new sysctl is pretty much
> random and not reflcting
> this is used in one fast path.

I will investigate its proper location...

>
> I suggest you wait for net-next being reopened, we are all busy
> attending netdev 0x18 conference.

Roger that. Thanks for your suggestions.

Thanks,
Jason