netdev - Re: [PATCH net-next v3 2/2] net: sched: add Flow Queue PIE packet scheduler

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Date:   Mon, 13 Jan 2020 12:44:38 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>, gautamramk@...il.com
Cc:     netdev@...r.kernel.org,
        "Mohit P. Tahiliani" <tahiliani@...k.edu.in>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        "David S . Miller" <davem@...emloft.net>,
        Dave Taht <dave.taht@...il.com>,
        Leslie Monis <lesliemonis@...il.com>,
        "Sachin D . Patil" <sdp.sachin@...il.com>,
        "V . Saicharan" <vsaicharan1998@...il.com>,
        Mohit Bhasi <mohitbhasi1998@...il.com>
Subject: Re: [PATCH net-next v3 2/2] net: sched: add Flow Queue PIE packet scheduler

Jakub Kicinski <kuba@...nel.org> writes:

> On Fri, 10 Jan 2020 11:56:57 +0530, gautamramk@...il.com wrote:
>> From: "Mohit P. Tahiliani" <tahiliani@...k.edu.in>
>> 
>> Principles:
>>   - Packets are classified on flows.
>>   - This is a Stochastic model (as we use a hash, several flows might
>>                                 be hashed on the same slot)
>>   - Each flow has a PIE managed queue.
>>   - Flows are linked onto two (Round Robin) lists,
>>     so that new flows have priority on old ones.
>>   - For a given flow, packets are not reordered.
>>   - Drops during enqueue only.
>>   - ECN capability is off by default.
>>   - ECN threshold is at 10% by default.
>>   - Uses timestamps to calculate queue delay by default.
>> 
>> Usage:
>> tc qdisc ... fq_pie [ limit PACKETS ] [ flows NUMBER ]
>>                     [ alpha NUMBER ] [ beta NUMBER ]
>>                     [ target TIME us ] [ tupdate TIME us ]
>>                     [ memory_limit BYTES ] [ quantum BYTES ]
>>                     [ ecnprob PERCENTAGE ] [ [no]ecn ]
>>                     [ [no]bytemode ] [ [no_]dq_rate_estimator ]
>> 
>> defaults:
>>   limit: 10240 packets, flows: 1024
>>   alpha: 1/8, beta : 5/4
>>   target: 15 ms, tupdate: 15 ms (in jiffies)
>>   memory_limit: 32 Mb, quantum: device MTU
>>   ecnprob: 10%, ecn: off
>>   bytemode: off, dq_rate_estimator: off
>
> Some reviews below, but hopefully someone who knows more about qdiscs
> will still review :)

I looked it over, and didn't find anything you hadn't already pointed
out below. It's pretty obvious that this started out as a copy of
sch_fq_codel. Which is good, because that's pretty solid. And bad,
because that means it introduces another almost-identical qdisc without
sharing any of the code...

I think it would be worthwhile to try to consolidate things at some
point. Either by just merging code from fq_{codel,pie}, but another
option would be to express fq_codel and fq_pie using the fq{,_impl}.h
includes. Maybe even sch_cake as well, but that may take a bit more
work. Not sure if we should require this before merging fq_pie, or just
leave it as a possible enhancement for later? WDYT?

-Toke