linux-kernel - Re: [PATCH V6 3/6] sched/fair: Fair server interface

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Message-ID: <e4efd69c-b155-4c13-99c3-9603f5769f93@kernel.org>
Date: Thu, 11 Apr 2024 17:02:41 +0200
From: Daniel Bristot de Oliveira <bristot@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>,
 Daniel Bristot de Oliveira <bristot@...nel.org>
Cc: Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
 linux-kernel@...r.kernel.org, Luca Abeni <luca.abeni@...tannapisa.it>,
 Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>,
 Thomas Gleixner <tglx@...utronix.de>, Joel Fernandes
 <joel@...lfernandes.org>, Vineeth Pillai <vineeth@...byteword.org>,
 Shuah Khan <skhan@...uxfoundation.org>, Phil Auld <pauld@...hat.com>,
 Suleiman Souhlal <suleiman@...gle.com>,
 Youssef Esmat <youssefesmat@...gle.com>
Subject: Re: [PATCH V6 3/6] sched/fair: Fair server interface

On 4/11/24 16:43, Peter Zijlstra wrote:
> On Fri, Apr 05, 2024 at 07:28:02PM +0200, Daniel Bristot de Oliveira wrote:
>> Add an interface for fair server setup on debugfs.
>>
>> Each CPU has three files under /debug/sched/fair_server/cpu{ID}:
>>
>>  - runtime: set runtime in ns
>>  - period:  set period in ns
>>  - defer:   on/off for the defer mechanism
>>
>> This then leaves /proc/sys/kernel/sched_rt_{period,runtime}_us to set
>> bounds on admission control.
>>
>> The interface also add the server to the dl bandwidth accounting.
> 
> I suppose most people will want to use it like:
> 
>   for i in /debug/sched/fair_server/cpu*
>   do
> 	  echo $PERIOD > ${i}/period
> 	  ecoh $RUNTIME > ${i}/runtime
>   done
> 
> And I think we agreed to keep this loop in userspace, but memory is
> vague.

correct, we agreed to keep loop in user-space. It is important to have per-cpu
for large systems, like we have at red hat customers (kubernets): each container can
have a different setup... and they do. Like, DPDK people would like to keep some
few us runtime, while other CPUs it is better to keep the default.

> The 'defer' thing is dubious though, I don't suppose anybody would ever
> want to actually change that, other than you while poking around at this
> code, right?

In a setup where all real-time tasks are DL (without fixed-priority tasks (FIFO/RR))
the defer = 0 makes more sense because the bandwidth is reserved anyways, and the
DL server would have a relatively low prio (long period).

Believe it or not, we are getting there in some cases with automation systems :-)

If it does not hurt, I would like keep it... Otherwise, we can think about it in
the future.

-- Daniel