linux-kernel - Re: [External] Re: [PATCH] sched/fair: fix broken bandwidth control with nohz

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Date:   Tue, 29 Mar 2022 10:58:15 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        linux-kernel@...r.kernel.org, duanxiongchun@...edance.com,
        songmuchun@...edance.com, Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [External] Re: [PATCH] sched/fair: fix broken bandwidth control
 with nohz_full

On 2022/3/29 00:44, Steven Rostedt wrote:
> On Mon, 28 Mar 2022 17:56:07 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
>>> echo $$ > test/cgroup.procs
>>> taskset -c 1 bash -c "while true; do let i++; done"  --> will be throttled  
>>
>> Ofcourse.. I'm arguing that bandiwdth control and NOHZ_FULL are somewhat
>> mutually exclusive, use-case wise. So I really don't get why you'd want
>> them both.
> 
> Is it?
> 
> One use case I can see for having both is for having a deadline task that
> needs to get something done in a tight deadline. NOHZ_FULL means "do not
> interrupt this task when it is the top priority task on the CPU and is
> running in user space".

Yes, this is similar with our use-case.

> 
> Why is it mutually exclusive to have a deadline task that does not want to
> be interrupted by timer interrupts?
> 
> Just because the biggest pushers of NOHZ_FULL is for those that are running
> RT tasks completely in user space and event want to fault if it ever goes
> into the kernel, doesn't mean that's the only use case.
> 
> Chengming brought up VMs. That's a case to want to control the bandwidth,
> but also not interrupt them with timer interrupts when they are running as
> the top priority task on a CPU.

Agree. NOHZ_FULL means don't want timer interrupts when running mostly in
user space or guest mode, bandwidth control just means need to go into kernel
to schedule out only when its quota used up. We shouldn't make them mutually
exclusive.

Thanks.

> 
> -- Steve