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

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Message-ID: <YkHal1m3pnxGoQ1Y@hirez.programming.kicks-ass.net>
Date:   Mon, 28 Mar 2022 17:56:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Chengming Zhou <zhouchengming@...edance.com>
Cc:     mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, 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 Mon, Mar 28, 2022 at 11:40:25PM +0800, Chengming Zhou wrote:

> > NOHZ_FULL is for use-cases that 'never' intend to go into the kernel,
> > your use-case actively relies on going into the kernel. Hence the
> > confusion.
> 
> In fact, I put a testcase at the end of git message, in which only run
> a userspace loop workload:
> 
> cd /sys/fs/cgroup
> echo "+cpu" > cgroup.subtree_control
> 
> mkdir test
> echo "105000 100000" > test/cpu.max
> 
> 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.

NOHZ_FULL says, "I 'never' intend to go to the kernel"

bandwidth control says: "I expect to be sharing the system and must be
interrupted to not consume too much time", which very much implies: "I
will go into the kernel".

The trade-off we make to make NOHZ_FULL work, makes system enter/exit
*far* more expensive. There's also people asking to outright kill a task
that causes entry under NOHZ_FULL.

So yes, you can configure it, but why does it make sense?