linux-kernel - Re: [RFC][PATCH 3/3] sched: User Mode Concurency Groups

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Message-ID: <YeVpdj/OP+qfAztn@hirez.programming.kicks-ass.net>
Date:   Mon, 17 Jan 2022 14:04:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Peter Oskolkov <posk@...k.io>
Cc:     mingo@...hat.com, tglx@...utronix.de, 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,
        linux-mm@...ck.org, linux-api@...r.kernel.org, x86@...nel.org,
        pjt@...gle.com, posk@...gle.com, avagin@...gle.com,
        jannh@...gle.com, tdelisle@...terloo.ca
Subject: Re: [RFC][PATCH 3/3] sched: User Mode Concurency Groups

On Tue, Dec 21, 2021 at 05:19:00PM +0000, Peter Oskolkov wrote:
> On Tue, Dec 14, 2021 at 09:44:48PM +0100, Peter Zijlstra wrote:
> > +static struct task_struct *umcg_get_task(u32 tid)
> > +{
> > +	struct task_struct *tsk = NULL;
> > +
> > +	if (tid) {
> > +		rcu_read_lock();
> > +		tsk = find_task_by_vpid(tid);
> > +		if (tsk && current->mm == tsk->mm && tsk->umcg_task)
> 
> This essentially limits all operations to a single mm/process.
> Fine for now, but a fast remote context switch is also a valid use
> case. It is not directly related to userspace scheduling, so I'm
> just mentioning it here. Maybe server-to-server cross-process
> context switches should be allowed for the same user/cgroup? (Again,
> this is for later to consider).

Doing cross-address-space UMCG will be a massive effort, too much
(pretty much everything) in this implementation assumes things are
directly addressable.

> > +			get_task_struct(tsk);
> > +		else
> > +			tsk = NULL;
> > +		rcu_read_unlock();
> > +	}
> > +
> > +	return tsk;
> > +}