Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 17 Jan 2022 14:04:54 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Peter Oskolkov <posk@posk.io>
Cc:     mingo@redhat.com, tglx@linutronix.de, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-api@vger.kernel.org, x86@kernel.org,
        pjt@google.com, posk@google.com, avagin@google.com,
        jannh@google.com, tdelisle@uwaterloo.ca
Subject: Re: [RFC][PATCH 3/3] sched: User Mode Concurency Groups
Message-ID: <YeVpdj/OP+qfAztn@hirez.programming.kicks-ass.net>
References: <20211214204445.665580974@infradead.org>
 <20211214205358.701701555@infradead.org>
 <20211221171900.GA580323@dev-hv>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20211221171900.GA580323@dev-hv>
Precedence: bulk

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;
> > +}