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Date:   Fri, 17 Apr 2020 12:57:54 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Tim Chen <tim.c.chen@linux.intel.com>
Cc:     vpillai <vpillai@digitalocean.com>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        mingo@kernel.org, tglx@linutronix.de, pjt@google.com,
        torvalds@linux-foundation.org, linux-kernel@vger.kernel.org,
        fweisbec@gmail.com, keescook@chromium.org, kerrnel@google.com,
        Phil Auld <pauld@redhat.com>, Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>, aubrey.li@linux.intel.com,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Joel Fernandes <joelaf@google.com>, joel@joelfernandes.org,
        Aaron Lu <aaron.lu@linux.alibaba.com>
Subject: Re: [RFC PATCH 07/13] sched: Add core wide task selection and
 scheduling.
Message-ID: <20200417105754.GY20730@hirez.programming.kicks-ass.net>
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On Thu, Apr 16, 2020 at 04:32:28PM -0700, Tim Chen wrote:
> 
> 
> On 4/14/20 6:35 AM, Peter Zijlstra wrote:
> > On Wed, Mar 04, 2020 at 04:59:57PM +0000, vpillai wrote:
> >> +static struct task_struct *
> >> +pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> >> +{
> >> +	struct task_struct *next, *max = NULL;
> >> +	const struct sched_class *class;
> >> +	const struct cpumask *smt_mask;
> >> +	int i, j, cpu;
> >> +	bool need_sync = false;
> > 
> > AFAICT that assignment is superfluous. Also, you violated the inverse
> > x-mas tree.
> > 
> >> +
> >> +	cpu = cpu_of(rq);
> >> +	if (cpu_is_offline(cpu))
> >> +		return idle_sched_class.pick_next_task(rq);
> > 
> > Are we actually hitting this one?
> > 
> 
> I did hit this race when I was testing taking cpu offline and online,
> which prompted the check of cpu being offline.

This is the schedule from the stop task to the idle task I presume,
there should really not be any other. And at that point the rq had
better be empty, so why didn't the normal task selection work?