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Date:   Tue, 1 Dec 2020 14:11:33 -0500
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
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Subject: Re: [PATCH -tip 22/32] sched: Split the cookie and setup per-task
 cookie on fork
Message-ID: <20201201191133.GA221478@google.com>
References: <20201117232003.3580179-1-joel@joelfernandes.org>
 <20201117232003.3580179-23-joel@joelfernandes.org>
 <20201125111541.GU2414@hirez.programming.kicks-ass.net>
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On Wed, Nov 25, 2020 at 12:15:41PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 17, 2020 at 06:19:52PM -0500, Joel Fernandes (Google) wrote:
> 
> > +/*
> > + * Ensure that the task has been requeued. The stopper ensures that the task cannot
> > + * be migrated to a different CPU while its core scheduler queue state is being updated.
> > + * It also makes sure to requeue a task if it was running actively on another CPU.
> > + */
> > +static int sched_core_task_join_stopper(void *data)
> > +{
> > +	struct sched_core_task_write_tag *tag = (struct sched_core_task_write_tag *)data;
> > +	int i;
> > +
> > +	for (i = 0; i < 2; i++)
> > +		sched_core_tag_requeue(tag->tasks[i], tag->cookies[i], false /* !group */);
> > +
> > +	return 0;
> > +}
> > +
> > +static int sched_core_share_tasks(struct task_struct *t1, struct task_struct *t2)
> > +{
> 
> > +	stop_machine(sched_core_task_join_stopper, (void *)&wr, NULL);
> 
> > +}
> 
> This is *REALLY* terrible...

I pulled this bit from your original patch. Are you concerned about the
stop_machine? Sharing a core is a slow path for our usecases (and as far as I
know, for everyone else's). We can probably do something different if that
requirement changes.