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Date:   Fri, 16 Sep 2022 07:41:13 -0700
From:   Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Ricardo Neri <ricardo.neri@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Ben Segall <bsegall@google.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Len Brown <len.brown@intel.com>, Mel Gorman <mgorman@suse.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Valentin Schneider <vschneid@redhat.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, "Tim C . Chen" <tim.c.chen@intel.com>
Subject: Re: [RFC PATCH 01/23] sched/task_struct: Introduce classes of tasks
Message-ID: <20220916144112.GA29395@ranerica-svr.sc.intel.com>
References: <20220909231205.14009-1-ricardo.neri-calderon@linux.intel.com>
 <20220909231205.14009-2-ricardo.neri-calderon@linux.intel.com>
 <YyHbOqoH+V6FUY68@hirez.programming.kicks-ass.net>
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On Wed, Sep 14, 2022 at 03:46:34PM +0200, Peter Zijlstra wrote:
> On Fri, Sep 09, 2022 at 04:11:43PM -0700, Ricardo Neri wrote:
> 
> >  include/linux/sched.h | 7 +++++++
> >  init/Kconfig          | 9 +++++++++
> >  2 files changed, 16 insertions(+)
> > 
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index e7b2f8a5c711..acc33dbaa47c 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -117,6 +117,8 @@ struct task_group;
> >  					 __TASK_TRACED | EXIT_DEAD | EXIT_ZOMBIE | \
> >  					 TASK_PARKED)
> >  
> > +#define TASK_CLASS_UNCLASSIFIED		-1
> 
> > +#ifdef CONFIG_SCHED_TASK_CLASSES
> > +	/* Class of task that the scheduler uses for task placement decisions */
> > +	short				class;
> > +#endif

Thanks for your feedback Peter!
> 
> You're missing a hunk for init/init_task.c for this non-zero init value.

Ah, yes. I'll add it.
> 
> Does we really have to use a signed type and non-zero init value?

At least on Intel processors, class 0 is a valid class. The scheduler needs to
have a notion of unclassified tasks and decide how to handle them, IMO.

Intel processors currently support 8-bit, unsigned classes. I doubt other
architectures will ever support more than 256 classes. Short can handle all the
possible classification values and also the unclassified case.

On the other hand, class 0 could be the default classification unless hardware
classifies differently. 0 would be special and need to be documented clearly.
This would work for Intel processors.

Thanks and BR,
Ricardo