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Date:   Tue, 27 Sep 2022 15:01:07 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
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
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On Fri, Sep 16, 2022 at 07:41:13AM -0700, Ricardo Neri wrote:

> 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.

You can always do: class = hw_class + 1; that makes 0 'special' and the
hardware class can be trivially reconstructed by subtracting 1.