linux-kernel - Re: [PATCH v4 18/24] sched/task_struct: Add helpers for IPC classification

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Date:   Sun, 25 Jun 2023 13:23:18 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Ionela Voinescu <ionela.voinescu@....com>
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ricardo Neri <ricardo.neri@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Len Brown <len.brown@...el.com>, Mel Gorman <mgorman@...e.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Lukasz Luba <lukasz.luba@....com>,
        Zhao Liu <zhao1.liu@...el.com>,
        "Yuan, Perry" <Perry.Yuan@....com>, x86@...nel.org,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        "Tim C . Chen" <tim.c.chen@...el.com>,
        Zhao Liu <zhao1.liu@...ux.intel.com>
Subject: Re: [PATCH v4 18/24] sched/task_struct: Add helpers for IPC
 classification

On Thu, Jun 22, 2023 at 11:20:47AM +0100, Ionela Voinescu wrote:
> Hi,
> 
> On Monday 12 Jun 2023 at 21:24:16 (-0700), Ricardo Neri wrote:
> > The raw classification that hardware provides for a task may not
> > be directly usable by the scheduler: the classification may change too
> > frequently or architecture-specific implementations may need to consider
> > additional factors. For instance, some processors with Intel Thread
> > Director need to consider the state of the SMT siblings of a core.
> > 
> > Provide per-task helper variables that architectures can use to
> > postprocess the classification that hardware provides.
> > 
> > Cc: Ben Segall <bsegall@...gle.com>
> > Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
> > Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> > Cc: Ionela Voinescu <ionela.voinescu@....com>
> > Cc: Joel Fernandes (Google) <joel@...lfernandes.org>
> > Cc: Len Brown <len.brown@...el.com>
> > Cc: Lukasz Luba <lukasz.luba@....com>
> > Cc: Mel Gorman <mgorman@...e.de>
> > Cc: Perry Yuan <Perry.Yuan@....com>
> > Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > Cc: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Cc: Tim C. Chen <tim.c.chen@...el.com>
> > Cc: Valentin Schneider <vschneid@...hat.com>
> > Cc: Zhao Liu <zhao1.liu@...ux.intel.com>
> > Cc: x86@...nel.org
> > Cc: linux-pm@...r.kernel.org
> > Cc: linux-kernel@...r.kernel.org
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
> > ---
> > Changes since v3:
> >  * None
> > 
> > Changes since v2:
> >  * None
> > 
> > Changes since v1:
> >  * Used bit-fields to fit all the IPC class data in 4 bytes. (PeterZ)
> >  * Shortened names of the helpers.
> >  * Renamed helpers with the ipcc_ prefix.
> >  * Reworded commit message for clarity
> > ---
> >  include/linux/sched.h | 12 +++++++++++-
> >  1 file changed, 11 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 0e1c38ad09c2..719147460ca8 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1541,7 +1541,17 @@ struct task_struct {
> >  	 * A hardware-defined classification of task that reflects but is
> >  	 * not identical to the number of instructions per cycle.
> >  	 */
> > -	unsigned short			ipcc;
> > +	unsigned int			ipcc : 9;
> > +	/*
> > +	 * A candidate classification that arch-specific implementations
> > +	 * qualify for correctness.
> > +	 */
> > +	unsigned int			ipcc_tmp : 9;
> > +	/*
> > +	 * Counter to filter out transient candidate classifications
> > +	 * of a task.
> > +	 */
> > +	unsigned int			ipcc_cntr : 14;
> >  #endif
> >  
> 
> Why does this need to be split in task_struct? Isn't this architecture
> specific? IMO the scheduler should never make use of class information
> itself. It only receives the value though the call of an arch function
> and passes it as an argument to an arch function to get a performance
> score. So the way one interprets the class value (splits it in relevant
> and helper bits) should be defined and considered in the architecture
> specific code.

This is an excellent observation. The scheduler is unconcerned with what
the classes mean. I'll remove these auxiliary members.

This also removes 2 patches from the series.

Thanks and BR,
Ricardo