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

[Ricardo Neri]

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@xxxxxxxxxx>
> > Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> > Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> > Cc: Ionela Voinescu <ionela.voinescu@xxxxxxx>
> > Cc: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > Cc: Len Brown <len.brown@xxxxxxxxx>
> > Cc: Lukasz Luba <lukasz.luba@xxxxxxx>
> > Cc: Mel Gorman <mgorman@xxxxxxx>
> > Cc: Perry Yuan <Perry.Yuan@xxxxxxx>
> > Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > Cc: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Cc: Tim C. Chen <tim.c.chen@xxxxxxxxx>
> > Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
> > Cc: Zhao Liu <zhao1.liu@xxxxxxxxxxxxxxx>
> > Cc: x86@xxxxxxxxxx
> > Cc: linux-pm@xxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> > ---
> > 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