Re: [PATCH v4] sched/fair: unlink misfit task from cpu overutilized

[Qais Yousef]

On 01/26/23 12:42, Dietmar Eggemann wrote:
> On 19/01/2023 17:42, Vincent Guittot wrote:
> > By taking into account uclamp_min, the 1:1 relation between task misfit
> > and cpu overutilized is no more true as a task with a small util_avg may
> > not fit a high capacity cpu because of uclamp_min constraint.
> > 
> > Add a new state in util_fits_cpu() to reflect the case that task would fit
> > a CPU except for the uclamp_min hint which is a performance requirement.
> > 
> > Use -1 to reflect that a CPU doesn't fit only because of uclamp_min so we
> > can use this new value to take additional action to select the best CPU
> > that doesn't match uclamp_min hint.
> > 
> > Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > ---
> > 
> > Change since v3:
> > - Keep current condition for uclamp_max_fits in util_fits_cpu()
> > - Update some comments
> 
> We had already this discussion whether this patch can also remove
> Capacity Inversion (CapInv).
> 
> After studying the code again, I'm not so sure anymore.
> 
> This patch:
> 
> (1) adds a dedicated return value (-1) to util_fits_cpu() when:
> 
> `util fits 80% capacity_of() && util < uclamp_min && uclamp_min >
> capacity_orig_thermal (region c)`
> 
> (2) Enhancements to the CPU selection in sic() and feec() to cater for
> this new return value.

-1 means that the task fits, but only uclamp_min hint fails. ie: the task util
is small enough to run on this cpu, but it would like to run faster and this
cpu can't satisfy this request at the moment.

> 
> IMHO this doesn't make the intention of CapInv in util_fits_cpu()
> obsolete, which is using:
> 
> in CapInv:
> 
>   capacity_orig         = capacity_orig_of() - thermal_load_avg
>   capacity_orig_thermal = capacity_orig_of() - thermal_load_avg
> 
> not in CapInv:
> 
>   capacity_orig         = capacity_orig_of()
>   capacity_orig_thermal = capacity_orig_of() - th_pressure
> 
> Maybe I still miss a bit of the story?

Vincent approach is different to mine. I tried to hide all the complexity in
util_fits_cpu() so all users don't care.

But with Vincent changes, now the decision is delegated to the caller to decide
what to do if thermal pressure is causing trouble.

IOW, I expect this line only stay after Vincent patch

	capacity_orig_thermal = capacity_orig_of() - th_pressure

HTH


Cheers

--
Qais Yousef

> 
> v3 hints to removing the bits in the next version:
> 
> https://lkml.kernel.org/r/20230115001906.v7uq4ddodrbvye7d@airbuntu
> 
> >  kernel/sched/fair.c | 105 ++++++++++++++++++++++++++++++++++----------
> >  1 file changed, 82 insertions(+), 23 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index d4db72f8f84e..54e14da53274 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4561,8 +4561,8 @@ static inline int util_fits_cpu(unsigned long util,
> >  	 * handle the case uclamp_min > uclamp_max.
> >  	 */
> >  	uclamp_min = min(uclamp_min, uclamp_max);
> > -	if (util < uclamp_min && capacity_orig != SCHED_CAPACITY_SCALE)
> > -		fits = fits && (uclamp_min <= capacity_orig_thermal);
> > +	if (fits && (util < uclamp_min) && (uclamp_min > capacity_orig_thermal))
> > +		return -1;
> 
> Or does the definition 'return -1 if util fits but uclamp doesn't' make
> the distinction between capacity_orig and capacity_orig_thermal obsolete
> and so CapInv?
> 
> [...]
> 
> >  static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
> > @@ -6138,6 +6142,7 @@ static inline bool cpu_overutilized(int cpu)
> >  	unsigned long rq_util_min = uclamp_rq_get(cpu_rq(cpu), UCLAMP_MIN);
> >  	unsigned long rq_util_max = uclamp_rq_get(cpu_rq(cpu), UCLAMP_MAX);
> >  
> > +	/* Return true only if the utilization doesn't fits CPU's capacity */
> 
> small typo: s/doesn't fits/doesn't fit
> 
> [...]
> 
> > @@ -6946,12 +6952,28 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
> >  
> >  		if (!available_idle_cpu(cpu) && !sched_idle_cpu(cpu))
> >  			continue;
> > -		if (util_fits_cpu(task_util, util_min, util_max, cpu))
> > +
> > +		fits = util_fits_cpu(task_util, util_min, util_max, cpu);
> > +
> > +		/* This CPU fits with all requirements */
> > +		if (fits > 0)
> >  			return cpu;
> > +		/*
> > +		 * Only the min performance hint (i.e. uclamp_min) doesn't fit.
> > +		 * Look for the CPU with best capacity.
> > +		 */
> > +		else if (fits < 0)
> > +			cpu_cap = capacity_orig_of(cpu) - thermal_load_avg(cpu_rq(cpu));
> 
> Still don't grasp why we use thermal_load_avg() here? Looks to me that
> this would only match the CapInv case in util_fits_cpu().
> 
> [...]
>