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Date:   Thu, 12 Jan 2023 14:26:03 +0000
From:   Qais Yousef <qyousef@layalina.io>
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     mingo@kernel.org, peterz@infradead.org, dietmar.eggemann@arm.com,
        rafael@kernel.org, viresh.kumar@linaro.org, vschneid@redhat.com,
        linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org,
        lukasz.luba@arm.com, wvw@google.com, xuewen.yan94@gmail.com,
        han.lin@mediatek.com, Jonathan.JMChen@mediatek.com
Subject: Re: [PATCH v2] sched/fair: unlink misfit task from cpu overutilized
Message-ID: <20230112142603.yj5dtleksi6sfrzp@airbuntu>
References: <20221228165415.3436-1-vincent.guittot@linaro.org>
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In-Reply-To: <20221228165415.3436-1-vincent.guittot@linaro.org>
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Hi Vincent

On 12/28/22 17:54, 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 of
> may not may not fit a high capacity cpu because of uclamp_min constraint.

Wouldn't it be better to split this into two patches

	* Unlink/Decouple misfit ...
	* Unlink/Decouple util_fits_cpu from HMP

?

> 
> 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.

This part has nothing to do with the commit subject. I think it's better to
split the patches if it's not too much work for you.

> 
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
> 
> Change since v1:
> - fix some wrong conditions
> - take into account more cases
> 
>  kernel/sched/fair.c | 99 +++++++++++++++++++++++++++++++++------------
>  1 file changed, 74 insertions(+), 25 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1649e7d71d24..57077f0a897e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4582,8 +4582,7 @@ static inline int util_fits_cpu(unsigned long util,
>  	 *     2. The system is being saturated when we're operating near
>  	 *        max capacity, it doesn't make sense to block overutilized.
>  	 */
> -	uclamp_max_fits = (capacity_orig == SCHED_CAPACITY_SCALE) && (uclamp_max == SCHED_CAPACITY_SCALE);
> -	uclamp_max_fits = !uclamp_max_fits && (uclamp_max <= capacity_orig);
> +	uclamp_max_fits = (uclamp_max <= capacity_orig) || (capacity_orig == SCHED_CAPACITY_SCALE);
>  	fits = fits || uclamp_max_fits;
>  
>  	/*
> @@ -4618,8 +4617,8 @@ 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;
>  
>  	return fits;
>  }
> @@ -4629,7 +4628,7 @@ static inline int task_fits_cpu(struct task_struct *p, int cpu)
>  	unsigned long uclamp_min = uclamp_eff_value(p, UCLAMP_MIN);
>  	unsigned long uclamp_max = uclamp_eff_value(p, UCLAMP_MAX);
>  	unsigned long util = task_util_est(p);
> -	return util_fits_cpu(util, uclamp_min, uclamp_max, cpu);
> +	return (util_fits_cpu(util, uclamp_min, uclamp_max, cpu) > 0);

So the big difference between your approach and my approach is that
task_fits_cpu() and asym_fits_cpu() now are very strict in regards to thermal
pressure since with your approach we delegate the smartness to the caller.

Should we add a comment for these 2 users to make it obvious we intentionally
ignore the '-1' value and why it is okay?

I'm not sure I can write a reasonable rationale myself. I'm actually worried
this might subtly break decisions made by select_idle_capacity() or feec() when
doing the LB.

Have you considered this?

>  }
>  
>  static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
> @@ -6864,6 +6863,7 @@ static int
>  select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
>  {
>  	unsigned long task_util, util_min, util_max, best_cap = 0;
> +	int fits, best_fits = 0;
>  	int cpu, best_cpu = -1;
>  	struct cpumask *cpus;
>  
> @@ -6879,12 +6879,28 @@ 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 capacity and performance requirements */
> +		if (fits > 0)
>  			return cpu;
> +		/*
> +		 * Only the min performance (i.e. uclamp_min) doesn't fit. Look
> +		 * for the CPU with highest performance capacity.
> +		 */
> +		else if (fits < 0)
> +			cpu_cap = capacity_orig_of(cpu) - thermal_load_avg(cpu_rq(cpu));
>  
> -		if (cpu_cap > best_cap) {
> +		/*
> +		 * First, select cpu which fits better (-1 being better than 0).
> +		 * Then, select the one with largest capacity at same level.
> +		 */
> +		if ((fits < best_fits) ||
> +		    ((fits == best_fits) && (cpu_cap > best_cap))) {
>  			best_cap = cpu_cap;
>  			best_cpu = cpu;
> +			best_fits = fits;
>  		}
>  	}
>  
> @@ -6897,7 +6913,7 @@ static inline bool asym_fits_cpu(unsigned long util,
>  				 int cpu)
>  {
>  	if (sched_asym_cpucap_active())
> -		return util_fits_cpu(util, util_min, util_max, cpu);
> +		return (util_fits_cpu(util, util_min, util_max, cpu) > 0);
>  
>  	return true;
>  }
> @@ -7264,6 +7280,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>  	unsigned long p_util_max = uclamp_is_used() ? uclamp_eff_value(p, UCLAMP_MAX) : 1024;
>  	struct root_domain *rd = this_rq()->rd;
>  	int cpu, best_energy_cpu, target = -1;
> +	int prev_fits = -1, best_fits = -1;
> +	unsigned long best_thermal_cap = 0;
> +	unsigned long prev_thermal_cap = 0;
>  	struct sched_domain *sd;
>  	struct perf_domain *pd;
>  	struct energy_env eenv;
> @@ -7299,6 +7318,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>  		unsigned long prev_spare_cap = 0;
>  		int max_spare_cap_cpu = -1;
>  		unsigned long base_energy;
> +		int fits, max_fits = -1;
>  
>  		cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask);
>  
> @@ -7351,7 +7371,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>  					util_max = max(rq_util_max, p_util_max);
>  				}
>  			}
> -			if (!util_fits_cpu(util, util_min, util_max, cpu))
> +
> +			fits = util_fits_cpu(util, util_min, util_max, cpu);
> +			if (!fits)
>  				continue;
>  
>  			lsub_positive(&cpu_cap, util);
> @@ -7359,7 +7381,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>  			if (cpu == prev_cpu) {
>  				/* Always use prev_cpu as a candidate. */
>  				prev_spare_cap = cpu_cap;
> -			} else if (cpu_cap > max_spare_cap) {
> +				prev_fits = fits;
> +			} else if ((fits > max_fits) ||
> +				   ((fits == max_fits) && (cpu_cap > max_spare_cap))) {
>  				/*
>  				 * Find the CPU with the maximum spare capacity
>  				 * among the remaining CPUs in the performance
> @@ -7367,6 +7391,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>  				 */
>  				max_spare_cap = cpu_cap;
>  				max_spare_cap_cpu = cpu;
> +				max_fits = fits;
>  			}
>  		}
>  
> @@ -7385,26 +7410,50 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>  			if (prev_delta < base_energy)
>  				goto unlock;
>  			prev_delta -= base_energy;
> +			prev_thermal_cap = cpu_thermal_cap;
>  			best_delta = min(best_delta, prev_delta);
>  		}
>  
>  		/* Evaluate the energy impact of using max_spare_cap_cpu. */
>  		if (max_spare_cap_cpu >= 0 && max_spare_cap > prev_spare_cap) {
> +			/* Current best energy cpu fits better */
> +			if (max_fits < best_fits)
> +				continue;
> +
> +			/*
> +			 * Both don't fit performance (i.e. uclamp_min) but
> +			 * best energy cpu has better performance.
> +			 */
> +			if ((max_fits < 0) &&
> +			    (cpu_thermal_cap <= best_thermal_cap))
> +				continue;
> +
>  			cur_delta = compute_energy(&eenv, pd, cpus, p,
>  						   max_spare_cap_cpu);
>  			/* CPU utilization has changed */
>  			if (cur_delta < base_energy)
>  				goto unlock;
>  			cur_delta -= base_energy;
> -			if (cur_delta < best_delta) {
> -				best_delta = cur_delta;
> -				best_energy_cpu = max_spare_cap_cpu;
> -			}
> +
> +			/*
> +			 * Both fit for the task but best energy cpu has lower
> +			 * energy impact.
> +			 */
> +			if ((max_fits > 0) &&

Shouldn't this be

			if ((max_fits > 0) && (max_fits == best_fits) &&
?

We should update best_delta unconditionally first time we hit max_fits = 1, no?

I think it's worth extending the comment with something along the lines of

			* ... except for the first time max_fits becomes 1
			* then we must update best_delta unconditionally

> +			    (cur_delta >= best_delta))
> +				continue;
> +
> +			best_delta = cur_delta;
> +			best_energy_cpu = max_spare_cap_cpu;
> +			best_fits = max_fits;
> +			best_thermal_cap = cpu_thermal_cap;
>  		}
>  	}
>  	rcu_read_unlock();
>  
> -	if (best_delta < prev_delta)
> +	if ((best_fits > prev_fits) ||
> +	    ((best_fits > 0) && (best_delta < prev_delta)) ||
> +	    ((best_fits < 0) && (best_thermal_cap > prev_thermal_cap)))
>  		target = best_energy_cpu;

Overall I think the approach is sound. I tested it on my pinebook pro and
couldn't catch obvious breakage at least.

I am still worried though about spilling the knowledge outside of
util_fits_cpu() is creating extra complexity in the callers and potentially
more fragility when these callers evolve overtime e.g:
task_fits_cpu()/asym_fits_cpu() gain a new user that must actually care about
the -1 return value.

I think we can still optimize the capacity inversion logic to use no loops
without having to spill the knowledge to the users/callers of util_fits_cpu(),
no?

That said except for the few comments I had this LGTM anyway. Thanks for your
effort!


Cheers

--
Qais Yousef

>  
>  	return target;
> @@ -10228,24 +10277,23 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
>  	 */
>  	update_sd_lb_stats(env, &sds);
>  
> -	if (sched_energy_enabled()) {
> -		struct root_domain *rd = env->dst_rq->rd;
> -
> -		if (rcu_dereference(rd->pd) && !READ_ONCE(rd->overutilized))
> -			goto out_balanced;
> -	}
> -
> -	local = &sds.local_stat;
> -	busiest = &sds.busiest_stat;
> -
>  	/* There is no busy sibling group to pull tasks from */
>  	if (!sds.busiest)
>  		goto out_balanced;
>  
> +	busiest = &sds.busiest_stat;
> +
>  	/* Misfit tasks should be dealt with regardless of the avg load */
>  	if (busiest->group_type == group_misfit_task)
>  		goto force_balance;
>  
> +	if (sched_energy_enabled()) {
> +		struct root_domain *rd = env->dst_rq->rd;
> +
> +		if (rcu_dereference(rd->pd) && !READ_ONCE(rd->overutilized))
> +			goto out_balanced;
> +	}
> +
>  	/* ASYM feature bypasses nice load balance check */
>  	if (busiest->group_type == group_asym_packing)
>  		goto force_balance;
> @@ -10258,6 +10306,7 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
>  	if (busiest->group_type == group_imbalanced)
>  		goto force_balance;
>  
> +	local = &sds.local_stat;
>  	/*
>  	 * If the local group is busier than the selected busiest group
>  	 * don't try and pull any tasks.
> -- 
> 2.17.1
>