linux-kernel - Re: [PATCH v8 1/9] sched/fair: fix unfairness at wakeup

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Message-ID: <Y3JqrQJWOyHMY+G2@google.com>
Date:   Mon, 14 Nov 2022 17:19:57 +0100
From:   Patrick Bellasi <patrick.bellasi@...bug.net>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
        linux-kernel@...r.kernel.org, parth@...ux.ibm.com,
        qyousef@...alina.io, chris.hyser@...cle.com,
        David.Laight@...lab.com, pjt@...gle.com, pavel@....cz,
        tj@...nel.org, qperret@...gle.com, tim.c.chen@...ux.intel.com,
        joshdon@...gle.com, timj@....org, kprateek.nayak@....com,
        yu.c.chen@...el.com, youssefesmat@...omium.org,
        joel@...lfernandes.org
Subject: Re: [PATCH v8 1/9] sched/fair: fix unfairness at wakeup

Hi Vincent!

On 10-Nov 18:50, Vincent Guittot wrote:

[...]
  
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 1fc198be1ffd..14879d429919 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2432,9 +2432,9 @@ extern void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags);
>  extern const_debug unsigned int sysctl_sched_nr_migrate;
>  extern const_debug unsigned int sysctl_sched_migration_cost;
>  
> -#ifdef CONFIG_SCHED_DEBUG
>  extern unsigned int sysctl_sched_latency;
>  extern unsigned int sysctl_sched_min_granularity;
> +#ifdef CONFIG_SCHED_DEBUG
>  extern unsigned int sysctl_sched_idle_min_granularity;
>  extern unsigned int sysctl_sched_wakeup_granularity;
>  extern int sysctl_resched_latency_warn_ms;
> @@ -2448,6 +2448,34 @@ extern unsigned int sysctl_numa_balancing_scan_period_max;
>  extern unsigned int sysctl_numa_balancing_scan_size;
>  #endif
>  
> +static inline unsigned long  get_sched_latency(bool idle)
                                ^^^^^^^^^^^^^^^^^

This can be confusing since it's not always returning the sysctl_sched_latency
value. It's also being used to tune the vruntime at wakeup time.

Thus, what about renaming this to something more close to what's used for, e.g.
   get_wakeup_latency(se)
?

Also, in the following patches we call this always with a false parametr.
Thus, perhaps in a following patch, we can better add something like:
   #define max_wakeup_latency get_wakeup_latency(false)
?

> +{
> +	unsigned long thresh;
> +
> +	if (idle)
> +		thresh = sysctl_sched_min_granularity;
> +	else
> +		thresh = sysctl_sched_latency;
> +
> +	/*
> +	 * Halve their sleep time's effect, to allow
> +	 * for a gentler effect of sleepers:
> +	 */
> +	if (sched_feat(GENTLE_FAIR_SLEEPERS))
> +		thresh >>= 1;
> +
> +	return thresh;
> +}
> +
> +static inline unsigned long  get_latency_max(void)
                                ^^^^^^^^^^^^^^^

This is always used to cap some form of vruntime deltas in:
 - check_preempt_tick()
 - wakeup_latency_gran()
 - wakeup_preempt_entity()
It's always smaller then the max_wakeup_latency (as defined above).

Thus, does not seems something like:
   wakeup_latency_threshold()
a better documenting naming?

> +{
> +	unsigned long thresh = get_sched_latency(false);
> +
> +	thresh -= sysctl_sched_min_granularity;
> +
> +	return thresh;
> +}

[...]

Best,
Patrick

-- 
#include <best/regards.h>

Patrick Bellasi