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Date:   Tue, 18 May 2021 20:52:51 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     tj@kernel.org, hannes@cmpxchg.org, lizefan.x@bytedance.com,
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        paulmck@kernel.org, rdunlap@infradead.org,
        akpm@linux-foundation.org, tglx@linutronix.de, macro@orcam.me.uk,
        viresh.kumar@linaro.org, mike.kravetz@oracle.com,
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        cgroups@vger.kernel.org, kernel-team@android.com
Subject: Re: [PATCH v2 1/1] cgroup: make per-cgroup pressure stall tracking
 configurable
Message-ID: <20210518185251.GI5618@worktop.programming.kicks-ass.net>
References: <20210518020200.1790058-1-surenb@google.com>
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On Mon, May 17, 2021 at 07:02:00PM -0700, Suren Baghdasaryan wrote:

> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index cc25a3cff41f..4b8e72640ac9 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -148,6 +148,7 @@
>  static int psi_bug __read_mostly;
>  
>  DEFINE_STATIC_KEY_FALSE(psi_disabled);
> +DEFINE_STATIC_KEY_FALSE(psi_cgroups_disabled);

I'm thinking the whole thing will be easier/clearer when you make this:

DEFINE_STATIC_KEY_TRUE(psi_cgroups_enabled);

>  
>  #ifdef CONFIG_PSI_DEFAULT_DISABLED
>  static bool psi_enable;
> @@ -211,6 +212,9 @@ void __init psi_init(void)
>  		return;
>  	}
>  
> +	if (!cgroup_psi_enabled())
> +		static_branch_enable(&psi_cgroups_disabled);

	if (!cgroup_psi_enabled())
		static_branch_disable(&psi_cgroups_enabled);

> +
>  	psi_period = jiffies_to_nsecs(PSI_FREQ);
>  	group_init(&psi_system);
>  }
> @@ -744,23 +748,23 @@ static void psi_group_change(struct psi_group *group, int cpu,
>  
>  static struct psi_group *iterate_groups(struct task_struct *task, void **iter)
>  {
> +	if (*iter == &psi_system)
> +		return NULL;
> +
>  #ifdef CONFIG_CGROUPS
> +	if (!static_branch_likely(&psi_cgroups_disabled)) {

	if (static_branch_likely(&psi_cgroups_enabled)) {

> +		struct cgroup *cgroup = NULL;
>  
> +		if (!*iter)
> +			cgroup = task->cgroups->dfl_cgrp;
> +		else
> +			cgroup = cgroup_parent(*iter);
>  
> +		if (cgroup && cgroup_parent(cgroup)) {
> +			*iter = cgroup;
> +			return cgroup_psi(cgroup);
> +		}
>  	}
>  #endif
>  	*iter = &psi_system;
>  	return &psi_system;

But yes, very nice.