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Date:   Fri, 14 May 2021 13:41:54 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     tj@kernel.org, hannes@cmpxchg.org, lizefan.x@bytedance.com,
        mingo@redhat.com, juri.lelli@redhat.com,
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        minchan@kernel.org, corbet@lwn.net, bristot@redhat.com,
        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,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org, kernel-team@android.com
Subject: Re: [PATCH 1/1] cgroup: make per-cgroup pressure stall tracking
 configurable
Message-ID: <YJ5iAvqAmIhzJRot@hirez.programming.kicks-ass.net>
References: <20210513175349.959661-1-surenb@google.com>
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On Thu, May 13, 2021 at 10:53:49AM -0700, Suren Baghdasaryan wrote:

> +bool cgroup_psi_enabled(void)
> +{
> +	return (cgroup_feature_disable_mask & (1 << OPT_FEATURE_PRESSURE)) == 0;
> +}

> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index cc25a3cff41f..c73efd7d4fba 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -747,9 +747,12 @@ static struct psi_group *iterate_groups(struct task_struct *task, void **iter)
>  #ifdef CONFIG_CGROUPS
>  	struct cgroup *cgroup = NULL;
>  
> -	if (!*iter)
> +	if (!*iter) {
> +		/* Skip to psi_system if per-cgroup accounting is disabled */
> +		if (!cgroup_psi_enabled())
> +			goto update_sys;
>  		cgroup = task->cgroups->dfl_cgrp;
> -	else if (*iter == &psi_system)
> +	} else if (*iter == &psi_system)
>  		return NULL;
>  	else
>  		cgroup = cgroup_parent(*iter);
> @@ -758,6 +761,7 @@ static struct psi_group *iterate_groups(struct task_struct *task, void **iter)
>  		*iter = cgroup;
>  		return cgroup_psi(cgroup);
>  	}
> +update_sys:
>  #else
>  	if (*iter)
>  		return NULL;

I'm confused; shouldn't that do the same as that #else branch? Also, can
you pretty please make cgroup_psi_enabled() a static_key ?