linux-kernel - Re: [PATCH resend 5/8] sched: cgroup cookie API for core scheduling

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Message-ID: <YGLt/ltwa92lfCDK@hirez.programming.kicks-ass.net>
Date:   Tue, 30 Mar 2021 11:23:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        torvalds@...ux-foundation.org, fweisbec@...il.com,
        keescook@...omium.org, Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
        graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
        rostedt@...dmis.org, benbjiang@...cent.com,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
        Dhaval Giani <dhaval.giani@...cle.com>, chris.hyser@...cle.com,
        Josh Don <joshdon@...gle.com>, Hao Luo <haoluo@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>, dhiatt@...italocean.com
Subject: Re: [PATCH resend 5/8] sched: cgroup cookie API for core scheduling

On Wed, Mar 24, 2021 at 05:40:17PM -0400, Joel Fernandes (Google) wrote:
> From: Josh Don <joshdon@...gle.com>
> 
> This adds the API to set/get the cookie for a given cgroup. This
> interface lives at cgroup/cpu.core_tag.
> 
> The cgroup interface can be used to toggle a unique cookie value for all
> descendent tasks, preventing these tasks from sharing with any others.
> See Documentation/admin-guide/hw-vuln/core-scheduling.rst for a full
> rundown of both this and the per-task API.

I refuse to read RST. Life's too short for that.

> +u64 cpu_core_tag_read_u64(struct cgroup_subsys_state *css,
> +			  struct cftype *cft)
> +{
> +	return !!css_tg(css)->core_tagged;
> +}
> +
> +int cpu_core_tag_write_u64(struct cgroup_subsys_state *css, struct cftype *cft,
> +			   u64 val)
> +{
> +	static DEFINE_MUTEX(sched_core_group_mutex);
> +	struct task_group *tg = css_tg(css);
> +	struct cgroup_subsys_state *css_tmp;
> +	struct task_struct *p;
> +	unsigned long group_cookie;
> +	int ret = 0;
> +
> +	if (val > 1)
> +		return -ERANGE;
> +
> +	if (!static_branch_likely(&sched_smt_present))
> +		return -EINVAL;
> +
> +	mutex_lock(&sched_core_group_mutex);
> +
> +	if (!tg->core_tagged && val) {
> +		/* Tag is being set. Check ancestors and descendants. */
> +		if (cpu_core_get_group_cookie(tg) ||
> +		    cpu_core_check_descendants(tg, true /* tag */)) {
> +			ret = -EBUSY;
> +			goto out_unlock;
> +		}

So the desired semantics is to only allow a single tag on any upwards
path? Isn't that in conflict with the cgroup requirements?

TJ?

> +	} else if (tg->core_tagged && !val) {
> +		/* Tag is being reset. Check descendants. */
> +		if (cpu_core_check_descendants(tg, true /* tag */)) {

I'm struggling to understand this. If, per the above, you cannot set
when either a parent is already set or a child is set, then how can a
child be set to refuse clearing?

> +			ret = -EBUSY;
> +			goto out_unlock;
> +		}
> +	} else {
> +		goto out_unlock;
> +	}