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Date:   Fri, 5 Feb 2021 12:41:58 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
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        Tom Lendacky <thomas.lendacky@amd.com>
Subject: Re: [PATCH v10 2/5] sched: CGroup tagging interface for core
 scheduling
Message-ID: <YB0vBpumsoraM23g@hirez.programming.kicks-ass.net>
References: <20210123011704.1901835-1-joel@joelfernandes.org>
 <20210123011704.1901835-3-joel@joelfernandes.org>
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On Fri, Jan 22, 2021 at 08:17:01PM -0500, Joel Fernandes (Google) wrote:
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -736,6 +736,7 @@ void __put_task_struct(struct task_struct *tsk)
>  	exit_creds(tsk);
>  	delayacct_tsk_free(tsk);
>  	put_signal_struct(tsk->signal);
> +	sched_tsk_free(tsk);
>  
>  	if (!profile_handoff_task(tsk))
>  		free_task(tsk);

> +struct sched_core_task_cookie {
> +	refcount_t refcnt;
> +	struct work_struct work; /* to free in WQ context. */;
> +};

> +static void sched_core_put_cookie_work(struct work_struct *ws);
> +
> +/* Caller has to call sched_core_get() if non-zero value is returned. */
> +static unsigned long sched_core_alloc_task_cookie(void)
> +{
> +	struct sched_core_task_cookie *ck =
> +		kmalloc(sizeof(struct sched_core_task_cookie), GFP_KERNEL);
> +
> +	if (!ck)
> +		return 0;
> +	refcount_set(&ck->refcnt, 1);
> +	INIT_WORK(&ck->work, sched_core_put_cookie_work);
> +
> +	return (unsigned long)ck;
> +}

> +static void sched_core_put_task_cookie(unsigned long cookie)
> +{
> +	struct sched_core_task_cookie *ptr =
> +		(struct sched_core_task_cookie *)cookie;
> +
> +	if (refcount_dec_and_test(&ptr->refcnt))
> +		kfree(ptr);
> +}

> +static void sched_core_put_cookie_work(struct work_struct *ws)
> +{
> +	struct sched_core_task_cookie *ck =
> +		container_of(ws, struct sched_core_task_cookie, work);
> +
> +	sched_core_put_task_cookie((unsigned long)ck);
> +	sched_core_put();
> +}

> +void sched_tsk_free(struct task_struct *tsk)
> +{
> +	struct sched_core_task_cookie *ck;
> +
> +	sched_core_put_cookie((struct sched_core_cookie *)tsk->core_cookie);
> +
> +	if (!tsk->core_task_cookie)
> +		return;
> +
> +	ck = (struct sched_core_task_cookie *)tsk->core_task_cookie;
> +	queue_work(system_wq, &ck->work);
> +}

*groan*.. so the purpose of that work is to be able to disable
core-scheduling after the last such task dies.

Maybe add sched_core_put_async() instead?