linux-kernel - Re: [PATCH -tip 22/32] sched: Split the cookie and setup per-task cookie on fork

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Message-ID: <20201125125447.GV2414@hirez.programming.kicks-ass.net>
Date:   Wed, 25 Nov 2020 13:54:47 +0100
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, kerrnel@...gle.com,
        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,
        pjt@...gle.com, rostedt@...dmis.org, derkling@...gle.com,
        benbjiang@...cent.com,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Junaid Shahid <junaids@...gle.com>, jsbarnes@...gle.com,
        chris.hyser@...cle.com, Ben Segall <bsegall@...gle.com>,
        Josh Don <joshdon@...gle.com>, Hao Luo <haoluo@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH -tip 22/32] sched: Split the cookie and setup per-task
 cookie on fork

On Tue, Nov 17, 2020 at 06:19:52PM -0500, Joel Fernandes (Google) wrote:
> +/* Per-task interface */
> +static unsigned long sched_core_alloc_task_cookie(void)
> +{
> +	struct sched_core_cookie *ptr =
> +		kmalloc(sizeof(struct sched_core_cookie), GFP_KERNEL);
> +
> +	if (!ptr)
> +		return 0;
> +	refcount_set(&ptr->refcnt, 1);
> +
> +	/*
> +	 * NOTE: sched_core_put() is not done by put_task_cookie(). Instead, it
> +	 * is done after the stopper runs.
> +	 */
> +	sched_core_get();
> +	return (unsigned long)ptr;
> +}
> +
> +static bool sched_core_get_task_cookie(unsigned long cookie)
> +{
> +	struct sched_core_cookie *ptr = (struct sched_core_cookie *)cookie;
> +
> +	/*
> +	 * NOTE: sched_core_put() is not done by put_task_cookie(). Instead, it
> +	 * is done after the stopper runs.
> +	 */
> +	sched_core_get();
> +	return refcount_inc_not_zero(&ptr->refcnt);
> +}
> +
> +static void sched_core_put_task_cookie(unsigned long cookie)
> +{
> +	struct sched_core_cookie *ptr = (struct sched_core_cookie *)cookie;
> +
> +	if (refcount_dec_and_test(&ptr->refcnt))
> +		kfree(ptr);
> +}

> +	/*
> +	 * NOTE: sched_core_get() is done by sched_core_alloc_task_cookie() or
> +	 *       sched_core_put_task_cookie(). However, sched_core_put() is done
> +	 *       by this function *after* the stopper removes the tasks from the
> +	 *       core queue, and not before. This is just to play it safe.
> +	 */

So for no reason what so ever you've made the code more difficult?