linux-kernel - Re: [PATCH v10 2/5] sched: CGroup tagging interface for core scheduling

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Message-ID: <YBwGRpnISmwkJ2i8@hirez.programming.kicks-ass.net>
Date:   Thu, 4 Feb 2021 15:35:50 +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>
Subject: Re: [PATCH v10 2/5] sched: CGroup tagging interface for core
 scheduling

On Fri, Jan 22, 2021 at 08:17:01PM -0500, Joel Fernandes (Google) wrote:
> +static inline void __sched_core_erase_cookie(struct sched_core_cookie *cookie)
> +{
> +	lockdep_assert_held(&sched_core_cookies_lock);
> +
> +	/* Already removed */
> +	if (RB_EMPTY_NODE(&cookie->node))
> +		return;
> +
> +	rb_erase(&cookie->node, &sched_core_cookies);
> +	RB_CLEAR_NODE(&cookie->node);
> +}
> +
> +/* Called when a task no longer points to the cookie in question */
> +static void sched_core_put_cookie(struct sched_core_cookie *cookie)
> +{
> +	unsigned long flags;
> +
> +	if (!cookie)
> +		return;
> +
> +	if (refcount_dec_and_test(&cookie->refcnt)) {
> +		raw_spin_lock_irqsave(&sched_core_cookies_lock, flags);
> +		__sched_core_erase_cookie(cookie);
> +		raw_spin_unlock_irqrestore(&sched_core_cookies_lock, flags);
> +		kfree(cookie);
> +	}
> +}

> +static void __sched_core_update_cookie(struct task_struct *p)
> +{

> +	raw_spin_lock(&sched_core_cookies_lock);

> +		/*
> +		 * Cookie exists, increment refcnt. If refcnt is currently 0,
> +		 * we're racing with a put() (refcnt decremented but cookie not
> +		 * yet removed from the tree). In this case, we can simply
> +		 * perform the removal ourselves and retry.
> +		 * sched_core_put_cookie() will still function correctly.
> +		 */
> +		if (unlikely(!refcount_inc_not_zero(&match->refcnt))) {
> +			__sched_core_erase_cookie(match);
> +			goto retry;
> +		}

refcount_dec_and_lock() avoids that complication.