linux-kernel - Re: [PATCH 2/6] mm/swap: Introduce alternative per-cpu LRU cache locking

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Message-ID: <20210921220358.GN4323@worktop.programming.kicks-ass.net>
Date:   Wed, 22 Sep 2021 00:03:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nicolas Saenz Julienne <nsaenzju@...hat.com>
Cc:     akpm@...ux-foundation.org, frederic@...nel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        tglx@...utronix.de, cl@...ux.com, juri.lelli@...hat.com,
        mingo@...hat.com, mtosatti@...hat.com, nilal@...hat.com,
        mgorman@...e.de, ppandit@...hat.com, williams@...hat.com,
        bigeasy@...utronix.de, anna-maria@...utronix.de,
        linux-rt-users@...r.kernel.org
Subject: Re: [PATCH 2/6] mm/swap: Introduce alternative per-cpu LRU cache
 locking

On Tue, Sep 21, 2021 at 06:13:20PM +0200, Nicolas Saenz Julienne wrote:
> +static inline void lru_cache_lock(struct lru_cache_locks *locks)
> +{
> +	if (static_branch_unlikely(&remote_pcpu_cache_access)) {
> +		/* Avoid migration between this_cpu_ptr() and spin_lock() */
> +		migrate_disable();
> +		spin_lock(this_cpu_ptr(&locks->spin));
> +	} else {
> +		local_lock(&locks->local);
> +	}
> +}

> +static inline void lru_cache_unlock(struct lru_cache_locks *locks)
> +{
> +	if (static_branch_unlikely(&remote_pcpu_cache_access)) {
> +		spin_unlock(this_cpu_ptr(&locks->spin));
> +		migrate_enable();
> +	} else {
> +		local_unlock(&locks->local);
> +	}
> +}

*why* use migrate_disable(), that's horrible!