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Date:   Wed, 22 Sep 2021 00:03:58 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Nicolas Saenz Julienne <nsaenzju@redhat.com>
Cc:     akpm@linux-foundation.org, frederic@kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        tglx@linutronix.de, cl@linux.com, juri.lelli@redhat.com,
        mingo@redhat.com, mtosatti@redhat.com, nilal@redhat.com,
        mgorman@suse.de, ppandit@redhat.com, williams@redhat.com,
        bigeasy@linutronix.de, anna-maria@linutronix.de,
        linux-rt-users@vger.kernel.org
Subject: Re: [PATCH 2/6] mm/swap: Introduce alternative per-cpu LRU cache
 locking
Message-ID: <20210921220358.GN4323@worktop.programming.kicks-ass.net>
References: <20210921161323.607817-1-nsaenzju@redhat.com>
 <20210921161323.607817-3-nsaenzju@redhat.com>
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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!