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

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Message-ID: <0ec733daf2daaf8a6f5b1fbf56676b9892d5bf73.camel@redhat.com>
Date:   Wed, 22 Sep 2021 10:47:07 +0200
From:   nsaenzju@...hat.com
To:     Peter Zijlstra <peterz@...radead.org>
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 Wed, 2021-09-22 at 00:03 +0200, Peter Zijlstra wrote:
> 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!

I was trying to be mindful of RT. They don't appreciate people taking spinlocks
just after having disabled preemption.

I think getting local_lock(&locks->local) is my only option then. But it adds
an extra redundant spinlock in the RT+NOHZ_FULL case.

-- 
Nicolás Sáenz