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

[nsaenzju]

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