Re: [PATCH -mm v2 3/3] slub: make dead caches discard free slabs immediately

[Peter Zijlstra]

On Fri, Apr 01, 2016 at 01:55:40PM +0300, Vladimir Davydov wrote:
> > > +	if (deactivate) {
> > > +		/*
> > > +		 * Disable empty slabs caching. Used to avoid pinning offline
> > > +		 * memory cgroups by kmem pages that can be freed.
> > > +		 */
> > > +		s->cpu_partial = 0;
> > > +		s->min_partial = 0;
> > > +
> > > +		/*
> > > +		 * s->cpu_partial is checked locklessly (see put_cpu_partial),
> > > +		 * so we have to make sure the change is visible.
> > > +		 */
> > > +		kick_all_cpus_sync();
> > > +	}
> > 
> > Argh! what the heck! and without a single mention in the changelog.
> 
> This function is only called when a memory cgroup is removed, which is
> rather a rare event. I didn't think it would cause any pain. Sorry.

Suppose you have a bunch of CPUs running HPC/RT code and someone causes
the admin CPUs to create/destroy a few cgroups.

> > Why are you spraying IPIs across the entire machine? Why isn't
> > synchronize_sched() good enough, that would allow you to get rid of the
> > local_irq_save/restore as well.
> 
> synchronize_sched() is slower. Calling it for every per memcg kmem cache
> would slow down cleanup on cgroup removal.

Right, but who cares? cgroup removal isn't a fast path by any standard.

> Regarding local_irq_save/restore - synchronize_sched() wouldn't allow us
> to get rid of them, because unfreeze_partials() must be called with irqs
> disabled.

OK, I figured it was because it needed to be serialized against this
kick_all_cpus_sync() IPI.

> Come to think of it, kick_all_cpus_sync() is used as a memory barrier
> here, so as to make sure that after it's finished all cpus will use the
> new ->cpu_partial value, which makes me wonder if we could replace it
> with a simple smp_mb. I mean, this_cpu_cmpxchg(), which is used by
> put_cpu_partial to add a page to per-cpu partial list, must issue a full
> memory barrier (am I correct?), so we have two possibilities here:

Nope, this_cpu_cmpxchg() does not imply a memory barrier.