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

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Message-ID: <20160401114129.GR3430@twins.programming.kicks-ass.net>
Date:	Fri, 1 Apr 2016 13:41:29 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vladimir Davydov <vdavydov@...tuozzo.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm v2 3/3] slub: make dead caches discard free slabs
 immediately

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.