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Date:   Wed, 19 Aug 2020 09:25:47 -0400
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Uladzislau Rezki <urezki@gmail.com>
Cc:     "Paul E. McKenney" <paulmck@kernel.org>, qiang.zhang@windriver.com,
        Josh Triplett <josh@joshtriplett.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        rcu <rcu@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] rcu: shrink each possible cpu krcp
Message-ID: <20200819132547.GA3875610@google.com>
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On Wed, Aug 19, 2020 at 01:22:25PM +0200, Uladzislau Rezki wrote:
> On Tue, Aug 18, 2020 at 08:04:20PM -0400, Joel Fernandes wrote:
> > On Tue, Aug 18, 2020 at 6:02 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> > 
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index b8ccd7b5af82..6decb9ad2421 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -2336,10 +2336,15 @@ int rcutree_dead_cpu(unsigned int cpu)
> > > >  {
> > > >         struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> > > >         struct rcu_node *rnp = rdp->mynode;  /* Outgoing CPU's rdp & rnp. */
> > > > +       struct kfree_rcu_cpu *krcp;
> > > >
> > > >         if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
> > > >                 return 0;
> > > >
> > > > +       /* Drain the kcrp of this CPU. IRQs should be disabled? */
> > > > +       krcp = this_cpu_ptr(&krc)
> > > > +       schedule_delayed_work(&krcp->monitor_work, 0);
> > > > +
> > > >
> > > > A cpu can be offlined and its krp will be stuck until a shrinker is involved.
> > > > Maybe be never.
> > >
> > > Does the same apply to its kmalloc() per-CPU caches?  If so, I have a
> > > hard time getting too worried about it.  ;-)
> > 
> > Looking at slab_offline_cpu() , that calls cancel_delayed_work_sync()
> > on the cache reaper who's job is to flush the per-cpu caches. So I
> > believe during CPU offlining, the per-cpu slab caches are flushed.
> > 
> SLAB does it for sure, same as page allocator. There are special CPU-offline
> callbacks for both cases to perform cleanup when CPU dies.

Got it, thanks for confirming, makes sense.

thanks,

 - Joel