linux-kernel - Re: [PATCH] rcu: shrink each possible cpu krcp

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Message-ID: <CAEXW_YRZ6RM90+aYA0JQ1war0n-D0M4peXJZE2_Uqf07xvF+5g@mail.gmail.com>
Date:   Tue, 18 Aug 2020 20:04:20 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Uladzislau Rezki <urezki@...il.com>, qiang.zhang@...driver.com,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        rcu <rcu@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rcu: shrink each possible cpu krcp

On Tue, Aug 18, 2020 at 6:02 PM Paul E. McKenney <paulmck@...nel.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.

thanks,

 - Joel