linux-kernel - Re: [PATCH RFC tip/core/rcu] Add shrinker to shift to fast/inefficient GP mode

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Date:   Fri, 8 May 2020 07:47:16 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Hillf Danton <hdanton@...a.com>
Cc:     linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
        akpm@...ux-foundation.org, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH RFC tip/core/rcu] Add shrinker to shift to
 fast/inefficient GP mode

On Fri, May 08, 2020 at 09:37:43PM +0800, Hillf Danton wrote:
> 
> On Thu, 7 May 2020 08:49:10 Paul E. McKenney wrote:
> >  
> > > +static void rcu_mph_info(void *data)
> >
> > This pointer will always be &rcu_state, so why not ignore the pointer
> > and use "rcu_state" below?
> >
> Yes you're right.
> 
> > RCU grace periods are inherently global, so I don't know of any way
> > for RCU to focus on a given NUMA node.  All or nothing.
> 
> Or is it feasible to expose certain RCU thing to VM, say, with which kswapd
> can kick grace period every time the kthreads think it's needed? That way
> the work to gauge memory pressure can be off RCU's shoulders.

A pair of functions RCU provides is easy for me.  ;-)

							Thanx, Paul

> > But on the
> > other hand, speeding up RCU grace periods will also help specific
> > NUMA nodes, so I believe that it is all good.
> > 
> > > +{
> > > +	struct rcu_state *state = data;
> > > +
> > > +	WRITE_ONCE(state->mph_end, jiffies + HZ / 10);
> > > +	smp_store_release(&state->mph, true);
> > > +	rcu_force_quiescent_state();
> > > +}
> > > +
> > > +static struct mph_subscriber rcu_mph_subscriber = {
> > > +	.info = rcu_mph_info,
> > > +	.data = &rcu_state,
> > 
> > Then this ".data" entry can be omitted, correct?
> 
> Yes :)
> 
> Hillf
>