linux-kernel - Re: [PATCH 11/11] rcu/nocb: Simplify (de-)offloading state machine

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Message-ID: <ZoVBTCcbiTr0zWoF@localhost.localdomain>
Date: Wed, 3 Jul 2024 14:17:16 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Boqun Feng <boqun.feng@...il.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Neeraj Upadhyay <neeraj.upadhyay@....com>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>, rcu <rcu@...r.kernel.org>
Subject: Re: [PATCH 11/11] rcu/nocb: Simplify (de-)offloading state machine

Le Tue, Jul 02, 2024 at 04:19:36PM -0700, Boqun Feng a écrit :
> On Thu, May 30, 2024 at 03:45:52PM +0200, Frederic Weisbecker wrote:
> > Now that the (de-)offloading process can only apply to offline CPUs,
> > there is no more concurrency between rcu_core and nocb kthreads. Also
> > the mutation now happens on empty queues.
> > 
> > Therefore the state machine can be reduced to a single bit called
> > SEGCBLIST_OFFLOADED. Simplify the transition as follows:
> > 
> > * Upon offloading: queue the rdp to be added to the rcuog list and
> >   wait for the rcuog kthread to set the SEGCBLIST_OFFLOADED bit. Unpark
> >   rcuo kthread.
> > 
> > * Upon de-offloading: Park rcuo kthread. Queue the rdp to be removed
> >   from the rcuog list and wait for the rcuog kthread to clear the
> >   SEGCBLIST_OFFLOADED bit.
> > 
> > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > ---
> [...]
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > index 24daf606de0c..72a2990d2087 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> [...]
> > @@ -1079,29 +1080,14 @@ static int rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
> >  		 * but we stick to paranoia in this rare path.
> >  		 */
> >  		rcu_nocb_lock_irqsave(rdp, flags);
> > -		rcu_segcblist_clear_flags(&rdp->cblist, SEGCBLIST_KTHREAD_GP);
> > -		rcu_nocb_unlock_irqrestore(rdp, flags);
> > +		rcu_segcblist_clear_flags(&rdp->cblist, SEGCBLIST_OFFLOADED);
> > +		raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
> >  
> 
> Dropping rdp->nocb_lock unconditionally means we are the holder of it,
> right? If so, I think we better replace the above
> rcu_nocb_lock_irqsave() with raw_spin_lock_irqsave().

Good point, I'll do that.

Thanks.