netdev - Re: [PATCH net-next v2 12/18] openvswitch: Move ovs_frag_data_storage into the struct ovs_pcpu

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Message-ID: <20250417101820.Cd0BZc0G@linutronix.de>
Date: Thu, 17 Apr 2025 12:18:20 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Paolo Abeni <pabeni@...hat.com>
Cc: Aaron Conole <aconole@...hat.com>, netdev@...r.kernel.org,
	linux-rt-devel@...ts.linux.dev,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Simon Horman <horms@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Eelco Chaudron <echaudro@...hat.com>,
	Ilya Maximets <i.maximets@....org>, dev@...nvswitch.org
Subject: Re: [PATCH net-next v2 12/18] openvswitch: Move
 ovs_frag_data_storage into the struct ovs_pcpu_storage

On 2025-04-17 11:48:03 [+0200], Paolo Abeni wrote:
> 
> 
> On 4/17/25 11:08 AM, Sebastian Andrzej Siewior wrote:
> > On 2025-04-17 10:01:17 [+0200], Paolo Abeni wrote:
> >> @Sebastian: I think the 'owner' assignment could be optimized out at
> >> compile time for non RT build - will likely not matter for performances,
> >> but I think it will be 'nicer', could you please update the patches to
> >> do that?
> > 
> > If we don't assign the `owner' then we can't use the lock even on !RT
> > because lockdep should complain. What about this then:
> > 
> > diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c
> > index a3989d450a67f..b8f766978466d 100644
> > --- a/net/openvswitch/datapath.c
> > +++ b/net/openvswitch/datapath.c
> > @@ -294,8 +294,11 @@ void ovs_dp_process_packet(struct sk_buff *skb, struct sw_flow_key *key)
> >  	sf_acts = rcu_dereference(flow->sf_acts);
> >  	/* This path can be invoked recursively: Use the current task to
> >  	 * identify recursive invocation - the lock must be acquired only once.
> > +	 * Even with disabled bottom halves this can be preempted on PREEMPT_RT.
> > +	 * Limit the provecc to RT to avoid assigning `owner' if it can be
> > +	 * avoided.
> >  	 */
> > -	if (ovs_pcpu->owner != current) {
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT) && ovs_pcpu->owner != current) {
> >  		local_lock_nested_bh(&ovs_pcpu_storage.bh_lock);
> >  		ovs_pcpu->owner = current;
> >  		ovs_pcpu_locked = true;
> > @@ -687,9 +690,11 @@ static int ovs_packet_cmd_execute(struct sk_buff *skb, struct genl_info *info)
> >  
> >  	local_bh_disable();
> >  	local_lock_nested_bh(&ovs_pcpu_storage.bh_lock);
> > -	this_cpu_write(ovs_pcpu_storage.owner, current);
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > +		this_cpu_write(ovs_pcpu_storage.owner, current);
> 
> Perhaps implement the above 2 lines in an helper, to keep the code tidy?
> otherwise LGTM.

I've been thinking about it but the two cases are slightly different.
Maybe open coding right next to the comment isn't that bad.

> Thanks,
> 
> Paolo

Sebastian