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: <20250417090810.ps1WZHQQ@linutronix.de>
Date: Thu, 17 Apr 2025 11:08:10 +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 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);
 	err = ovs_execute_actions(dp, packet, sf_acts, &flow->key);
-	this_cpu_write(ovs_pcpu_storage.owner, NULL);
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		this_cpu_write(ovs_pcpu_storage.owner, NULL);
 	local_unlock_nested_bh(&ovs_pcpu_storage.bh_lock);
 	local_bh_enable();
 	rcu_read_unlock();

> Thanks!
> 
> Paolo

Sebastian