linux-kernel - [RFC PATCH net-next 0/2] net: Use SMP threads for backlog NAPI.

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Message-Id: <20230814093528.117342-1-bigeasy@linutronix.de>
Date:   Mon, 14 Aug 2023 11:35:26 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Wander Lairson Costa <wander@...hat.com>
Subject: [RFC PATCH net-next 0/2] net: Use SMP threads for backlog NAPI.

The RPS code and "deferred skb free" both send IPI/ function call
to a remote CPU in which a softirq is raised. This leads to a warning on
PREEMPT_RT because raising softiqrs from function call led to undesired
behaviour in the past. I had duct tape in RT for the "deferred skb free"
and Wander Lairson Costa reported the RPS case.

Patch #1 creates per-CPU threads for the backlog NAPI. It follows the
	 threaded NAPI model and solves the issue and simplifies the
         code.
Patch #2 gets rid of the warning. Since the ksoftirqd changes the
         situtation isn't as bad as it was. Still, it would be better to
	 keep it in the context where it originated.

Sebastian