netdev - Re: [PATCH net-next v2 3/3] net: dev: Make rps

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Date:   Fri, 4 Feb 2022 20:17:48 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Toke Høiland-Jørgensen 
        <toke@...e.dk>
Subject: Re: [PATCH net-next v2 3/3] net: dev: Make rps_lock() disable
 interrupts.

On Fri,  4 Feb 2022 21:12:59 +0100 Sebastian Andrzej Siewior wrote:
> Disabling interrupts and in the RPS case locking input_pkt_queue is
> split into local_irq_disable() and optional spin_lock().
> 
> This breaks on PREEMPT_RT because the spinlock_t typed lock can not be
> acquired with disabled interrupts.
> The sections in which the lock is acquired is usually short in a sense that it
> is not causing long und unbounded latiencies. One exception is the
> skb_flow_limit() invocation which may invoke a BPF program (and may
> require sleeping locks).
> 
> By moving local_irq_disable() + spin_lock() into rps_lock(), we can keep
> interrupts disabled on !PREEMPT_RT and enabled on PREEMPT_RT kernels.
> Without RPS on a PREEMPT_RT kernel, the needed synchronisation happens
> as part of local_bh_disable() on the local CPU.
> ____napi_schedule() is only invoked if sd is from the local CPU. Replace
> it with __napi_schedule_irqoff() which already disables interrupts on
> PREEMPT_RT as needed. Move this call to rps_ipi_queued() and rename the
> function to napi_schedule_rps as suggested by Jakub.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Reviewed-by: Jakub Kicinski <kuba@...nel.org>