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From:   Eric Dumazet <edumazet@google.com>
Date:   Wed, 17 Jan 2018 14:02:43 -0800
Message-ID: <CANn89iJmA-21uL+pPBiV6zsrR7nY6RJ6bamDCgPctMoNWpa4Hw@mail.gmail.com>
Subject: Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd context
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        David Miller <davem@davemloft.net>,
        Mike Galbraith <efault@gmx.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Dmitry Safonov <dima@arista.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Hannes Frederic Sowa <hannes@stressinduktion.org>,
        Ingo Molnar <mingo@kernel.org>,
        Sasha Levin <alexander.levin@verizon.com>,
        Paolo Abeni <pabeni@redhat.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Radu Rendec <rrendec@arista.com>,
        Rik van Riel <riel@redhat.com>,
        Stanislaw Gruszka <sgruszka@redhat.com>,
        Wanpeng Li <wanpeng.li@hotmail.com>
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On Wed, Jan 17, 2018 at 2:00 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
> On Wed, 17 Jan 2018, Linus Torvalds wrote:
>
>> On Wed, Jan 17, 2018 at 1:54 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>> > raise_softirq() -> raise_softirq_irqoff()
>> >
>> >         set_softirq_bit();
>> >
>> >         if (!in_interrupt())
>> >                 wake_softirqd();
>> >
>> > So if the caller is not in hard or soft interrupt context, which includes
>> > bottom half disabled regions softirqd is woken.
>>
>> That does seem unnecessarily expensive, and maybe we could just do it
>> with thread flag (TIF_NOTIFY_RESUME or whatever).
>>
>> In fact, that was what I *thought* we did. Maybe I just remember some
>> historical behavior.
>>
>> Since networking seems to largely prefer softirqd anyway, maybe that
>> wake_softirqd() is the right thing to do anyway.
>
> Well, but we only do it when we are not in a bh disabled region. The places
> where thread context raises the network softirqs is usually inside a bh
> disabled region, so the softirq is executed on local_bh_enable(). The
> thread is woken up rarely.

There is also the netif_rx_ni() stuff.

Can't remember right now why it is not using
local_bh_{diable,enable}() pair instead
of preempt_disable() ... if (local_softirq_pending()) do_softirq();