linux-kernel - Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd context

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Date:   Wed, 17 Jan 2018 14:02:43 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        David Miller <davem@...emloft.net>,
        Mike Galbraith <efault@....de>,
        Peter Zijlstra <peterz@...radead.org>,
        Dmitry Safonov <dima@...sta.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Ingo Molnar <mingo@...nel.org>,
        Sasha Levin <alexander.levin@...izon.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Radu Rendec <rrendec@...sta.com>,
        Rik van Riel <riel@...hat.com>,
        Stanislaw Gruszka <sgruszka@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd context

On Wed, Jan 17, 2018 at 2:00 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Wed, 17 Jan 2018, Linus Torvalds wrote:
>
>> On Wed, Jan 17, 2018 at 1:54 PM, Thomas Gleixner <tglx@...utronix.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();