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

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Message-Id: <20180117.172447.1568697383040902713.davem@davemloft.net>
Date:   Wed, 17 Jan 2018 17:24:47 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     edumazet@...gle.com
Cc:     tglx@...utronix.de, torvalds@...ux-foundation.org, efault@....de,
        peterz@...radead.org, dima@...sta.com, frederic@...nel.org,
        linux-kernel@...r.kernel.org, 0x7f454c46@...il.com,
        akpm@...ux-foundation.org, fweisbec@...il.com,
        hannes@...essinduktion.org, mingo@...nel.org,
        alexander.levin@...izon.com, pabeni@...hat.com,
        paulmck@...ux.vnet.ibm.com, rrendec@...sta.com, riel@...hat.com,
        sgruszka@...hat.com, wanpeng.li@...mail.com
Subject: Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd
 context

From: Eric Dumazet <edumazet@...gle.com>
Date: Wed, 17 Jan 2018 14:02:43 -0800

> 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();

Hmmm, that code predates the initial GIT repository build.

I do remember we had some back and forth with that stuff.