netdev - Re: [PATCH net 2/2] net: gro: Let the timeout timer expire in softirq context with `threadirqs'

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Message-ID: <CANn89iKg5E1EDwHs1d1ppBbAxix+C8bbti4W-avuG1Yqe26aBQ@mail.gmail.com>
Date:   Wed, 17 Mar 2021 15:50:11 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH net 2/2] net: gro: Let the timeout timer expire in softirq
 context with `threadirqs'

On Wed, Mar 17, 2021 at 3:10 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Thu, Apr 16 2020 at 15:59, Sebastian Andrzej Siewior wrote:
> > any comments from the timer department?
>
> Yes.
>
> > On 2019-11-27 18:37:19 [+0100], To Eric Dumazet wrote:
> >> On 2019-11-27 09:11:40 [-0800], Eric Dumazet wrote:
> >> > Resent in non HTML mode :/
> >> don't worry, mutt handles both :)
> >>
> >> > Long story short, why hrtimer are not by default using threaded mode
> >> > in threadirqs mode ?
> >>
> >> Because it is only documented to thread only interrupts. Not sure if we
> >> want change this.
> >> In RT we expire most of the hrtimers in softirq context for other
> >> reasons. A subset of them still expire in hardirq context.
> >>
> >> > Idea of having some (but not all of them) hard irq handlers' now being
> >> > run from BH mode,
> >> > is rather scary.
> >>
> >> As I explained in my previous email: All IRQ-handlers fire in
> >> threaded-mode if enabled. Only the hrtimer is not affected by this
> >> change.
> >>
> >> > Also, hrtimers got the SOFT thing only in 4.16, while the GRO patch
> >> > went in linux-3.19
> >> >
> >> > What would be the plan for stable trees ?
> >> No idea yet. We could let __napi_schedule_irqoff() behave like
> >> __napi_schedule().
>
> It's not really a timer departement problem. It's an interrupt problem.
>
> With force threaded interrupts we don't call the handler with interrupts
> disabled. What sounded a good idea long ago, is actually bad.
>
> See https://lore.kernel.org/r/87eegdzzez.fsf@nanos.tec.linutronix.de
>
> Any leftover issues on a RT kernel are a different story, but for !RT
> this is the proper fix.
>
> I'll spin up a proper patch and tag it for stable...

Your patch looks much better indeed, thanks !