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

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Date:   Thu, 11 Jan 2018 17:32:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Dmitry Safonov <dima@...sta.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Miller <davem@...emloft.net>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>,
        Paolo Abeni <pabeni@...hat.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Radu Rendec <rrendec@...sta.com>,
        Rik van Riel <riel@...hat.com>,
        Stanislaw Gruszka <sgruszka@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd
 context

On Thu, Jan 11, 2018 at 08:20:18AM -0800, Eric Dumazet wrote:
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index 2f5e87f1bae22f3df44fa4493fcc8b255882267f..d2f20daf77d14dc8ebde00d7c4a0237152d082ba
> 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -192,7 +192,7 @@ EXPORT_SYMBOL(__local_bh_enable_ip);
> 
>  /*
>   * We restart softirq processing for at most MAX_SOFTIRQ_RESTART times,
> - * but break the loop if need_resched() is set or after 2 ms.
> + * but break the loop after 2 ms.
>   * The MAX_SOFTIRQ_TIME provides a nice upper bound in most cases, but in
>   * certain cases, such as stop_machine(), jiffies may cease to
>   * increment and so we need the MAX_SOFTIRQ_RESTART limit as
> @@ -299,8 +299,7 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
> 
>         pending = local_softirq_pending();
>         if (pending) {
> -               if (time_before(jiffies, end) && !need_resched() &&
> -                   --max_restart)
> +               if (time_before(jiffies, end) && --max_restart)
>                         goto restart;
> 
>                 wakeup_softirqd();

You just introduced a 2ms preempt-disable region I think, that's not
cool for PREEMPT and a plain bug on PREEMPT_RT.