Date: Thu, 11 Jan 2018 17:32:04 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Eric Dumazet <edumazet@google.com>
Cc: Dmitry Safonov <dima@arista.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Miller <davem@davemloft.net>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Hannes Frederic Sowa <hannes@stressinduktion.org>,
        Ingo Molnar <mingo@kernel.org>,
        "Levin, Alexander (Sasha Levin)" <alexander.levin@verizon.com>,
        Paolo Abeni <pabeni@redhat.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Radu Rendec <rrendec@arista.com>,
        Rik van Riel <riel@redhat.com>,
        Stanislaw Gruszka <sgruszka@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Wanpeng Li <wanpeng.li@hotmail.com>
Subject: Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd
 context
Message-ID: <20180111163204.GE6176@hirez.programming.kicks-ass.net>
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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.