linux-kernel - Re: [PATCH 4/9] softirq: Allow early break

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Date:   Fri, 5 May 2023 14:10:26 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Liu Jian <liujian56@...wei.com>
Cc:     corbet@....net, paulmck@...nel.org, frederic@...nel.org,
        quic_neeraju@...cinc.com, joel@...lfernandes.org,
        josh@...htriplett.org, boqun.feng@...il.com, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        qiang1.zhang@...el.com, jstultz@...gle.com, tglx@...utronix.de,
        sboyd@...nel.org, davem@...emloft.net, kuba@...nel.org,
        pabeni@...hat.com, peterz@...radead.org, frankwoo@...gle.com,
        Rhinewuwu@...gle.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH 4/9] softirq: Allow early break

On Fri, May 5, 2023 at 1:24 PM Liu Jian <liujian56@...wei.com> wrote:
>
> From: Peter Zijlstra <peterz@...radead.org>
>
> Allow terminating the softirq processing loop without finishing the
> vectors.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Liu Jian <liujian56@...wei.com>
> ---
>  kernel/softirq.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index 48a81d8ae49a..e2cad5d108c8 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -582,6 +582,9 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
>                                prev_count, preempt_count());
>                         preempt_count_set(prev_count);
>                 }
> +
> +               if (pending && __softirq_needs_break(start))
> +                       break;

This means that some softirq may starve, because

for_each_set_bit(vec_nr, &pending, NR_SOFTIRQS)

Always iterate using the same order (of bits)




>         }
>
>         if (!IS_ENABLED(CONFIG_PREEMPT_RT) &&
> @@ -590,13 +593,14 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
>
>         local_irq_disable();
>
> -       pending = local_softirq_pending();
> -       if (pending) {
> -               if (!__softirq_needs_break(start) && --max_restart)
> -                       goto restart;
> +       if (pending)
> +               or_softirq_pending(pending);
> +       else if ((pending = local_softirq_pending()) &&
> +                !__softirq_needs_break(start) &&
> +                --max_restart)
> +               goto restart;
>
> -               wakeup_softirqd();
> -       }
> +       wakeup_softirqd();
>
>         account_softirq_exit(current);
>         lockdep_softirq_end(in_hardirq);
> --
> 2.34.1
>