linux-kernel - Re: [RFC PATCH v2] Softirq:avoid large sched delay from the pending softirqs

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Message-ID: <CAKc596J7T7nbft1AEfLN09Seo1mYB_j4t=P5CXxDt6RuNyCYoA@mail.gmail.com>
Date:   Wed, 22 Jul 2020 12:58:49 +0800
From:   jun qian <qianjun.kernel@...il.com>
To:     tglx@...utronix.de, peterz@...radead.org, will@...nel.org,
        luto@...nel.org, linux-kernel@...r.kernel.org,
        Uladzislau Rezki <urezki@...il.com>
Cc:     laoar.shao@...il.com
Subject: Re: [RFC PATCH v2] Softirq:avoid large sched delay from the pending softirqs

Hi Peter & Uladzislau

Are there any issues that have not been considered in this patch? Can
you give me some suggestions on this issue. If the situation I
described is indeed a problem，how about this modification. Thanks a
lot.

On Mon, Jul 20, 2020 at 9:09 PM <qianjun.kernel@...il.com> wrote:
>
> From: jun qian <qianjun.kernel@...il.com>
>
> When get the pending softirqs, it need to process all the pending
> softirqs in the while loop. If the processing time of each pending
> softirq is need more than 2 msec in this loop, or one of the softirq
> will running a long time, according to the original code logic, it
> will process all the pending softirqs without wakeuping ksoftirqd,
> which will cause a relatively large scheduling delay on the
> corresponding CPU, which we do not wish to see. The patch will check
> the total time to process pending softirq, if the time exceeds 2 ms
> we need to wakeup the ksofirqd to aviod large sched delay.
>
> Signed-off-by: jun qian <qianjun.kernel@...il.com>
> ---
>  kernel/softirq.c | 20 +++++++++++++++++---
>  1 file changed, 17 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index c4201b7f..f8e5be9 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -210,7 +210,7 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
>   * we want to handle softirqs as soon as possible, but they
>   * should not be able to lock up the box.
>   */
> -#define MAX_SOFTIRQ_TIME  msecs_to_jiffies(2)
> +#define MAX_SOFTIRQ_TIME 2000  /* In microseconds */
>  #define MAX_SOFTIRQ_RESTART 10
>
>  #ifdef CONFIG_TRACE_IRQFLAGS
> @@ -248,7 +248,8 @@ static inline void lockdep_softirq_end(bool in_hardirq) { }
>
>  asmlinkage __visible void __softirq_entry __do_softirq(void)
>  {
> -       unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
> +       ktime_t end, start;
> +       s64 delta;
>         unsigned long old_flags = current->flags;
>         int max_restart = MAX_SOFTIRQ_RESTART;
>         struct softirq_action *h;
> @@ -256,6 +257,8 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
>         __u32 pending;
>         int softirq_bit;
>
> +       start = ktime_get();
> +
>         /*
>          * Mask out PF_MEMALLOC as the current task context is borrowed for the
>          * softirq. A softirq handled, such as network RX, might set PF_MEMALLOC
> @@ -299,6 +302,15 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
>                 }
>                 h++;
>                 pending >>= softirq_bit;
> +
> +               end = ktime_get();
> +               delta = ktime_to_us(end - start);
> +               /*
> +                * the softirq's action has been running for too much time
> +                * so it may need to wakeup the ksoftirqd
> +                */
> +               if (delta > MAX_SOFTIRQ_TIME && need_resched())
> +                       break;
>         }
>
>         if (__this_cpu_read(ksoftirqd) == current)
> @@ -307,7 +319,9 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
>
>         pending = local_softirq_pending();
>         if (pending) {
> -               if (time_before(jiffies, end) && !need_resched() &&
> +               end = ktime_get();
> +               delta = ktime_to_us(end - start);
> +               if (delta < MAX_SOFTIRQ_TIME && !need_resched() &&
>                     --max_restart)
>                         goto restart;
>
> --
> 1.8.3.1
>