linux-kernel - Re: [PATCH] softirq: wake up ktimer thread in softirq context

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Date:   Tue, 20 Dec 2022 09:10:45 -0800
From:   Alison Chaiken <achaiken@...ora.tech>
To:     "Chang, Junxiao" <junxiao.chang@...el.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rt-users@...r.kernel.org" <linux-rt-users@...r.kernel.org>,
        "bigeasy@...utronix.de" <bigeasy@...utronix.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "Peh, Hock Zhang" <hock.zhang.peh@...el.com>,
        Glenn Elliott <glenn@...ora.tech>,
        Rami Jaber <rjaber@...ora.tech>
Subject: Re: [PATCH] softirq: wake up ktimer thread in softirq context

On Tue, Dec 20, 2022 at 2:44 AM Chang, Junxiao <junxiao.chang@...el.com> wrote:
> Any comment? This patch is for 6.1-rt, issue could be reproduced with 5.19-rt kernel as well.

In

https://lore.kernel.org/linux-rt-users/CAFzL-7v-NSFKAsyhxReEES7bMomSTwYyrZscnjbkydEP3CTXmQ@mail.gmail.com/

we reported an occasional problem with an x86 system entering a deep C
state even though timers were pending.   Perhaps your patch would
prevent this transition.

> This issue is easier to reproduced when there is heavy network workload which introduces a lot of softirq events. If hrtimer interrupt is triggered in softirq context, with current RT kernel, it will not wake up ktimers thread which handles hrtimer event because in function __irq_exit_rcu, "in_interrupt()" is true:
>
> static inline void __irq_exit_rcu(void)
> {
> ...
>         preempt_count_sub(HARDIRQ_OFFSET);
>         if (!in_interrupt()) {
>                 if (local_softirq_pending())
>                         invoke_softirq();
>
>                 if (IS_ENABLED(CONFIG_PREEMPT_RT) && local_pending_timers())
>                         wake_timersd();
>         }
> ...
> }
>

Isn't removing the check for IS_ENABLED(CONFIG_PREEMPT_RT) inadvisable?

> Then ktimers threads stays in sleep state, hrtimer function will not be called although hrtimer interrupt has been triggered. Ktimers thread might be woken up in next timer interrupt which introduces long delay.
>
> Any comments are welcome.
>
> Regards,
> Junxiao

-- Alison Chaiken
Aurora Innovation


 -----Original Message-----
> From: Chang, Junxiao <junxiao.chang@...el.com>
> Sent: Thursday, December 8, 2022 3:56 PM
> To: linux-kernel@...r.kernel.org
> Cc: linux-rt-users@...r.kernel.org; bigeasy@...utronix.de; tglx@...utronix.de; rostedt@...dmis.org; Chang, Junxiao <junxiao.chang@...el.com>; Peh, Hock Zhang <hock.zhang.peh@...el.com>
> Subject: [PATCH] softirq: wake up ktimer thread in softirq context
>
> Occiaionally timer interrupt might be triggered in softirq context, ktimer thread should be woken up with RT kernel, or else ktimer thread might stay in sleep state although timer interrupt has been triggered.
>
> This change fixes a latency issue that timer handler is delayed for more than 4ms in network related test.
>
> Fixes: 2165d27554e8 ("softirq: Use a dedicated thread for timer wakeups.")
> Reported-by: Peh, Hock Zhang <hock.zhang.peh@...el.com>
> Signed-off-by: Junxiao Chang <junxiao.chang@...el.com>
> ---
>  kernel/softirq.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/softirq.c b/kernel/softirq.c index ab1fe34326bab..34ae39e4a3d10 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -664,13 +664,10 @@ static inline void __irq_exit_rcu(void)  #endif
>         account_hardirq_exit(current);
>         preempt_count_sub(HARDIRQ_OFFSET);
> -       if (!in_interrupt()) {
> -               if (local_softirq_pending())
> -                       invoke_softirq();
> -
> -               if (IS_ENABLED(CONFIG_PREEMPT_RT) && local_pending_timers())
> -                       wake_timersd();
> -       }
> +       if (!in_interrupt() && local_softirq_pending())
> +               invoke_softirq();
> +       if (!(in_nmi() || in_hardirq()) && local_pending_timers())
> +               wake_timersd();
>
>         tick_irq_exit();
>  }
> --
> 2.25.1
>