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

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Message-ID: <CAFzL-7t+Lq3ZJk90w=75yP++DbD1TGo16GRZkDzUNCyKpUH1TA@mail.gmail.com>
Date:   Tue, 20 Dec 2022 11:28:32 -0800
From:   Alison Chaiken <achaiken@...ora.tech>
To:     "bigeasy@...utronix.de" <bigeasy@...utronix.de>
Cc:     "Chang, Junxiao" <junxiao.chang@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rt-users@...r.kernel.org" <linux-rt-users@...r.kernel.org>,
        "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>
Subject: Re: [PATCH] softirq: wake up ktimer thread in softirq context

On 2022-12-20 10:44:07 [+0000], Chang, Junxiao wrote:
> > Any comment? This patch is for 6.1-rt, issue could be reproduced with 5.19-rt kernel as well.

On Tue, Dec 20, 2022 at 10:03 AM bigeasy@...utronix.de
<bigeasy@...utronix.de> wrote:
> Thanks for the ping. I did see the initial email and I didn't get to it
> yet. I need to re-test, confirm and then apply.
> The ktimer patch is not in v5.15 and this is currently the latest one
> maintained by the stable team. I don't know which one will be the
> following LTS kernel but this one needs to have this addressed. The
> v5.19 is not receiving any updates.

We backported the timersd patch to 5.15, where is where we have
observed the C-state transition despite pending timers.    The
reported trace sequence does show that __do_softirq() starts just
before the hrtimer_interrupt when the problem occurs:

When the timers are delayed, the trouble appears to begin when the
hrtimer_interrupt results in execution of hrtimer_run_queues() instead
of raise_hrtimer_softirq():
 <userspace>-13534   [007] 16947.069504: enqueue_hrtimer
          <idle>-0       [007] 16947.069547:
hrtimer_next_event_without: 16947.067643167
          <idle>-0       [007] 16947.069553: enqueue_hrtimer
          <idle>-0       [007] 16947.069567: enqueue_hrtimer
          <idle>-0       [007] 16947.069575:
hrtimer_next_event_without: 16947.067643167
          <idle>-0       [007] 16947.069579: enqueue_hrtimer
          <idle>-0       [007] 16947.078270: hrtimer_interrupt
          <idle>-0       [007] 16947.078278: hrtimer_run_queues.
          <idle>-0       [007] 16947.078300: enqueue_hrtimer
       ktimers/7-80      [007] 16947.078308: __do_softirq
     ksoftirqd/7-81      [007] 16947.078338: ksoftirqd_should_run 0
          <idle>-0       [007] 16947.078361:
hrtimer_next_event_without: 16947.067643167
          <idle>-0       [007] 16947.079323: hrtimer_interrupt
          <idle>-0       [007] 16947.079328: hrtimer_run_queues.
          <idle>-0       [007] 16947.079334: enqueue_hrtimer
     ksoftirqd/7-81      [007] 16947.079359: ksoftirqd_should_run 128
     ksoftirqd/7-81      [007] 16947.079360: __do_softirq
     ksoftirqd/7-81      [007] 16947.079361: hrtimer_interrupt
     ksoftirqd/7-81      [007] 16947.079361: raise_hrtimer_softirq.
     ksoftirqd/7-81      [007] 16947.079364: ksoftirqd_should_run 0
          <idle>-0       [007] 16947.079375:
hrtimer_next_event_without: 9223372036854775807
          <idle>-0       [007] 16947.079376:
tick_nohz_get_sleep_length: 86.838 ms
          <idle>-0       [007] 16947.079378: enqueue_hrtimer

Note the bogus value printed by hrtimer_next_event_without().

> Given the current timing, I will look into this in January.
>
> > Regards,
> > Junxiao
>
> Sebastian

-- Alison Chaiken
Aurora Innovation