Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230811170049.308866-1-frederic@kernel.org> <20230811170049.308866-4-frederic@kernel.org>
In-Reply-To: <20230811170049.308866-4-frederic@kernel.org>
From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Fri, 11 Aug 2023 19:37:55 +0200
Message-ID: <CAJZ5v0jUnOC4gABpmg3pX0KWXvYcYEzsHcjY1LqH8qzJ94QsvQ@mail.gmail.com>
Subject: Re: [PATCH 03/10] cpuidle: Report illegal tick stopped while polling
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Rafael J . Wysocki" <rafael@kernel.org>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Anna-Maria Behnsen <anna-maria@linutronix.de>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8BIT
Precedence: bulk

On Fri, Aug 11, 2023 at 7:01 PM Frederic Weisbecker <frederic@kernel.org> wrote:
>
> poll_idle() can't be called while the tick is stopped because it enables
> interrupts and only polls on TIF_NEED_RESCHED, which doesn't tell if an
> interrupt queues a timer that would require a tick re-programming.
>
> There is no point anyway to poll with the tick stopped so add a check
> to make sure it never happens.

I'd rather update governors so they never use polling states when the
tick has been stopped and then add the WARN_ON().

> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> ---
>  drivers/cpuidle/poll_state.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
> index 9b6d90a72601..009f46f121ae 100644
> --- a/drivers/cpuidle/poll_state.c
> +++ b/drivers/cpuidle/poll_state.c
> @@ -7,6 +7,7 @@
>  #include <linux/sched.h>
>  #include <linux/sched/clock.h>
>  #include <linux/sched/idle.h>
> +#include <linux/tick.h>
>
>  #define POLL_IDLE_RELAX_COUNT  200
>
> @@ -19,6 +20,13 @@ static int __cpuidle poll_idle(struct cpuidle_device *dev,
>
>         dev->poll_time_limit = false;
>
> +       /*
> +        * This re-enables IRQs and only polls on TIF_NEED_RESCHED.
> +        * A timer queued by an interrupt here may go unnoticed if
> +        * the tick is stopped.
> +        */
> +       WARN_ON_ONCE(tick_nohz_tick_stopped());
> +
>         raw_local_irq_enable();
>         if (!current_set_polling_and_test()) {
>                 unsigned int loop_count = 0;
> --
> 2.34.1
>