linux-kernel - Re: [PATCH 1/2] PM-runtime: move runtime accounting on ktime_get_mono_fast

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Date:   Tue, 5 Feb 2019 09:09:50 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Biju Das <biju.das@...renesas.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Thara Gopinath <thara.gopinath@...aro.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH 1/2] PM-runtime: move runtime accounting on ktime_get_mono_fast_ns()

On Mon, 4 Feb 2019 at 17:25, Vincent Guittot <vincent.guittot@...aro.org> wrote:
>
> Similarly to what happened whith autosuspend, a deadlock has been raised
> with runtime accounting in the sequence:
>
> change_clocksource
>     ...
>     write_seqcount_begin
>     ...
>     timekeeping_update
>         ...
>         sh_cmt_clocksource_enable
>             ...
>             rpm_resume
>                 update_pm_runtime_accounting
>                     ktime_get
>                         do
>                             read_seqcount_begin
>                         while read_seqcount_retry
>     ....
>     write_seqcount_end
>
> Move runtime accounting on ktime_get_mono_fast_ns()
> With ktime_get_mono_fast_ns, the timestamp is not guaranteed to be
> monotonic across an update of timekeeping and as a result time can goes
> backward. Add a test to skip accounting for such situation which should
> stay exceptional.
>
> Fixes: a08c2a5a3194 ("PM-runtime: Replace jiffies-based accounting with ktime-based accounting")
> Reported-by: Biju Das <biju.das@...renesas.com>
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>

Reviewed-by: Ulf Hansson <ulf.hansson@...aro.org>

Kind regards
Uffe


> ---
>  drivers/base/power/runtime.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
> index 1caa394..50740da 100644
> --- a/drivers/base/power/runtime.c
> +++ b/drivers/base/power/runtime.c
> @@ -66,13 +66,22 @@ static int rpm_suspend(struct device *dev, int rpmflags);
>   */
>  void update_pm_runtime_accounting(struct device *dev)
>  {
> -       u64 now = ktime_to_ns(ktime_get());
> +       u64 now = ktime_get_mono_fast_ns();
> +       u64 last = dev->power.accounting_timestamp;
>         u64 delta;
>
> -       delta = now - dev->power.accounting_timestamp;
> -
>         dev->power.accounting_timestamp = now;
>
> +       /*
> +        * Because ktime_get_mono_fast_ns() is not monotonic during
> +        * timekeeping update, we must ensure that now is after the last saved
> +        * timesptamp
> +        */
> +       if (now < last)
> +               return;
> +
> +       delta = now - last;
> +
>         if (dev->power.disable_depth > 0)
>                 return;
>
> @@ -1312,7 +1321,7 @@ void pm_runtime_enable(struct device *dev)
>
>                 /* About to enable runtime pm, set accounting_timestamp to now */
>                 if (!dev->power.disable_depth)
> -                       dev->power.accounting_timestamp = ktime_to_ns(ktime_get());
> +                       dev->power.accounting_timestamp = ktime_get_mono_fast_ns();
>         } else {
>                 dev_warn(dev, "Unbalanced %s!\n", __func__);
>         }
> --
> 2.7.4
>