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SubjectRe: [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@linaro.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@bp.renesas.com>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>

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

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