linux-kernel - Re: [PATCH] printk: add sleep time into timestamp

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Date:	Thu, 27 Mar 2014 20:18:05 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Neil Zhang <zhangwm@...vell.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] printk: add sleep time into timestamp

On Thu, Mar 27, 2014 at 1:17 AM, Neil Zhang <zhangwm@...vell.com> wrote:
> Add sleep time into timestamp to reflect the actual time since
> sched_clock will be stopped during suspend.

So why is this change necessary?

Further, since the sleep time may be updated a bit late in the resume
cycle (in many cases we cannot access the RTC until irqs are enabled
back on), you may see messages that show pre-suspend times when really
they occur after we resume (but before the sleep time is incremented).

More comments below....

> This patch depends on the following patch.
> timekeeping: check params before use them
>
> Signed-off-by: Neil Zhang <zhangwm@...vell.com>
> ---
>  kernel/printk/printk.c |   15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 4dae9cb..2dc6145 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -250,6 +250,17 @@ static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
>  static char *log_buf = __log_buf;
>  static u32 log_buf_len = __LOG_BUF_LEN;
>
> +static u64 print_clock(void)
> +{
> +       struct timespec ts;
> +       u64 ts_nsec = local_clock();
> +
> +       get_xtime_and_monotonic_and_sleep_offset(NULL, NULL, &ts);

So this will cause deadlocks anytime we print from the timekeeping
core, since we may hold a write on the timekeeper lock, and this patch
makes every printk try to take a read-lock on the timekeeper lock.

I'd suggest you use monotonic_to_bootbased() here instead of hacking
up this hrtimer specific interface, but even so, right now that call
doesn't take the timekeeper lock, but probably should, so its not a
good long term plan.

I'm still not convinced this change needs to be done, but a  better
solution here would be to add infrastructure that when the sleep time
is updated we update an offset that the is adding to the local_clock()
however, you probably want to be careful since you don't want sleep
time in normal local_clock/sched_clock calls since it would mess up
scheduling.

thanks
-john
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