Re: [PATCH V2] printk: add sleep time into timestamp

From: John Stultz
Date: Fri Apr 04 2014 - 20:58:26 EST

On 03/31/2014 08:20 PM, Neil Zhang wrote:
> Add sleep time into timestamp to reflect the actual time since
> sched_clock will be stopped during suspend.
> Thanks John Stultz for suggestion to use monotonic_to_bootbased.
> Signed-off-by: Neil Zhang <zhangwm@xxxxxxxxxxx>
> ---
> kernel/printk/printk.c | 16 ++++++++++++++--
> 1 file changed, 14 insertions(+), 2 deletions(-)
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 4dae9cb..121f792 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -250,6 +250,18 @@ 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();
> +
> + ts = ns_to_timespec(ts_nsec);
> + monotonic_to_bootbased(&ts);
> + ts_nsec = timespec_to_ns(&ts);
> +

Note that this is still problematic as monotonic_to_bootbased currently
doesn't take the required locks to ensure it reads the data atomically.

If it were fixed to take the proper locks, this would risk a deadlock
since there are places where the timekeeping code takes a write on the
seqlock and then may call printk (or WARN_ON).

I suspect if this is going to be done, we need to have the timekeeping
provide the print_clock subsystem the boottime offset when it is changed
so the print_clock logic can manage this w/o grabbing timekeeping locks.
But as I mentioned in reply to Lei's similar patches, I'm not a huge fan
of the added complexity that will be required here when userspace like
syslog can timestamps kernel messages already (so additional rational
will be needed to move this forward).

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