Re: [PATCH] printk: Print cpu number along with time

Greg Kroah-Hartman
2014-04-24T19:55:30Z

On Thu, Apr 24, 2014 at 03:40:24PM -0400, Jörn Engel wrote:
> On Wed, 23 April 2014 20:52:47 -0400, Jörn Engel wrote:
> >
> > I use the patch below for some time now. While it doesn't avoid the
> > log pollution in the first place, it lessens the impact somewhat.
>
> Added a config option and ported it to current -linus. Andrew, would
> you take this patch?
>
> ---
>
> Sometimes the printk log is heavily interleaving between different cpus.
> This is particularly bad when you have two backtraces at the same time,
> but can be annoying in other cases as well. With an explicit cpu
> number, a simple grep can disentangle the mess for you.
>
> Signed-off-by: Joern Engel <joern@logfs.org>
> ---
> kernel/printk/printk.c | 19 ++++++++++++++++---
> lib/Kconfig.debug | 9 +++++++++
> 2 files changed, 25 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index a45b50962295..b9e464924825 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -200,6 +200,7 @@ struct printk_log {
> u16 len; /* length of entire record */
> u16 text_len; /* length of text buffer */
> u16 dict_len; /* length of dictionary buffer */
> + u16 cpu; /* cpu the message was generated on */
> u8 facility; /* syslog facility */
> u8 flags:5; /* internal record flags */
> u8 level:3; /* syslog level */
> @@ -346,6 +347,7 @@ static void log_store(int facility, int level,
> msg->facility = facility;
> msg->level = level & 7;
> msg->flags = flags & 0x1f;
> + msg->cpu = smp_processor_id();
> if (ts_nsec > 0)
> msg->ts_nsec = ts_nsec;
> else
> @@ -859,7 +861,7 @@ static bool printk_time;
> #endif
> module_param_named(time, printk_time, bool, S_IRUGO | S_IWUSR);
>
> -static size_t print_time(u64 ts, char *buf)
> +static size_t print_time(u64 ts, u16 cpu, char *buf)
> {
> unsigned long rem_nsec;
>
> @@ -868,11 +870,20 @@ static size_t print_time(u64 ts, char *buf)
>
> rem_nsec = do_div(ts, 1000000000);
>
> +#ifdef CONFIG_PRINTK_CPU
> + if (!buf)
> + return snprintf(NULL, 0, "[%5lu.000000,%02x] ",

%02x for a cpu? What happens on machines with 8k cpus?

And is this really an issue? Debugging by using printk is fun, but not
really something that people need to add a cpu number to. Why not just
use a tracepoint in your code to get the needed information instead?

thanks,

greg k-h