Re: CONFIG_PRINTK_TIME woes

[Andrew Morton <akpm@osdl.org>]

tony.luck@intel.com wrote:
>
> At the other extreme ... the current use of sched_clock() with
>  potentially nano-second resolution is way over the top.  Logging
>  to a serial console at 115200 a typical line from printk will take
>  2-4 milli-seconds to print ... so there would seem to be little
>  benefit from a sub-millisecond resolution (in fact at 250HZ jiffies
>  are on the ragged edge of being good enough).
> 
>  If that isn't sufficient ... it should be possible to make a cut-down,
>  lockless version of do_gettimeofday that meets Andrew's suggestion
>  of good resolution with occasional theoretical weirdness.  But before
>  we go there ... I'd like to hear whether there are usage models that
>  really need better resolution than jiffies can provide?

I think so.  Say you're debugging or performance tuning filesystem requests
and I/O completions, etc.  You disable the console with `dmesg -n', run the
test then do `dmesg -s 1000000 > foo'.  Having somewhat-sub-millisecond
timestamping in the resulting trace is required.