Re: [PATCH/RFC 3/3] ath5k: trace resets

[Bruno Randolf <br1@einfach.org>]

On Tue July 20 2010 23:52:00 Bob Copeland wrote:
> > again, here my same concerns: printing the reasons for resets is
> > something which is useful on embedded boards and production setups which
> > can't have tracing enabled. which is why i want to object against this
> > change!
> 
> What Johannes said wrt performance: during tracing, only the binary
> representations of the data are written to the trace ring buffer, so
> unlike the current debug code, we aren't doing printk formatting until
> the trace buffer is read.  You can save the raw binary data from the
> trace and do formatting on another machine.

that's true, but try to run a kernel with tracing compiled in and NOT runtime 
enabled on a small embedded board (soekris net48xx for example) and you'll see 
the difference. without tracing you can get 22Mbps, with tracing max 15Mbps 
UDP thruput. that means you cannot run a kernel with tracing enabled on a 
production system, which also means you cannot log the reasons for a reset 
there any more. and most of the problems we what to trace (e.g. stuck queue) 
happen only after days or weeks of operation in production environments...
 
> Another advantage is better granularity: if you only care about watching
> tx on the cab queue, you can dynamically filter based on the tracepoint
> arguments, something like:
> 
>   # echo "qnum == 6" > /debug/tracing/events/ath5k/ath5k_tx/filter
> 
> With the debug printks, you have to hack the driver or grep and hope
> the printk buffer didn't overflow and spill what you were looking for.

no doubt, i can see the advantages...

so let's go ahead with tracing, since we can always build less performant 
tracing kernels when we want to track down problems.

bruno