Jan Kiszka wrote:
Philippe Gerum wrote:

Jan Kiszka wrote:

Philippe Gerum wrote:


Jan Kiszka wrote:


Philippe Gerum wrote:



Jan Kiszka wrote:



Hi,

while XENO_OPT_DEBUG is generally a useful switch for tracing
potential
issues in the core and the skins, it also introduces high
latencies via
the queue debugging feature (due to checks iterating over whole
queues).

This patch introduces separate control over queue debugging so
that you
can have debug checks without too dramatic slowdowns.


Maybe it's time to introduce debug levels, so that we could reuse them
in order to
add more (selectable) debug instrumentation; queue debugging could
then
be given a
certain level (likely something like CONFIG_XENO_DEBUG_LEVEL=8712 for
this one...), instead of going for a specific conditional each time we
introduce new checks?



Hmm, this means someone have to define what should be printed at which
level - tend to be hard decisions... Often it is at least as much
useful
to have debug groups so that specific parts can be excluded from
debugging. I'm pro such groups (one would be those queues e.g.) but
contra too many levels (2, at most 3).


Ack, selection by increasingly verbose/high-overhead groups is what I
have in mind.



At this chance, I would also suggest to introduce some ASSERT macro
(per
group, per level). That could be used to instrument the core with
runtime checks. But it could also be quickly removed at compilation
time, reducing the code size (e.g. checks at the nucleus layer against
buggy skins or at RTDM layer against rough drivers).


I'm not opposed to that, if we keep the noise / signal ratio of those
assertions at the reasonable low-level throughout the code, and don't
use this to enforce silly parametrical checks.



Then let's discuss how to implement and control this. Say we have some
macros for marking code as "depends on debug group X":

#if XENO_DEBUG_GROUP(group)
code;
#endif /* XENO_DEBUG_GROUP(group) */

XENO_IF_DEBUG_GROUP(group, code);

(or do you prefere XNPOD_xxx?)


This debug code may span feature/component boundaries, so XENO_ is better.


Additionally, we could introduce that assertion macro:

XENO_ASSERT(group, expression, failure_code);

But how to control the groups now? Via Kconfig bool options?

Yes, I think so. From some specialized Debug menu in the generic
portion. We would need this to keep the (unused) debug code out of
production systems.

And what

groups to define? Per subsytem? Or per disturbance level (latency
regression)? If we control the group selection via Kconfig, we could
define pseudo bool options like "All debug groups" or "Low-intrusive
debug groups" that select the fitting concrete groups.


We won't be able to anticipate on each and every debug spots we might
need in the future, and in any case, debug triggers may well span
multiple sub-systems. I'd go for defining levels depending on the
throroughness/complexity of their checks.



To keep it simple:

XNDBG_LIGHT /* simple checks with low constant overhead */
XNDBG_HEAVY /* complex checks with high or unknown overhead */

Those two could become #defines and would have to be provide as first
argument to our debug macros.

Or we directly merge the attribute into the macro name:

XENO_DEBUG_LIGHT, XENO_IF_DEBUG_LIGHT(), XENO_ASSERT_LIGHT()
XENO_DEBUG_HEAVY, XENO_IF_DEBUG_HEAVY(), XENO_ASSERT_HEAVY()


There's no need to limit the number of debug levels, since defining what goes 
into
each of them is a matter of developer's appreciation, wrt induced overhead, and/or debug thoroughness. IOW, let's not impose any policy here. To keep things really simple, we'd also need to decouple the assertion mechanism from the debug thoroughness, E.g.

#if XENO_DEBUG_LEVEL > 0
#define XENO_ASSERT(cond,action,fmt,args...)  do { \
if (unlikely((cond) != 0)) \
    (action)(fmt , ##args); \
} while(0)
#else  /* DEBUG OFF */
#define XENO_ASSERT(cond,action,fmt,args...) do { } while(0)
#endif /* DEBUG ON */

#define XENO_BUGON(cond)  \
XENO_ASSERT(cond,xnpod_fatal,"assertion failed at %s:%d",__FILE__,__LINE__)

This way, we could reserve level #1 (and above by extension) to activate all assertions; if people need to even control the thoroughness of assertions, they could just resort to implementing the assertion code in a separate debug function testing for particular debug levels, then calling this routine as XENO_BUGON's "cond" argument. Keeping level #0 free from assertions would be nice for production systems. E.g.

#if XENO_DEBUG_LEVEL > 2
        check_all_nucleus_queues();
#endif
...
        XENO_BUGON(I_am_so_sorry);

and so on.

Setting a value for XENO_DEBUG_LEVEL would be trivial using Kconfig. Dynamic setting of the debug level through /proc could be obtained by forcing XENO_DEBUG_LEVEL to MAX_INT (i.e. when dynamic debug levels are selected from Kconfig), and testing some addition global variable within the debug code to filter out unwanted checks.


Alternatively, we could make the group selection a runtime switch,
controlled via a global bitmask that can be modified through /proc e.g.
Only switching of CONFIG_XENO_OPT_DEBUG would then remove all debugging
code, otherwise the execution of the checks would depend on the current
bitmask content.

We could cumulate this with the static selection.



Then this is also a perfect add-on - for later work. :)

Jan



--

Philippe.

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