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Re: [to-be-posted-soon] Multiple handlers per marker


Mathieu Desnoyers wrote:
* Mike Mason (mmlnx@us.ibm.com) wrote:
Mathieu Desnoyers wrote:
* Mathieu Desnoyers (compudj@krystal.dyndns.org) wrote:
* Mathieu Desnoyers (mathieu.desnoyers@polymtl.ca) wrote:
* Mike Mason (mmlnx@us.ibm.com) wrote:
Hi Mathieu,

Are you aware of any working being done to allow multiple handlers to be attached to a marker? Something like what kprobes allows. I've started to look into this and don't want to duplicate efforts.

Nope, but I know we will have to address this.

Something along the lines of walking an RCU list of function pointers,
calling them.

The only downside I see is that we will have to pass a va_list * instead
of real va args. The could make the marker site a little bit bigger and
will change the probe callback arguments.

What do you think about these ideas ?

If we can find a way to make the common case (only one probe connected)
_ultra_ fast, and yet architecture independent, that would be awesome. A
simple call is kind of hard to beat though.. So we may have to think
about a design with :

- One call at the marker site
- if 1 probe is installed :
- If the format string is empty, connect a probe without va args.
- If the format string is not empty, connect a "stage 1" probe that takes
the va args, starts/ends the va_list and calls _one_ function (let's
call it "stage 2" probe), that takes va_list as parameter.
- if more than 1 probe is installed :
- The stage 1 probe creates the va_list and passes it to each function
connected, iterated with an RCU list.


What do you think ?

Mathieu

I'm working on an implementation.

It's ready for testing. Please grab
http://ltt.polymtl.ca/lttng/patch-2.6.24-rc1-git13-lttng-0.10-pre18.tar.bz2
patch name :
markers-support-multiple-probes.patch
This patch alone doesn't apply cleanly at all on 2.6.24-rc1-git14. Are there other patches in this series I should apply first?


Yes, the following ones should suffice :


# instrumentation menu removal
add-kconfig-to-arch.patch
add-arch-supports-oprofile.patch
add-arch-supports-kprobes.patch
move-kconfig-instrumentation-to-arch.patch
#
kprobes-use-mutex-for-insn-pages.patch
kprobes-dont-use-kprobes-mutex-in-arch-code.patch
kprobes-declare-kprobes-mutex-static.patch
declare-array.patch
text-edit-lock-architecture-independent-code.patch
text-edit-lock-alternative-i386-and-x86_64.patch
text-edit-lock-kprobes-architecture-independent.patch
text-edit-lock-kprobes-i386.patch
text-edit-lock-kprobes-x86_64.patch
text-edit-lock-i386-standardize-debug-rodata.patch
text-edit-lock-x86_64-standardize-debug-rodata.patch
#
immediate-values-architecture-independent-code.patch
immediate-values-kconfig-embedded.patch
immediate-values-move-kprobes-i386-restore-interrupt-to-kdebug-h.patch
add-asm-compat-to-x86.patch
immediate-values-i386-optimization.patch
immediate-values-powerpc-optimization.patch
immediate-values-documentation.patch
#
linux-kernel-markers-immediate-values.patch
#
markers-support-multiple-probes.patch

Tell me if you still have rejects.

I applied the above patches to 2.6.24-rc1-git14. They applied fine with just a few offsets until the last patch, which yielded this result:


patching file include/linux/marker.h
Hunk #5 succeeded at 162 with fuzz 2.
patching file kernel/marker.c
Hunk #14 FAILED at 534.
Hunk #15 FAILED at 587.
Hunk #16 FAILED at 621.
Hunk #17 FAILED at 732.
Hunk #18 FAILED at 769.
Hunk #19 succeeded at 791 (offset 12 lines).
5 out of 19 hunks FAILED -- saving rejects to file kernel/marker.c.rej
patching file kernel/module.c
Hunk #1 succeeded at 1998 (offset -3 lines).
Hunk #2 succeeded at 2608 (offset -37 lines).
Hunk #3 succeeded at 2651 with fuzz 1 (offset -3 lines).
patching file include/linux/module.h
Hunk #1 FAILED at 468.
Hunk #2 succeeded at 572 (offset -2 lines).
1 out of 2 hunks FAILED -- saving rejects to file include/linux/module.h.rej
patching file samples/markers/probe-example.c

Mike


Mathieu



Mike

It still need to go through patchcheck.pl and some polishing, but it
seems to work fine for me with multiple probes (the sample marker,
sample probe and multiple instances of my lttng probes can
connect/disconnect without problem).
Currently, the "connect/disconnect" and "arm/disarm" operations are
separate. However, they could be merged. Any comment/preference on this?
Being separate, a probe provider can wait until the very last moment
before it activates its markers, with a minimalistic impact on the
system, but it is not such a strong argument.
Mathieu



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