Re: LBR unwinding for user defined dynamic trace point

[Milian Wolff <milian.wolff@kdab.com>]

[-- Attachment #1: Type: text/plain, Size: 2507 bytes --]

On Wednesday, September 16, 2015 8:56:14 PM CEST Andi Kleen wrote:
> On Wed, Sep 16, 2015 at 11:55:22AM +0200, Milian Wolff wrote:
> > On Tuesday, September 15, 2015 7:02:37 AM CEST Andi Kleen wrote:
> > > Milian Wolff <milian.wolff@kdab.com> writes:
> > > > When I do the same with the Dwarf unwinder, it works just fine. Using
> > > > the
> > > > LBR unwinder with a performance counter like instructions also works
> > > > fine. Does anyone know what the issue is here?
> > > 
> > > LBR is only supported for PMU sampling at this point.
> > 
> > OK, thanks. Can you give me some more information on why that is? Is it
> > fundamentally not possible, or simply not yet implemented?
> 
> For LBR callgraph it could be implemented. For some other LBR usages
> there would be limitations, as there is no LBR freezing for software
> trace points.

Hey Andi,

I read your articles on LBR on LWN, thanks a lot for that! I want to revive 
this older thread though:

Could you tell me what would be required to get LBR callgraphs supported for 
trace points? I noticed that it also does not work with static tracepoints or 
software events:

~~~~~~~~~~~~~~~~~~~~~~~
$ perf record -e faults --call-graph lbr ls
Error:
The sys_perf_event_open() syscall returned with 95 (Operation not supported) 
for event (faults).
/bin/dmesg may provide additional information.
No CONFIG_PERF_EVENTS=y kernel support configured?

$ perf record -e raw_syscalls:sys_enter --call-graph lbr ls
Error:
The sys_perf_event_open() syscall returned with 95 (Operation not supported) 
for event (raw_syscalls:sys_enter).
/bin/dmesg may provide additional information.
No CONFIG_PERF_EVENTS=y kernel support configured?
~~~~~~~~~~~~~~~~~~~~~~~

It does work fine for hardware events though:

~~~~~~~~~~~~~~~~~~~~~~~
$ perf record -e cache-misses --call-graph lbr ls
WARNING: No sample_id_all support, falling back to unordered processing
...
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.017 MB perf.data (6 samples) ]
~~~~~~~~~~~~~~~~~~~~~~~

Due to the reasons and advantages you point out in your article for LBR 
callgraphs, I'd love to see it supported for the above use-cases as well, if 
possible. If you can guide me, and it does not require me to patch the kernel 
itself, then I'm also willing to come up with a patch for perf.

Thanks
-- 
Milian Wolff | milian.wolff@kdab.com | Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt Experts

[-- Attachment #2: smime.p7s --]
[-- Type: application/pkcs7-signature, Size: 5903 bytes --]