[PATCH] tapset: add a tapset function for reading epoch time in ARM

[Josh Stone]

On 06/13/2014 11:14 PM, Jey Jay wrote:
> I have tested with 3.10 kernel on ARM. Till now didn't face any kernel
> panic issue.

Please try kprobes on functions called between timekeeper_seq's
write_seqcount_begin and write_seqcount_begin, and use your
getnstimeofday from those probes.  I don't expect a panic, but a
read_seqcount_retry inifinite loop.  If you have a watchdog, that may
notice the stall and panic though.

Note that even if we blacklisted kprobes on all such functions, it would
still be possible for a probe in interrupt code to hit during the middle
of those same write regions.

> Please guide me whether I can proceed or explore other options for
> collecting function execution time.

It looks like arm get_cycles() works since kernel commit 923df96b9f31b
(v3.6-rc1~70^2^3~1), with the export for read_current_timer added in
f3accb122f2c7 (v3.7-rc5~21^2~4^2~5).

So I'd suggest to start using that, either based on a KERNEL_VERSION
check, or better a new STAPCONF (see buildrun.cxx output_exportconf).


Josh