Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking

Andy Lutomirski
2016-05-03T00:40:19Z

On Mon, May 2, 2016 at 1:00 PM, Jiri Kosina <jikos@kernel.org> wrote:
> On Mon, 2 May 2016, Jiri Kosina wrote:
>
>> > FWIW, I just tried this:
>> >
>> > static bool is_entry_text(unsigned long addr)
>> > {
>> > return addr >= (unsigned long)__entry_text_start &&
>> > addr < (unsigned long)__entry_text_end;
>> > }
>> >
>> > it works. So the entry code is already annotated reasonably well :)
>> >
>> > I just hacked it up here:
>> >
>> > https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=stack&id=085eacfe0edfc18768e48340084415dba9a6bd21
>> >
>> > and it seems to work, at least for page faults. A better
>> > implementation would print out the entire contents of pt_regs so that
>> > people reading the stack trace will know the registers at the time of
>> > the exception, which might be helpful.
>>
>> Sorry for being dense, but how do you distinguish here between a "real"
>> kernel entry, that pushes pt_regs, and any "non-entry" function call that
>> passes pt_regs around?
>
> Umm, actually, the more tricky part is the other way around -- how do you
> make sure that whenever you are calling out from a code between
> __entry_text_start and __entry_text_end, pt_regs will be at the place
> you're looking for it? How's that guaranteed?

It's not guaranteed in my code. I think we'd want to add a little
table of call sites and their pt_regs offsets. This was just meant to
test that the general idea works (and it does indeed generate better
traces than the stock kernel, which gets it unconditionally wrong).

--Andy

>
> Thanks,
>
> --
> Jiri Kosina
> SUSE Labs
>

--
Andy Lutomirski
AMA Capital Management, LLC