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

Josh Poimboeuf
2016-04-29T20:27:16Z

On Fri, Apr 29, 2016 at 01:19:23PM -0700, Andy Lutomirski wrote:
> On Fri, Apr 29, 2016 at 1:11 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > On Fri, Apr 29, 2016 at 11:06:53AM -0700, Andy Lutomirski wrote:
> >> On Thu, Apr 28, 2016 at 1:44 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >> > A preempted function might not have had a chance to save the frame
> >> > pointer to the stack yet, which can result in its caller getting skipped
> >> > on a stack trace.
> >> >
> >> > Add a flag to indicate when the task has been preempted so that stack
> >> > dump code can determine whether the stack trace is reliable.
> >>
> >> I think I like this, but how do you handle the rather similar case in
> >> which a task goes to sleep because it's waiting on IO that happened in
> >> response to get_user, put_user, copy_from_user, etc?
> >
> > Hm, good question. I was thinking that page faults had a dedicated
> > stack, but now looking at the entry and traps code, that doesn't seem to
> > be the case.
> >
> > Anyway I think it shouldn't be a problem if we make sure that any kernel
> > function which might trigger a valid page fault (e.g.,
> > copy_user_generic_string) do the proper frame pointer setup first. Then
> > the stack should still be reliable.
> >
> > In fact I might be able to teach objtool to enforce that: any function
> > which uses an exception table should create a stack frame.
> >
> > Or alternatively, maybe set some kind of flag for page faults, similar
> > to what I did with this patch.
> >
>
> How about doing it the other way around: teach the unwinder to detect
> when it hits a non-outermost entry (i.e. it lands in idtentry, etc)
> and use some reasonable heuristic as to whether it's okay to keep
> unwinding. You should be able to handle preemption like that, too --
> the unwind process will end up in an IRQ frame.

How exactly would the unwinder detect if a text address is in an
idtentry? Maybe put all the idt entries in a special ELF section?

--
Josh