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

[Josh Poimboeuf]

On Fri, May 20, 2016 at 08:41:00AM -0700, Andy Lutomirski wrote:
> On Fri, May 20, 2016 at 7:05 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > On Thu, May 19, 2016 at 04:39:51PM -0700, Andy Lutomirski wrote:
> >> On Thu, May 19, 2016 at 4:15 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> >> > Note this example is with today's unwinder.  It could be made smarter to
> >> > get the RIP from the pt_regs so the '?' could be removed from
> >> > copy_page_to_iter().
> >> >
> >> > Thoughts?
> >>
> >> I think we should do that.  The silly sample patch I sent you (or at
> >> least that I think I sent you) did that, and it worked nicely.
> >
> > Yeah, we can certainly do something similar to make the unwinder
> > smarter.  It should be very simple with this approach: if it finds the
> > pt_regs() function on the stack, the (struct pt_regs *) pointer will be
> > right after it.
> 
> That seems barely easier than checking if it finds a function in
> .entry that's marked on the stack,

Don't forget you'd also have to look up the function's pt_regs offset in
the table.

> and the latter has no runtime cost.

Well, I had been assuming the three extra pushes and one extra pop for
interrupts would be negligible, but I'm definitely no expert there.  Any
idea how I can measure it?

> > I'm not sure about the idea of a table.  I get the feeling it would add
> > more complexity to both the entry code and the unwinder.  How would you
> > specify the pt_regs location when it's on a different stack?  (See the
> > interrupt macro: non-nested interrupts will place pt_regs on the task
> > stack before switching to the irq stack.)
> 
> Hmm.  I need to think about the interrupt stack case a bit.  Although
> the actual top of the interrupt stack has a nearly fixed format, and I
> have old patches to clean it up and make it actually be fixed.  I'll
> try to dust those off and resend them soon.

How would that solve the problem?  Would pt_regs be moved or copied to
the irq stack?

> > If you're worried about performance, I can remove the syscall
> > annotations.  They're optional anyway, since the pt_regs is always at
> > the same place on the stack for syscalls.
> >
> > I think three extra pushes wouldn't be a performance issue for
> > interrupts/exceptions.  And they'll go away when we finally bury
> > CONFIG_FRAME_POINTER.
> 
> I bet we'll always need to support CONFIG_FRAME_POINTER for some
> embedded systems.

Yeah, probably.

> I'll play with this a bit.

Thanks, looking forward to seeing what you come up with.

-- 
Josh