[RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking
jpoimboe at redhat.com
Sat May 21 02:41:28 AEST 2016
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 at redhat.com> 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 at redhat.com> 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
> 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.
> I'll play with this a bit.
Thanks, looking forward to seeing what you come up with.
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