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

[Andy Lutomirski]

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, and the latter has no runtime cost.

>
>> > diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
>> > index 9a9e588..f54886a 100644
>> > --- a/arch/x86/entry/calling.h
>> > +++ b/arch/x86/entry/calling.h
>> > @@ -201,6 +201,32 @@ For 32-bit we have the following conventions - kernel is built with
>> >         .byte 0xf1
>> >         .endm
>> >
>> > +       /*
>> > +        * Create a stack frame for the saved pt_regs.  This allows frame
>> > +        * pointer based unwinders to find pt_regs on the stack.
>> > +        */
>> > +       .macro CREATE_PT_REGS_FRAME regs=%rsp
>> > +#ifdef CONFIG_FRAME_POINTER
>> > +       pushq   \regs
>> > +       pushq   $pt_regs+1
>> > +       pushq   %rbp
>> > +       movq    %rsp, %rbp
>> > +#endif
>> > +       .endm
>>
>> I don't love this part.  It's going to hurt performance, and, given
>> that we need to change the unwinder anyway to make it useful, let's
>> just emit a table somewhere in .rodata and use it directly.
>
> 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.

>
> 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.

--Andy