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

Andy Lutomirski
2016-05-24T01:43:13Z

On Mon, May 23, 2016 at 4:02 PM, Jiri Kosina <jikos@kernel.org> wrote:
> On Fri, 20 May 2016, Andy Lutomirski wrote:
>
>> I think it would be negligible, at least for interrupts, since
>> interrupts are already extremely expensive. But I don't love adding
>> assembly code that makes them even slower. The real thing I dislike
>> about this approach is that it's not a normal stack frame, so you need
>> code in the unwinder to unwind through it correctly, which makes me
>> think that you're not saving much complexity by adding the pushes.
>
> I fail to see what is so special about the stack frame; it's in fact
> pretty normal.
>
> It has added semantic value for "those who know", but the others will
> (pretty much correctly) consider it to be a stackframe from a function
> call, and be done with it.
>
> What am I missing?

In Josh's code, the stack looks like:

...
interrupted frame
pt_regs
pointer to pt_regs
address of pt_regs dummy function
rbp
handler frame

A naive unwinder won't unwind this correctly, as there's no link back
to the interrupted frame's RIP. If the layout changed to:

...
interrupted frame
pt_regs
interrupted RIP
rbp
handler frame

then I think it would unwind correctly, but the pt_regs would be
invisible, which is IMO a bit unfortunate. It could also be (I
think):

...
interrupted frame
pt_regs
interrupted rbp
interrupted RIP
pointer to pt_regs
address of pt_regs dummy function
pointer to "interrupted RIP" stack slot
handler frame

but now this is *five* pushes for the dummy frame, which I think is
getting a bit out of hand.

--Andy