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

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Date:	Mon, 23 May 2016 18:42:49 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Jiri Kosina <jikos@...nel.org>
Cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
	live-patching@...r.kernel.org,
	Michael Ellerman <mpe@...erman.id.au>,
	Chris J Arges <chris.j.arges@...onical.com>,
	linuxppc-dev@...ts.ozlabs.org, Jessica Yu <jeyu@...hat.com>,
	Petr Mladek <pmladek@...e.com>, Jiri Slaby <jslaby@...e.cz>,
	Vojtech Pavlik <vojtech@...e.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Miroslav Benes <mbenes@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking

On Mon, May 23, 2016 at 4:02 PM, Jiri Kosina <jikos@...nel.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