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

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Date:	Thu, 23 Jun 2016 13:31:32 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Jiri Kosina <jikos@...nel.org>, 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>,
	Jessica Yu <jeyu@...hat.com>, linuxppc-dev@...ts.ozlabs.org,
	Petr Mladek <pmladek@...e.com>, Jiri Slaby <jslaby@...e.cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Vojtech Pavlik <vojtech@...e.com>,
	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 Thu, Jun 23, 2016 at 09:35:29AM -0700, Andy Lutomirski wrote:
> > So which is the least-bad option?  To summarize:
> >
> >   1) task flag(s) for preemption and page faults
> >
> >   2) turn pt_regs into a stack frame
> >
> >   3) annotate all calls from entry code in a table
> >
> >   4) encode rbp on entry
> >
> > They all have their issues, though I'm partial to #2.
> >
> > Any more hare-brained ideas? :-)
> 
> I'll try to take a closer look at #2 and see just how much I dislike
> all the stack frame munging.

Ok.

> Also, in principle, it's only the
> sleeping calls and the calls that make it into real (non-entry) kernel
> code that really want to be unwindable through this mechanism.

Yeah, that's true.  We could modify options 2 or 3 to be less absolute.
Though I think that makes them more prone to future breakage.

> FWIW, I don't care that much about preserving gdb's partial ability to
> unwind through pt_regs, especially because gdb really ought to be able
> to use DWARF, too.

Hm, that's a good point.  I really don't know if there are any other
external tools out there that would care.  Maybe we could try option 4
and then see if anybody complains.

-- 
Josh