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

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Message-ID: <CALCETrUV0sbmjHAFwFwJQuOUhrsXPW_egOaQ2k38tSm6ni=3qg@mail.gmail.com>
Date:	Mon, 2 May 2016 17:39:52 -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 2, 2016 at 1:00 PM, Jiri Kosina <jikos@...nel.org> wrote:
> On Mon, 2 May 2016, Jiri Kosina wrote:
>
>> > FWIW, I just tried this:
>> >
>> > static bool is_entry_text(unsigned long addr)
>> > {
>> >     return addr >= (unsigned long)__entry_text_start &&
>> >         addr < (unsigned long)__entry_text_end;
>> > }
>> >
>> > it works.  So the entry code is already annotated reasonably well :)
>> >
>> > I just hacked it up here:
>> >
>> > https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=stack&id=085eacfe0edfc18768e48340084415dba9a6bd21
>> >
>> > and it seems to work, at least for page faults.  A better
>> > implementation would print out the entire contents of pt_regs so that
>> > people reading the stack trace will know the registers at the time of
>> > the exception, which might be helpful.
>>
>> Sorry for being dense, but how do you distinguish here between a "real"
>> kernel entry, that pushes pt_regs, and any "non-entry" function call that
>> passes pt_regs around?
>
> Umm, actually, the more tricky part is the other way around -- how do you
> make sure that whenever you are calling out from a code between
> __entry_text_start and __entry_text_end, pt_regs will be at the place
> you're looking for it? How's that guaranteed?

It's not guaranteed in my code.  I think we'd want to add a little
table of call sites and their pt_regs offsets.  This was just meant to
test that the general idea works (and it does indeed generate better
traces than the stock kernel, which gets it unconditionally wrong).

--Andy

>
> Thanks,
>
> --
> Jiri Kosina
> SUSE Labs
>



-- 
Andy Lutomirski
AMA Capital Management, LLC