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

Andy Lutomirski
2016-06-23T22:01:04Z

On Thu, Jun 23, 2016 at 1:40 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> On Thu, Jun 23, 2016 at 01:31:32PM -0500, Josh Poimboeuf wrote:
>> 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.
>
> I'm starting to think hare-brained option 4 is the way to go. Any
> external tooling should really be relying on DWARF anyway.
>
> Here's a sneak preview. If this general approach looks ok to you, I'll
> go ahead and port all the in-tree unwinders and post a proper patch.
>
> Instead of using xor -1 on the pt_regs pointer, I just cleared the
> high-order bit. That makes the unwinding experience much more pleasant
> for a human stack walker, and also ensures that anybody trying to
> dereference it gets slapped with an oops, at least in the 48-bit address
> space era.
>
> diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
> index 9a9e588..bf397426 100644
> --- a/arch/x86/entry/calling.h
> +++ b/arch/x86/entry/calling.h
> @@ -201,6 +201,23 @@ For 32-bit we have the following conventions - kernel is built with
> .byte 0xf1
> .endm
>
> + /*
> + * This is a sneaky trick to help the unwinder find pt_regs on the
> + * stack. The frame pointer is replaced with an encoded pointer to
> + * pt_regs. The encoding is just a clearing of the highest-order bit,
> + * which makes it an invalid address and is also a signal to the
> + * unwinder that it's a pt_regs pointer in disguise.
> + *
> + * NOTE: This must be called *after* SAVE_EXTRA_REGS because it
> + * corrupts rbp.
> + */
> + .macro ENCODE_FRAME_POINTER ptregs_offset=0
> +#ifdef CONFIG_FRAME_POINTER
> + leaq \ptregs_offset(%rsp), %rbp
> + btr $63, %rbp
> +#endif
> + .endm
> +

Maybe optimize slightly:

.ifeq \ptregs_offset
mov %rsp, %rbp
.else
leaq \ptregs_offset(%rsp), %rbp
.endif