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

Josh Poimboeuf
2016-06-22T18:22:51Z

On Wed, Jun 22, 2016 at 10:59:23AM -0700, Andy Lutomirski wrote:
> > So I got a chance to look at this some more. I'm thinking that to make
> > this feature more consistently useful, we shouldn't only annotate
> > pt_regs frames for calls to handlers; other calls should be annotated as
> > well: preempt_schedule_irq, CALL_enter_from_user_mode,
> > prepare_exit_to_usermode, SWAPGS, TRACE_IRQS_OFF, DISABLE_INTERRUPTS,
> > etc. That way, the unwinder will always be able to find pt_regs from an
> > interrupt/exception, even if starting from one of these other calls.
> >
> > But then, things get ugly. You have to either setup and tear down the
> > frame for every possible call, or do a higher-level setup/teardown
> > across multiple calls, which invalidates several assumptions in the
> > entry code about the location of pt_regs on the stack.
> >
> > Also problematic is that several of the macros (like TRACE_IRQS_IRETQ)
> > make assumptions about the location of pt_regs. And they're used by
> > both syscall and interrupt code. So if we didn't create a frame pointer
> > header for syscalls, we'd basically need two versions of the macros: one
> > for irqs/exceptions and one for syscalls.
> >
> > So I think the cleanest way to handle this is to always allocate two
> > extra registers on the stack in ALLOC_PT_GPREGS_ON_STACK. Then all
> > entry code can assume that pt_regs is at a constant location, and all
> > the above problems go away. Another benefit is that we'd only need two
> > saves instead of three -- the pointer to pt_regs is no longer needed
> > since pt_regs is always immediately after the frame header.
> >
> > I worked up a patch to implement this -- see below. It writes the frame
> > pointer in all entry paths, including syscalls. This helps keep the
> > code simple.
> >
> > The downside is a small performance penalty: with getppid()-in-a-loop on
> > my laptop, the average syscall went from 52ns to 53ns, which is about a
> > 2% slowdown. But I doubt it would be measurable in a real-world
> > workload.
> >
> > It looks like about half the slowdown is due to the extra stack
> > allocation (which presumably adds a little d-cache pressure on the stack
> > memory) and the other half is due to the stack writes.
> >
> > I could remove the writes from the syscall path but it would only save
> > about half a ns, and it would make the code less robust. Plus it's nice
> > to have the consistency of having *all* pt_regs frames annotated.
>
> This is a bit messy, and I'm not really sure that the entry code
> should be have to operate under constraints like this. Also,
> convincing myself this works for NMI sounds unpleasant.
>
> Maybe we should go back to my idea of just listing the call sites in a table.

So are you suggesting something like:

.macro ENTRY_CALL func pt_regs_offset=0
call \func
1: .pushsection .entry_calls, "a"
.long 1b - .
.long \pt_regs_offset
.popsection
.endm

and then change every call in the entry code to ENTRY_CALL?

--
Josh