Re: [RFC PATCH v2] perf/x86: make perf callchain work without CONFIG_FRAME_POINTER

[Josh Poimboeuf]

On Wed, Apr 17, 2019 at 01:39:19AM +0800, Kairui Song wrote:
> On Tue, Apr 16, 2019 at 7:30 PM Kairui Song <kasong@xxxxxxxxxx> wrote:
> >
> > On Tue, Apr 16, 2019 at 12:59 AM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > >
> > > On Mon, Apr 15, 2019 at 05:36:22PM +0200, Peter Zijlstra wrote:
> > > >
> > > > I'll mostly defer to Josh on unwinding, but a few comments below.
> > > >
> > > > On Tue, Apr 09, 2019 at 12:59:42AM +0800, Kairui Song wrote:
> > > > > diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> > > > > index e2b1447192a8..6075a4f94376 100644
> > > > > --- a/arch/x86/events/core.c
> > > > > +++ b/arch/x86/events/core.c
> > > > > @@ -2355,6 +2355,12 @@ void arch_perf_update_userpage(struct perf_event *event,
> > > > >     cyc2ns_read_end();
> > > > >  }
> > > > >
> > > > > +static inline int
> > > > > +valid_perf_registers(struct pt_regs *regs)
> > > > > +{
> > > > > +   return (regs->ip && regs->bp && regs->sp);
> > > > > +}
> > > >
> > > > I'm unconvinced by this, with both guess and orc having !bp is perfectly
> > > > valid.
> > > >
> > > > >  void
> > > > >  perf_callchain_kernel(struct perf_callchain_entry_ctx *entry, struct pt_regs *regs)
> > > > >  {
> > > > > @@ -2366,11 +2372,17 @@ perf_callchain_kernel(struct perf_callchain_entry_ctx *entry, struct pt_regs *re
> > > > >             return;
> > > > >     }
> > > > >
> > > > > -   if (perf_callchain_store(entry, regs->ip))
> > > > > +   if (valid_perf_registers(regs)) {
> > > > > +           if (perf_callchain_store(entry, regs->ip))
> > > > > +                   return;
> > > > > +           unwind_start(&state, current, regs, NULL);
> > > > > +   } else if (regs->sp) {
> > > > > +           unwind_start(&state, current, NULL, (unsigned long *)regs->sp);
> > > > > +   } else {
> > > > >             return;
> > > > > +   }
> > > >
> > > > AFAICT if we, by pure accident, end up with !bp for ORC, then we
> > > > initialize the unwind wrong.
> > > >
> > > > Note that @regs is mostly trivially correct, except for that tracepoint
> > > > case. So I don't think we should magic here.
> > >
> > > Ah, I didn't quite understand this code before, and I still don't
> > > really, but I guess the issue is that @regs can be either real or fake.
> > >
> > > In the real @regs case, we just want to always unwind starting from
> > > regs->sp.
> > >
> > > But in the fake @regs case, we should instead unwind from the current
> > > frame, skipping all frames until we hit the fake regs->sp.  Because
> > > starting from fake/incomplete regs is most likely going to cause
> > > problems with ORC (or DWARF for other arches).
> > >
> > > The idea of a fake regs is fragile and confusing.  Is it possible to
> > > just pass in the "skip" stack pointer directly instead?  That should
> > > work for both FP and non-FP.  And I _think_ there's no need to ever
> > > capture regs->bp anyway -- the stack pointer should be sufficient.
> >
> > Hi, that will break some other usage, if perf_callchain_kernel is
> > called but it won't unwind to the callsite (could be produced by
> > attach an ebpf call to kprobe), things will also go wrong. It should
> > start with given registers when the register is valid.
> > And it's true with omit frame pointer BP value could be anything, so 0
> > is also valid, I think I need to find a better way to tell if we could
> > start with the registers value or direct start unwinding and skip
> > until got the stack.
> >
> 
> Hi, sorry I might have some misunderstanding. Adding an extra argument
> (eg. skip_sp) to indicate if it should just unwind from the current
> frame, and use SP as the "skip mark", should work well.
> 
> And I also think the "fake"/"real" reg is fragile, could we abuse
> another eflag (just like PERF_EFLAGS_EXACT) to indicate the regs are
> partially dumped fake registers? So perf_callchain_kernel just check
> if it's a "partial registers", and in such case it can start unwinding
> and skip until it get to SP. This make it easier to tell if the
> registers are "fake".

If you do the regs->eflags thing to mark the regs as fake in
(perf_arch_fetch_caller_regs()), then I don't think skip_sp would be
needed, because regs->sp could probably mark the skip point.

Instead I was actually hoping we could get rid of fake regs and
perf_arch_fetch_caller_regs() altogether, because it's a nasty hack.
But I don't know what else those fake regs are used for.

-- 
Josh