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

[Josh Poimboeuf <jpoimboe@redhat.com>]

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.

In other words, either regs should be "real", and skip_sp is NULL; or
regs should be NULL and skip_sp should have a value.

-- 
Josh