Messages in this thread | | | Date | Mon, 15 Apr 2019 11:58:59 -0500 | From | Josh Poimboeuf <> | Subject | Re: [RFC PATCH v2] perf/x86: make perf callchain work without CONFIG_FRAME_POINTER |
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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
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