On Wed, Apr 17, 2019 at 01:39:19AM +0800, Kairui Song wrote: > On Tue, Apr 16, 2019 at 7:30 PM Kairui Song <kas...@redhat.com> wrote: > > > > On Tue, Apr 16, 2019 at 12:59 AM Josh Poimboeuf <jpoim...@redhat.com> 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