Messages in this thread | ![/](/images/icornerl.gif) | | Date | Wed, 17 Apr 2019 21:15:48 -0500 | From | Josh Poimboeuf <> | Subject | Re: [RFC PATCH v2] perf/x86: make perf callchain work without CONFIG_FRAME_POINTER |
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On Wed, Apr 17, 2019 at 09:07:35AM +0200, Peter Zijlstra wrote: > On Tue, Apr 16, 2019 at 03:15:59PM -0500, Josh Poimboeuf wrote: > > 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. > > This is the generic perf generate a sample path. It doesn't know the > context. Normally it is an interrupt or exception of sorts and we simply > pass the pt_regs from that down the chain and all is good. > > It is just for a few software events, such as SW_CONTEXT_SWITCH, > SW_MIGRATIONS and all the TP muck that we do not have regs to pass down. > > These regs are used by: > > - SAMPLE_IP, > - SAMPLE_CALLCHAIN (this here issue), > - SAMPLE_REGS_INTR, > - a few misc bits > > There's actually comment on top of perf_fetch_caller_regs() that tries > to explain what is needed -- although I suppose that could do with an > update.
Ok, I guess it makes a little more sense now to my perf-ignorant brain. An improved comment would definitely help.
Sounds like using a reserved regs->flags bit to indicate fake regs is the way to go.
-- Josh
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