On Fri, Jun 14, 2013 at 3:01 PM, David Ahern <dsah...@gmail.com> wrote: > On 6/14/13 3:53 PM, Cody P Schafer wrote: >> >> On Thu, Jun 13, 2013 at 6:10 PM, David Ahern <dsah...@gmail.com> wrote: >>> >>> On 6/13/13 7:04 PM, Cody P Schafer wrote: >>>> >>>> Hi all, I'm trying to use perf kvm to profile linux early boot. >>>> >>>> However, perf kvm top always appears to behave as though it's in >>>> non-callgraph mode. >>> >>> During event collection perf kernel side does not walk the guest >>> callchain. >>> I believe it only collects IP. >>> >> >> Some further investigation shows that the user unwind code could be >> used, but [the following] is always false. >> if (!((evsel->attr.sample_type & PERF_SAMPLE_REGS_USER) && >> (evsel->attr.sample_type & PERF_SAMPLE_STACK_USER))) >> return 0; >> >> There don't appear to be any users which _set_ PERF_SAMPLE_REGS_USER >> or PERF_SAMPLE_STACK_USER, only ones that check it. Can we just use >> __perf_evsel__set_sample_bit() twice and have this magically work? >> >> So, a few questions: >> - can we get perf to dump the user regs & stack for kvm guest kernels? >> - does the in kernel stack unwinding _only_ trigger for host (ie: >> real) kernel mode samples? If so, does this restriction always make >> sense? >> - How did all this user unwind code get triggered if no one is >> setting PERF_SAMPLE_REGS_USER and PERF_SAMPLE_STACK_USER? Was it ever >> triggered? >> > > I am under the impression the limitations are these 2 snippets in > arch/x86/kernel/cpu/perf_event.c:
Yep, that's disabling the in-kernel callchain generation (the in-perf callchain code should still be an option without this). I wonder if one could use perf_callchain_user() to find the callchain of the guest kernel. Does anyone know the particular changes that need to be done to support kvm backtraces/the reasons it is specifically disabled? -- To unsubscribe from this list: send the line "unsubscribe linux-perf-users" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
