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?
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