Re: BUG: perf kvm top --callgraph not showing callgraph

[David Ahern <dsahern@gmail.com>]

On 6/14/13 3:53 PM, Cody P Schafer wrote:
> On Thu, Jun 13, 2013 at 6:10 PM, David Ahern <dsahern@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:

oid
perf_callchain_kernel(struct perf_callchain_entry *entry, struct pt_regs 
*regs)
{
     if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
         /* TODO: We don't support guest os callchain now */
         return;
     }

...

void
perf_callchain_user(struct perf_callchain_entry *entry, struct pt_regs 
*regs)
{
     struct stack_frame frame;
     const void __user *fp;

     if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
         /* TODO: We don't support guest os callchain now */
         return;
     }