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:
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;
}
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