Re: Getting empty callchain from perf_callchain_kernel()

[Peter Zijlstra <peterz@infradead.org>]

On Fri, May 17, 2019 at 09:46:00AM +0200, Peter Zijlstra wrote:
> On Thu, May 16, 2019 at 11:51:55PM +0000, Song Liu wrote:
> > Hi, 
> > 
> > We found a failure with selftests/bpf/tests_prog in test_stacktrace_map (on bpf/master
> > branch). 
> > 
> > After digging into the code, we found that perf_callchain_kernel() is giving empty
> > callchain for tracepoint sched/sched_switch. And it seems related to commit
> > 
> > d15d356887e770c5f2dcf963b52c7cb510c9e42d
> > ("perf/x86: Make perf callchains work without CONFIG_FRAME_POINTER")
> > 
> > Before this commit, perf_callchain_kernel() returns callchain with regs->ip. With
> > this commit, regs->ip is not sent for !perf_hw_regs(regs) case. 
> 
> So while I think the below is indeed right; we should store regs->ip
> regardless of the unwind path chosen, I still think there's something
> fishy if this results in just the 1 entry.
> 
> The sched/sched_switch event really should have a non-trivial stack.
> 
> Let me see if I can reproduce with just perf.

$ perf record -g -e "sched:sched_switch" -- make clean
$ perf report -D

12 904071759467 0x1790 [0xd0]: PERF_RECORD_SAMPLE(IP, 0x1): 7236/7236: 0xffffffff81c29562 period: 1 addr: 0
... FP chain: nr:10
.....  0: ffffffffffffff80
.....  1: ffffffff81c29562
.....  2: ffffffff81c29933
.....  3: ffffffff8111f688
.....  4: ffffffff81120b9d
.....  5: ffffffff81120ce5
.....  6: ffffffff8100254a
.....  7: ffffffff81e0007d
.....  8: fffffffffffffe00
.....  9: 00007f9b6cd9682a
... thread: sh:7236
...... dso: /lib/modules/5.1.0-12177-g41bbb9129767/build/vmlinux


IOW, it seems to 'work'.