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'.