Re: Getting empty callchain from perf_callchain_kernel()

[Kairui Song]

On Fri, May 17, 2019 at 4:11 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> 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'.
>

Hi, I think the actual problem is that bpf_get_stackid_tp (and maybe
some other bfp functions) is now broken, or, strating an unwind
directly inside a bpf program will end up strangely. It have following
kernel message:

WARNING: kernel stack frame pointer at 0000000070cad07c in
test_progs:1242 has bad value 00000000ffd4497e

And in the debug message:

[  160.460287] 000000006e117175: ffffffffaa23a0e8
(get_perf_callchain+0x148/0x280)
[  160.460287] 0000000002e8715f: 0000000000c6bba0 (0xc6bba0)
[  160.460288] 00000000b3d07758: ffff9ce3f9790000 (0xffff9ce3f9790000)
[  160.460289] 0000000055c97836: ffff9ce3f9790000 (0xffff9ce3f9790000)
[  160.460289] 000000007cbb140a: 000000010000007f (0x10000007f)
[  160.460290] 000000007dc62ac9: 0000000000000000 ...
[  160.460290] 000000006b41e346: 1c7da01cd70c4000 (0x1c7da01cd70c4000)
[  160.460291] 00000000f23d1826: ffffd89cffc3ae80 (0xffffd89cffc3ae80)
[  160.460292] 00000000f9a16017: 000000000000007f (0x7f)
[  160.460292] 00000000a8e62d44: 0000000000000000 ...
[  160.460293] 00000000cbc83c97: ffffb89d00d8d000 (0xffffb89d00d8d000)
[  160.460293] 0000000092842522: 000000000000007f (0x7f)
[  160.460294] 00000000dafbec89: ffffb89d00c6bc50 (0xffffb89d00c6bc50)
[  160.460296] 000000000777e4cf: ffffffffaa225d97 (bpf_get_stackid+0x77/0x470)
[  160.460296] 000000009942ea16: 0000000000000000 ...
[  160.460297] 00000000a08006b1: 0000000000000001 (0x1)
[  160.460298] 000000009f03b438: ffffb89d00c6bc30 (0xffffb89d00c6bc30)
[  160.460299] 000000006caf8b32: ffffffffaa074fe8 (__do_page_fault+0x58/0x90)
[  160.460300] 000000003a13d702: 0000000000000000 ...
[  160.460300] 00000000e2e2496d: ffff9ce300000000 (0xffff9ce300000000)
[  160.460301] 000000008ee6b7c2: ffffd89cffc3ae80 (0xffffd89cffc3ae80)
[  160.460301] 00000000a8cf6d55: 0000000000000000 ...
[  160.460302] 0000000059078076: ffffd89cffc3ae80 (0xffffd89cffc3ae80)
[  160.460303] 00000000c6aac739: ffff9ce3f1e18eb0 (0xffff9ce3f1e18eb0)
[  160.460303] 00000000a39aff92: ffffb89d00c6bc60 (0xffffb89d00c6bc60)
[  160.460305] 0000000097498bf2: ffffffffaa1f4791 (bpf_get_stackid_tp+0x11/0x20)
[  160.460306] 000000006992de1e: ffffb89d00c6bc78 (0xffffb89d00c6bc78)
[  160.460307] 00000000dacd0ce5: ffffffffc0405676 (0xffffffffc0405676)
[  160.460307] 00000000a81f2714: 0000000000000000 ...

# Note here is the invalid frame pointer
[  160.460308] 0000000070cad07c: ffffb89d00a1d000 (0xffffb89d00a1d000)
[  160.460308] 00000000f8f194e4: 0000000000000001 (0x1)
[  160.460309] 000000002134f42a: ffffd89cffc3ae80 (0xffffd89cffc3ae80)
[  160.460310] 00000000f9345889: ffff9ce3f1e18eb0 (0xffff9ce3f1e18eb0)
[  160.460310] 000000008ad22a42: 0000000000000000 ...
[  160.460311] 0000000073808173: ffffb89d00c6bce0 (0xffffb89d00c6bce0)
[  160.460312] 00000000c9effff4: ffffffffaa1f48d1 (trace_call_bpf+0x81/0x100)
[  160.460313] 00000000c5d8ebd1: ffffb89d00c6bcc0 (0xffffb89d00c6bcc0)
[  160.460315] 00000000bce0b072: ffffffffab651be0
(event_sched_migrate_task+0xa0/0xa0)
[  160.460316] 00000000355cf319: 0000000000000000 ...
[  160.460316] 000000003b67f2ad: ffffd89cffc3ae80 (0xffffd89cffc3ae80)
[  160.460316] 000000009a77e20b: ffff9ce3fba25000 (0xffff9ce3fba25000)
[  160.460317] 0000000032cf7376: 0000000000000001 (0x1)
[  160.460317] 000000000051db74: ffffb89d00c6bd20 (0xffffb89d00c6bd20)
[  160.460318] 0000000040eb3bf7: ffffffffaa22be81
(perf_trace_run_bpf_submit+0x41/0xb0)

Simply store the IP still won't really fix the problem, it just passed
the test. Just had a try to have bpf functions set the
X86_EFLAGS_FIXED for the flags and always dump the bp, it bypassed
this specified problem.

Using frame pointer unwinder for testing this, and seems ORC is fine with this.

-- 
Best Regards,
Kairui Song