Re: 4.19 dwarf unwinding broken

[Josh Hunt <johunt@akamai.com>]

On 10/3/19 3:03 AM, Jiri Olsa wrote:
> On Thu, Oct 03, 2019 at 12:54:09AM -0700, Josh Hunt wrote:
>> The following commit is breaking dwarf unwinding on 4.19 kernels:
> 
> how?

When doing something like:
perf record -p $(cat /var/run/app.pid) -g --call-graph dwarf -F 999 -- 
sleep 3

with 4.19.75 perf I see things like:

app_Thr00 26247 1810131.375329:     168288 cycles:ppp:

app_Thr01 26767 1810131.377449:     344415 cycles:ppp:

uvm:WorkerThread 26746 1810131.383052:        504 cycles:ppp:
         ffffffff9f77cce0 _raw_spin_lock+0x10 (/boot/vmlinux-4.19.46)
         ffffffff9f181527 __perf_event_task_sched_in+0xf7 
(/boot/vmlinux-4.19.46)
         ffffffff9f09a7b8 finish_task_switch+0x158 (/boot/vmlinux-4.19.46)
         ffffffff9f778276 __schedule+0x2f6 (/boot/vmlinux-4.19.46)
         ffffffff9f7787f2 schedule+0x32 (/boot/vmlinux-4.19.46)
         ffffffff9f77bb0a schedule_hrtimeout_range_clock+0x8a 
(/boot/vmlinux-4.19.46)
         ffffffff9f22ea12 poll_schedule_timeout.constprop.6+0x42 
(/boot/vmlinux-4.19.46)
         ffffffff9f22eeeb do_sys_poll+0x4ab (/boot/vmlinux-4.19.46)
         ffffffff9f22fb7b __se_sys_poll+0x5b (/boot/vmlinux-4.19.46)
         ffffffff9f0023de do_syscall_64+0x4e (/boot/vmlinux-4.19.46)
         ffffffff9f800088 entry_SYSCALL_64+0x68 (/boot/vmlinux-4.19.46)
---

and with 4.19.75 perf with e5adfc3e7e77 reverted those empty call stacks 
go away and also other call stacks show more thread details:

uvm:WorkerThread 26746 1810207.336391:          1 cycles:ppp:
         ffffffff9f181505 __perf_event_task_sched_in+0xd5 
(/boot/vmlinux-4.19.46)
         ffffffff9f09a7b8 finish_task_switch+0x158 (/boot/vmlinux-4.19.46)
         ffffffff9f778276 __schedule+0x2f6 (/boot/vmlinux-4.19.46)
         ffffffff9f7787f2 schedule+0x32 (/boot/vmlinux-4.19.46)
         ffffffff9f77bb0a schedule_hrtimeout_range_clock+0x8a 
(/boot/vmlinux-4.19.46)
         ffffffff9f22ea12 poll_schedule_timeout.constprop.6+0x42 
(/boot/vmlinux-4.19.46)
         ffffffff9f22eeeb do_sys_poll+0x4ab (/boot/vmlinux-4.19.46)
         ffffffff9f22fb7b __se_sys_poll+0x5b (/boot/vmlinux-4.19.46)
         ffffffff9f0023de do_syscall_64+0x4e (/boot/vmlinux-4.19.46)
         ffffffff9f800088 entry_SYSCALL_64+0x68 (/boot/vmlinux-4.19.46)
             7f7ef3f5c90d [unknown] (/lib/x86_64-linux-gnu/libc-2.23.so)
                  3eb5c99 poll+0xc9 (inlined)
                  3eb5c99 colib::ipc::EventFd::wait+0xc9 
(/usr/local/bin/app)
                  3296779 uvm::WorkerThread::run+0x129 (/usr/local/bin/app)
         ffffffffffffffff [unknown] ([unknown])

They also look the same as earlier kernel versions we have running.

In addition reading e8ba2906f6b's changelog sounded very similar to what 
I was seeing. This application launches a # of threads and is definitely 
already running before the invocation of perf.

Thanks for looking at this.

Josh
> 
> jirka
> 
>>
>> commit e5adfc3e7e774ba86f7bb725c6eef5f32df8630e
>> Author: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
>> Date:   Tue Aug 7 12:09:01 2018 +0300
>>
>>      perf map: Synthesize maps only for thread group leader
>>
>> It looks like this was fixed later by:
>>
>> commit e8ba2906f6b9054102ad035ac9cafad9d4168589
>> Author: John Keeping <john@metanate.com>
>> Date:   Thu Aug 15 11:01:45 2019 +0100
>>
>>      perf unwind: Fix libunwind when tid != pid
>>
>> but was not selected as a backport to 4.19. Are there plans to backport the
>> fix? If not, could we have the breaking commit reverted in 4.19 LTS?
>>
>> Thanks
>> Josh