Tutorial: Debugging with Intel® Distribution for GDB*

Debug a SYCL* Application on a GPU#

Use a simple SYCL application named Array Transform application to perform basic debugging operations, such as break, run, print, continue, info, disassemble, and next. This tutorial describes how to interact with SIMD lanes, as additional thread elements. The application being debugged is instructed to run on a GPU by setting the ONEAPI_DEVICE_SELECTOR=level_zero:gpu environment variable.

The debug array transform application used in this tutorial can be found in the Intel oneAPI sample repo or by way of the oneapi-cli sample browser tool. After you have installed and initialized the Intel oneAPI Base Toolkit (sourced setvars.sh), run oneapi-cli --help in your terminal command line. The sample includes a build script to create an application that can be debugged and run on either a CPU or a GPU (the compiler debug flags are set during the build).

Before you proceed, make sure you have completed all necessary setup steps described in the Get Started Guide.

Basic Debugging#

Note

For your convenience, all common Intel Distribution for GDB commands used in examples below are provided in the reference sheet.

Consider the array-transform.cpp example again:

52        h.parallel_for(data_range, [=](id<1> index) {
53            size_t id0 = GetDim(index, 0);
54            int element = in[index]; // breakpoint-here
55            int result = element + 50;
56            if (id0 % 2 == 0) {
57                result = result + 50; // then-branch
58            } else {
59                result = -1; // else-branch
60            }
61            out[index] = result;
62        });

If you have not already done so, start the debugger:

gdb-oneapi array-transform

Start the debugger, set two breakpoints inside the kernel (one for each conditional branch) as follows:

  1. break 57
    

    Expected output:

    Breakpoint 1 at 0x40583c: file /path/to/array-transform.cpp, line 57.
    
  2. break 59
    

    Expected output:

    Breakpoint 2 at 0x40584a: file /path/to/array-transform.cpp, line 59.
    

Note

Do not expect your output to exactly match that provided in the tutorial. The output may vary due to the nature of parallelism and different machine properties. The ellipsis […] denotes output omitted for brevity.

Note

The default choice for offloading is a Level Zero GPU device, if it is available. If you have not yet set the ONEAPI_DEVICE_SELECTOR environment variable, you can do so from GDB by typing:

set env ONEAPI_DEVICE_SELECTOR=level_zero:gpu

To display the value of the variable, execute:

show env ONEAPI_DEVICE_SELECTOR

To start the program, execute:

run

You should see the following output:

Starting program: /path/to/array-transform
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
intelgt: gdbserver-ze started for process 8194.
[New Thread 0x7fffed706700 (LWP 8213)]
[SYCL] Using device: [Intel(R) Data Center GPU Flex Series 140 [0x56c1]] from [Intel(R) Level-Zero]
[Switching to Thread 1.129 lane 1]

Thread 2.129 hit Breakpoint 2, with SIMD lanes [1 3 5 7], main::{lambda(auto:1&)#1}::operator()[...] at array-transform.cpp:59
59                result = -1;  // else-branch
(gdb)

The debugger has a mechanism called “Auto-Attach” that spawns an instance of gdbserver-ze to listen to and control the GPU for debug. In the example above, the auto-attach mechanism is triggered and the gdbserver-ze is added to the debugger as an inferior. An inferior in GDB represents the unit under debug. In our case, the host application process and the GPU device each correspond to an inferior.

Check the presence of gdbserver-ze as follows:

info inferiors

Expected output:

(gdb) info inferiors
  Num  Description              Connection                                Executable
  1    process 8194             1 (native)                                /path/to/array-transform
* 2    device [37:00.0]         2 (remote | gdbserver-ze --attach - 8194)
Type "info devices" to see details of the devices.

Execute the info devices command to see the further details of the device.

info devices

Expected output:

  Location   Sub-device   Vendor Id   Target Id   Cores   Device Name
* [37:00.0]  -            0x8086      0x56c1      128     Intel(R) Data Center GPU Flex Series 140 [0x56c1]

Note

The auto-attach feature sets schedule-multiple to on, which allows all threads of all inferiors to be resumed during the same session. For example, when you run the continue command, all inferiors will continue.

The breakpoint event is received from Inferior 2, which represents the GPU. The thread ID 2.129:1 points to the thread 129 of the inferior 2 and indicates that the first active SIMD lane is now in focus.

The breakpoint at line 59 is hit first. The order of branch execution is defined by the Intel® Graphics Compiler.

Note

The behavior of the debugger may vary if the compiled code is optimized. For the best debugging experience, the compiler flags -g -O0 are recommended.

Check which SIMD lanes are currently active with the following command. The -stopped flag filters out GPU threads that are currently unavailable (e.g. not utilized by the program). We recommend using it to obtain a more concise output. We also recommend using the with print frame-arguments none -- prefix to reduce the overhead of the command, which can be noticeably large because of having to fetch the state of a large number of GPU threads.

with print frame-arguments none -- info threads -stopped

In the example, thread 2.129 has 4 active SIMD lanes: 1, 3, 5, and 7. The asterisk ‘*’ marks the current SIMD lane. See the expected output below.

Note

SIMD lane enumeration starts from 0.

(gdb) with print frame-arguments none -- info threads -stopped
  Id              Target Id                                          Frame
  1.1             Thread 0x7ffff598fb80 (LWP 8194) "array-transform" [...]
  1.2             Thread 0x7fffed706700 (LWP 8213) "array-transform" [...]
* 2.129:1         Thread 1.129                                       <frame> at array-transform.cpp:59
  2.129:[3 5 7]   Thread 1.129                                       <frame> at array-transform.cpp:59
  2.137:[1 3 5 7] Thread 1.137                                       <frame> at array-transform.cpp:59
  2.145:[1 3 5 7] Thread 1.145                                       <frame> at array-transform.cpp:59
  2.153:[1 3 5 7] Thread 1.153                                       <frame> at array-transform.cpp:59
  2.193:[1 3 5 7] Thread 1.193                                       <frame> at array-transform.cpp:59
  2.201:[1 3 5 7] Thread 1.201                                       <frame> at array-transform.cpp:59
  2.209:[1 3 5 7] Thread 1.209                                       <frame> at array-transform.cpp:59
  2.217:[1 3 5 7] Thread 1.217                                       <frame> at array-transform.cpp:59

To switch the focus to a different SIMD lane, use the thread <thread_ID> command. Thread ID is specified by a triple: inferior.thread:lane. See examples of working with particular lanes:

    1. thread 2.129:3
      

      Example output:

      [Switching to thread 2.129:3 (Thread 1.129 lane 3)]
      #0  main::{lambda(auto:1&)#1}::operator()[...] at array-transform.cpp:59
      59                result = -1;  // else-branch
      
    2. print element
      

      Example output:

      $1 = 111
      
    1. thread 2.129:5
      

      Example output:

      [Switching to thread 2.129:5 (Thread 1.129 lane 5)]
      #0  main::{lambda(auto:1&)#1}::operator()[...] at array-transform.cpp:59
      59                result = -1;  // else-branch
      
    2. print element
      

      Example output:

      $2 = 113
      

Note

To filter threads for a specific device or sub-device one needs to:

  1. Obtain the corresponding inferior number via info inferiors command:

    (gdb) info inferiors
         Num  Description              Connection                                 Executable
         1    process 25855            1 (native)                                 [...]
       * 2    device [3a:00.0].0       2 (remote | gdbserver-ze --attach - 25855)
         3    device [3a:00.0].1       2 (remote | gdbserver-ze --attach - 25855)
         Type "info devices" to see details of the devices.
    
  2. Run the info threads command and supply the obtained inferior numbers, followed by a star-wildcard thread range .*:

    (gdb) with print frame-arguments none -- info threads 2.* 3.*
         Id           Target Id         Frame
         2.1:[0-15]   Thread 1.1        <frame> at array-transform.cpp:53
         2.2:[0-15]   Thread 1.2        <frame> at array-transform.cpp:53
         ...
         3.1:[0-15]   Thread 2.1        <frame> at array-transform.cpp:53
         3.2:[0-15]   Thread 2.2        <frame> at array-transform.cpp:53
         ...
    

Note

In the thread ID, the inferior number can be skipped. In this case, the current inferior ID is used. The thread number can also be skipped in case of switching to a lane in the current thread. Thus, the command below can be used to switch to the desired SIMD lane:

thread :7

Expected output:

[Switching to thread 2.129:7 (Thread 1.129 lane 7)]
#0  main::{lambda(auto:1&)#1}::operator()[...] at array-transform.cpp:59
59                result = -1;  // else-branch

As you are now inside the kernel running on the GPU, you can look into the assembly code and GPU registers, for example, to understand the cause of unexpected application behavior. Get the GPU assembly code to inspect generated instructions by executing the following command:

disassemble

See an example output below:

Dump of assembler code for function _ZZZ4mainENKUlRT_E_clIN4sycl3_V17handlerEEEDaS0_ENKUlNS4_2idILi1EEEE_clES7_:
   0xffff8000ffe87200 <+0>:     (W)     shr (1|M16)              a0.2<1>:ud    r126.7<0;1,0>:ud  0x4:ud              {F@1}
   0xffff8000ffe87210 <+16>:    (W)     add (1|M16)              r126.0<1>:ud  r125.2<0;1,0>:ud  0x0:ud
   0xffff8000ffe87220 <+32>:    (W)     send.ugm (1|M16)         null     r126    r125:1  a0.2        0x4200C504           {ExBSO,A@1,$0} // wr:1+1, rd:0; store.ugm.d32x8t.a32.ss[a0.2]
   0xffff8000ffe87230 <+48>:    (W)     mov (1|M16)              r125.3<1>:ud  r125.2<0;1,0>:ud                 {$0.src}
   0xffff8000ffe87240 <+64>:    (W)     add (1|M16)              r125.2<1>:ud  r125.2<0;1,0>:ud  0x180:ud
   0xffff8000ffe87250 <+80>:    (W)     add (1|M16)              r126.0<1>:ud  r125.3<0;1,0>:ud  0x40:ud              {I@2}
   0xffff8000ffe87260 <+96>:    (W)     send.ugm (1|M16)         null     r126    r60:4   a0.2        0x4200E504           {ExBSO,A@1,$1} // wr:1+4, rd:0; store.ugm.d32x32t.a32.ss[a0.2]
   0xffff8000ffe87270 <+112>:   (W)     add (1|M16)              r126.0<1>:ud  r125.3<0;1,0>:ud  0xC0:ud              {$1.src}
   0xffff8000ffe87280 <+128>:   (W)     send.ugm (1|M16)         null     r126    r64:4   a0.2        0x4200E504           {ExBSO,A@1,$2} // wr:1+4, rd:0; store.ugm.d32x32t.a32.ss[a0.2]

To learn more about GEN assembly and registers, refer to the “Introduction to GEN assembly” article.

To display a list of GPU registers, run the following command:

info registers

You can use registers to see the state of the application or inspect arithmetic instructions, such as which operands are used and where the result is located.

Additionally, you can inspect the execution mask ($emask register), which shows active lanes. To print the result in binary format, use the /t format flag as follows:

print/t $emask

Example output:

$3 = 10101010

Recall that you have stopped at line 59, the else-branch of the condition that checks evenness of the work-item index. Hence, every other SIMD lane is inactive, as indicated by the $emask bit pattern.

To move forward and stop at the then-branch, set the scheduler-locking mode to step and execute the next command. The set scheduler-locking step command keeps the other threads stopped while the current thread is stepping:

set scheduler-locking step
next

You should see the following output:

[Switching to SIMD lane 0]

Thread 2.129 hit Breakpoint 1, with SIMD lanes [0 2 4 6], main::{lambda(auto:1&)#1}::operator()[...] at array-transform.cpp:57
57                result = result + 50;  // then-branch

Due to the breakpoint event, the SIMD lane focus switches to the first active lane in the then-branch, which is SIMD lane 0. Other threads of inferior 2 stayed at the line 59:

with print frame-arguments none -- info threads -stopped

Example output:

 Id              Target Id                                          Frame
  1.1             Thread 0x7ffff598fb80 (LWP 8194) "array-transform" [...]
  1.2             Thread 0x7fffed706700 (LWP 8213) "array-transform" [...]
* 2.129:0         Thread 1.129                                       <frame> at array-transform.cpp:57
  2.129:[2 4 6]   Thread 1.129                                       <frame> at array-transform.cpp:57
  2.137:[1 3 5 7] Thread 1.137                                       <frame> at array-transform.cpp:59
  2.145:[1 3 5 7] Thread 1.145                                       <frame> at array-transform.cpp:59
  2.153:[1 3 5 7] Thread 1.153                                       <frame> at array-transform.cpp:59
  2.193:[1 3 5 7] Thread 1.193                                       <frame> at array-transform.cpp:59
  2.201:[1 3 5 7] Thread 1.201                                       <frame> at array-transform.cpp:59
  2.209:[1 3 5 7] Thread 1.209                                       <frame> at array-transform.cpp:59
  2.217:[1 3 5 7] Thread 1.217                                       <frame> at array-transform.cpp:59

Since the thread is vectorized, you can also inspect the vector of a local variable:

x /8dw &result

Example output:

0xffffd556ab1627e0:     158     -1      160     -1
0xffffd556ab1627f0:     162     -1      164     -1

SIMD Lanes#

To investigate the program state from the point of view of SIMD lanes without switching, use the thread apply command. You can specify a SIMD lane as a number:

thread apply 2.129:2 print element

Example output:

Thread 2.129:2 (Thread 1.129 lane 2):
$5 = 110

You can also specify a SIMD lane as a range. In this case, only active SIMD lanes from the range are considered:

thread apply 2.129:2-5 print element

Example output:

Thread 2.129:2 (Thread 1.129 lane 2):
$11 = 110
warning: SIMD lane 3 is inactive in thread 2.129

Thread 2.129:4 (Thread 1.129 lane 4):
$12 = 112
warning: SIMD lane 5 is inactive in thread 2.129

To denote all active SIMD lanes, use the wildcard:

thread apply 2.129:* print element

Example output:

Thread 2.129:0 (Thread 1.129 lane 0):
$13 = 108

Thread 2.129:2 (Thread 1.129 lane 2):
$14 = 110

Thread 2.129:4 (Thread 1.129 lane 4):
$15 = 112

Thread 2.129:6 (Thread 1.129 lane 6):
$16 = 114

To apply the command to all active SIMD lanes of all threads, use the all-lanes parameter:

thread apply all-lanes print element

Example output:

Thread 2.217:7 (Thread 1.217 lane 7):
$17 = 155

Thread 2.217:5 (Thread 1.217 lane 5):
$18 = 153


[...]
Thread 2.129:2 (Thread 1.129 lane 2):
$47 = 110

Thread 2.129:0 (Thread 1.129 lane 0):
$48 = 108

Thread 1.2 (Thread 0x7fffed706700 (LWP 8213) "array-transform"):
No symbol "element" in current context.

You can mix SIMD lane ranges with thread ranges and the thread wildcard. For example, to apply the command to all active lanes of all threads of inferior 2, you can use any of the following commands:

  • thread apply 2.127-129:*
    
  • thread apply 2.*:*
    

If the current inferior is 2, the inferior number can be skipped:

  • thread apply 127-129:*
    
  • thread apply *:*
    

If you need a formatted output for a set of threads, thread apply might be used together with the printf command, as in the following examples.

  • A more compact output in comparison to thread apply *:* print element:

    (gdb) thread apply *:* -q printf "%d.%d:%d element=%d\n",$_inferior,$_thread,$_simd_lane,element
    2.129:0 element=108
    2.129:1 element=109
    2.129:2 element=110
    2.129:3 element=111
    2.129:4 element=112
    2.129:5 element=113
    2.129:6 element=114
    2.129:7 element=115
    2.137:0 element=124
    2.137:1 element=125
    2.137:2 element=126
    2.137:3 element=127
    2.137:4 element=128
    2.137:5 element=129
    2.137:6 element=130
    2.137:7 element=131
    2.145:0 element=140
    <...>
    

    In the above command, -q flag is used to suspend the standard thread information, usually printed by thread apply. To print the thread context in a compact way, three convenience variables were used:

    • $_inferior to get the inferior number;

    • $_thread to get the thread number within the inferior;

    • $_simd_lane to get the SIMD lane.

  • To get a more hierarchical view, one can supply thread apply *, which applies a command to all threads of the current inferior, with the command thread apply :* <printing command>. The latter applies the printing command to every active SIMD lane of a thread, selected by the former thread apply *. The result might look as the following:

    (gdb) thread apply * -s thread apply :* -q printf "dim0{%d}=%d \n",$_simd_lane,id0
    
    Thread 2.129:0 (Thread 1.129 lane 0):
    dim0{0}=8
    dim0{1}=9
    dim0{2}=10
    dim0{3}=11
    dim0{4}=12
    dim0{5}=13
    dim0{6}=14
    dim0{7}=15
    
    Thread 2.137:0 (Thread 1.137 lane 0):
    dim0{0}=24
    dim0{1}=25
    dim0{2}=26
    dim0{3}=27
    dim0{4}=28
    dim0{5}=29
    dim0{6}=30
    dim0{7}=31
    
    Thread 2.145:0 (Thread 1.145 lane 0):
    dim0{0}=40
    <...>
    

Work-Item Coordinates#

The GPGPU execution model defines a work-item as one of parallel executions of a kernel function.

Use the convenience variables $_thread_workgroup, $_workitem_local_id, and $_workitem_global_id to get the coordinates of the work-item processed by the current context, defined by the current thread and its current lane.

(gdb) print $_thread_workgroup
$1 = {<x>: 0, <y>: 0, <z>: 0}
(gdb) print $_workitem_local_id
$2 = {<x>: 56, <y>: 0, <z>: 0}
(gdb) print $_workitem_global_id
$3 = {<x>: 56, <y>: 0, <z>: 0}

Please note that the above convenience variables show work-item coordinates using X-Y-Z notation, as per execution model of the device, while SYCL execution model defines coordinates in notation of dimensions 1-2-3. SYCL RT often performs an optimization, such that SYCL dimensions are transposed and 1-2-3 corresponds to Z-Y-X.

Note

The coordinates are available only for work-items that are currently being processed. If a work-item has not yet been started or was already finished, we cannot find a thread which has processed it.

Find a Specific Work-item#

Using the convenience variables you can find a thread and its lane, which works on a specific work-item.

  • The first option to find the work-item is to define a conditional breakpoint. However, for a program with many threads, it could take time, till the breakpoint is hit. In the following example, we set the conditional breakpoint for the work-item with the global ID {37,0,0}:

    (gdb) break 54 if $_workitem_global_id=={37,0,0}
    Breakpoint 1 at 0x407093: file /home/gta/sources/oneAPI-samples/Tools/ApplicationDebugger/array-transform/src/array-transform.cpp, line 54.
    (gdb) run
    Starting program: /home/gta/sources/oneAPI-samples/Tools/ApplicationDebugger/array-transform/build/array-transform
    [...]
    [SYCL] Using device: [Intel(R) Data Center GPU Flex 140] from [Intel(R) Level-Zero]
    [Switching to Thread 1.209 lane 5]
    
    Thread 2.209 hit Breakpoint 1.2, with SIMD lane 5, main::{lambda(auto:1&)#1}::operator()<sycl::_V1::handler>(sycl::_V1::handler&) const::{lambda(sycl::_V1::id<1>)#1}::operator()(sycl::_V1::id<1>) const (this=0xffffd556ab1e5810, index=...) at array-transform.cpp:54
    54              int element = in[index];  // breakpoint-here
    (gdb) print $_workitem_global_id
    $1 = {<x>: 37, <y>: 0, <z>: 0}
    (gdb)
    
  • The second option is to use the thread apply command and store the found thread ID and lane number into the convenience variables $thr and $lane. The $found variable shows whether the search was successful. In the following example, we search for a work-item with the global ID {47,0,0}, and then switch to the found thread and lane:

    (gdb) thread apply *:* -q -s set $found=($_workitem_global_id == {47,0,0}) ? ($thr=$_thread) && ($lane = $_simd_lane) : $found
    (gdb) print $found
    $191 = true
    (gdb) thread $thr:$lane
    [Switching to thread 2.145:7 (Thread 1.145 lane 7)]
    #0  main::[...]
    54              int element = in[index];  // breakpoint-here
    (gdb) print $_workitem_global_id
    $192 = {<x>: 47, <y>: 0, <z>: 0}
    

Filter Threads by a Work-group#

By combining thread apply and eval, we can filter threads by a specific expression. In the following, we filter by $_thread_workgroup=={0,0,0}.

First, we construct a convenience variable $ids that holds a stringified list of qualified ids (<inferior num>.<thread num>), which belong to the work-group:

(gdb) set $ids=""
(gdb) thread apply * -s -q eval "set $ids=($_thread_workgroup == {0,0,0}) ? \"%s %d.%d\" : \"%s\"",$ids,$_inferior,$_thread, $ids
(gdb) print $ids
$2 = " 2.129 2.137 2.145 2.153 2.193 2.201 2.209 2.217"

Note that the variable $ids must be initialized with an empty string first. The eval GDB command is used here to append the list of already found ids to the newly found one, or leave it without change, if the condition does not hold.

Now the convenience variable $ids contains the list of filtered thread ids.

To call info threads for these ids, we need to use eval again, since the info threads command cannot take a list of threads stored in a convenience variable:

(gdb) eval "info threads %s", $ids
  Id          Target Id         Frame
* 2.129:0     Thread 1.129      main::[...] at array-transform.cpp:54
  2.129:[1-7] Thread 1.129      main::[...] at array-transform.cpp:54
  2.137:[0-7] Thread 1.137      main::[...] at array-transform.cpp:54
  2.145:[0-7] Thread 1.145      main::[...] at array-transform.cpp:54
  2.153:[0-7] Thread 1.153      main::[...] at array-transform.cpp:54
  2.193:[0-7] Thread 1.193      main::[...] at array-transform.cpp:54
  2.201:[0-7] Thread 1.201      main::[...] at array-transform.cpp:54
  2.209:[0-7] Thread 1.209      main::[...] at array-transform.cpp:54
  2.217:[0-7] Thread 1.217      main::[...] at array-transform.cpp:54

Breakpoint Actions#

You can define a set of actions for a breakpoint to be executed when the breakpoint is hit. By default, the actions are executed in the context of the SIMD lane selected after the hit.

  1. Quit the current debugging session and start a new one:

    quit
    
    gdb-oneapi array-transform
    
  2. Define two temporary breakpoints with actions for the if and else branches:

      1. Set a temporary breakpoint:

        tbreak 59
        

        Example output:

        Temporary breakpoint 1 at 0x40584a: file /path/to/array-transform.cpp, line 59.
        
      2. Define an action:

        commands
        

        When you are asked to type commands, enter the following:

        print element
        end
        

        When you are done with each command, finish with the end keyword.

      1. Set another temporary breakpoint:

        tbreak 57
        

        Example output:

        Temporary breakpoint 2 at 0x40583c: file /path/to/array-transform.cpp, line 57.
        
      2. Define an action to be executed for all SIMD lines by adding the /a modifier:

        commands /a
        

        When you are asked to type commands, enter the following:

        print element
        end
        

Start the program:

run

Example output:

[...]
Thread 2.129 hit Temporary breakpoint 1, with SIMD lanes [1 3 5 7], main::{lambda(auto:1&)#1}::operator()[...] at array-transform.cpp:59
59                result = -1;  // else-branch
$1 = 109

Continue to hit both breakpoints:

continue

Example output:

Continuing.
Thread 2.129 hit Temporary breakpoint 2, with SIMD lanes [0 2 4 6], main::{lambda(auto:1&)#1}::operator()[...] at array-transform.cpp:57
57                result = result + 50;  // then-branch
$2 = 108
$3 = 110
$4 = 112
$5 = 114

The action for the breakpoint at the else branch was executed for a single SIMD lane 1, while the action at the then branch was executed for all active SIMD lanes.

Note

For conditional breakpoints, the actions are executed only for SIMD lanes that meet the condition.

Conditional Breakpoints#

Quit the debugging session and start the program from the beginning:

quit
ONEAPI_DEVICE_SELECTOR=level_zero:gpu gdb-oneapi array-transform

This time set a breakpoint at line 57 with the condition element==106:

break 57 if element == 106

Example output:

Breakpoint 1 at 0x40583c: file /path/to/array-transform.cpp, line 57.

Run the program (execute the run command) and check if the output looks as follows:

Starting program: <path_to_array-transform>


[...]


[Switching to Thread 1.193 lane 6]

Thread 2.193 hit Breakpoint 1, with SIMD lane 6, main::{lambda(auto:1&)#1}::operator()[...] at array-transform.cpp:57
57                result = result + 50;  // then-branch
(gdb)

The condition is true for the lane 6 in thread 2.193.

Note

A breakpoint condition is evaluated only for active SIMD lanes, meaning that (gdb) break 57 if element == 107 does not cause a stop, since element == 107 is true for the lane 7 in thread 2.193, which is inactive at line 57.