authors: Bobby R. Bruce

Trace-based Debugging

Introduction

The simplest method of debugging is to have gem5 print out traces of what it’s doing. The simulator contains many DPRINTF statements that print trace messages describing potentially interesting events. Each DPRINTF is associated with a debug flag (e.g., Bus, Cache, Ethernet, Disk, etc.). To turn on the messages for a particular flag, use the --debug-flags command line argument. Multiple flags can be specified by giving a list of strings, e.g.:

build/ALPHA/gem5.opt --debug-flags=Bus,Cache configs/examples/fs.py

would turn on a group of debug flags related to instruction execution but leave out Tick (timing) information. This is useful if you want to compare execution between two runs where the same instructions execute but at different rates.

Note that the gem5.fast binary does not support tracing; part of what makes it faster than gem5.opt is that the DPRINTF code is compiled out.

The --debug-flags command line option should come after the gem5 executable but before the simulation script. This is because debug flags are handled by gem5 itself, and whether command line options are before or after the simulation script determine if they’re for gem5 or the script.

Debugging Options
-----------------
--debug-break=TIME[,TIME]
                        Tick to create a breakpoint
--debug-help            Print help on debug flags
--debug-flags=FLAG[,FLAG]
                        Sets the flags for debug output (-FLAG disables a
                        flag)
--debug-start=TIME      Start debug output at TIME (must be in ticks)
--debug-file=FILE       Sets the output file for debug [Default: cout]
--debug-ignore=EXPR     Ignore EXPR sim objects

The complete list of debug/trace flags can be seen by running gem5 with the --debug-help option.

If you find that events of interest are not being traced, feel free to add DPRINTFs yourself. You can add new debug flags simply by adding DebugFlag() command to any SConscript file (preferably the one nearest where you are using the new flag). If you use a debug flag in a C++ source file, you would need to include the header file debug/<name of debug flag>.hh in that file.

For more complex bugs, the trace can be useful in simply identifying points in the simulation where more in-depth investigation is needed. The --debug-break option lets you re-run your simulation under a debugger and stop on a particular tick as identified by the trace. You can also schedule breakpoints and enable or disable debug flags from within the debugger itself. See the page on Debugger Based Debugging for more information.

The Exec debug flag

The Exec compound debug flag is very useful because it turns on instruction tracing in gem5. It makes the simulator print a disassembled version of each instruction as it finishes executing, along with other useful information like the time, pc, the address if it was a memory instruction, etc. These individual pieces of information can be turned on and off with the base debug flags Exec controls. For example, you can disable the use of function symbol names in place of absolute PC addresses (if they’re available) by turning off the ExecSymbol flag (e.g., --debug-flags=Exec,-ExecSymbol).

If some supposedly innocuous change has caused gem5 to stop working correctly, you can compare trace outputs from before and after the change using the tracediff script in the src/util directory. Comments in the script describe how to use it.

Reducing trace file size

Trace file can become very large very quickly, but they also compress very well (e.g. about 90%). If you’d like to make gem5 output a compressed trace, just add a .gz extension to the output file name. For example --debug-file=trace.out will produce an uncompressed file as normal, but --debug-file=trace.out.gz will produce a gzip compressed file. You can use the zcat program and pipes to process the output. The editor vim also can uncompress gzip compressed files in memory.

The tracediff and rundiff utilities

tracediff and rundiff utilities allow the simple diffing of two streams of trace data from gem5 to find any differences. It’s very handy for debugging why regression tests fail, figuring out why your minor code change seems to cause some unrelated execution problem, or comparing the execution of CPU models.

Both utilities are found in the util directory. rundiff is a simple diff-like program. Unlike regular diff, this script does not read in the entire input before comparing its inputs, so it can be used on lengthy outputs piped from other programs (e.g., gem5 traces). tracediff is a front end for rundiff that provides an easy way to run two similar copies of gem5 and diff their outputs. It takes a common gem5 command line with embedded alternatives and executes the two alternative commands in separate subdirectories with output piped to rundiff.

Script arguments are handled uniformly as follows:

In other words, the arguments should look like the command line you want to run, with ‘|’ used to list the alternatives for the parts that you want to differ between the two runs.

For example:

 % tracediff gem5.opt --opt1 '--opt2|--opt3' --opt4
# would compare these two runs:
gem5.opt --opt1 --opt2 --opt4
gem5.opt --opt1 --opt3 --opt4

% tracediff 'path1|path2#/m5.opt' --opt1 --opt2
# would compare these two runs:
path1/gem5.opt --opt1 --opt2
path2/gem5.opt --opt1 --opt2

If you want to add arguments to one run only, just put a ‘|’ in with text only on one side (--onlyOn1|). You can do this with multiple arguments together too (|-a -b -c adds three args to the second run only).

The -n argument to tracediff allows you to preview the two generated command lines without running them.

For tracediff to be useful some trace flags must be enabled. The most common trace flags to use with tracediff are --debug-flags=Exec,-ExecTicks which removes the timestamp from each trace making it suitable to diff when slight timing variations are present.

Tracediff is also useful for comparing CPU models when one fails and the other doesn’t. In this case it’s best to create a checkpoint before the problem occurs (this can be done by just creating a bunch of checkpoints and finding one that fails). If the failure occurs in kernel code, use the -ExecUser debug flag, on the other hand if it occurs in user code try the -ExecKernel debug flag to isolate user code in the trace. You can then compare the traces and see when the execution diverges.

Comparing traces across machines

Sometimes gem5 executions differ inexplicably across different environments, and you’d like to use rundiff to help pinpoint where they diverge. Rather than try and reproduce those environments on the same machine, you can use netcat with rundiff to compare traces from gem5 instances running on separate systems across the network.

First, start rundiff running on one machine, configured to compare the trace output from a local instance of gem5 with the output of a netcat “server”. Since the network is likely to be the bottleneck, we’ll compress the trace going across netcat, which means we need to uncompress it as it arrives. For example (choosing port number 33335 arbitrarily):

util/rundiff 'gem5.opt --debug-flag=Exec <gem5 args> |' 'nc -d -l 33335 | gunzip -c |' >& tracediff.out &

Now go to the second machine, start a copy of gem5 there, and ship its compressed trace output to the netcat instance running on the first machine. For example:

gem5.opt --debug-flag=Exec <gem5 args> |& gzip -c |& nc <hostname> 33335

Internal Exec tracing implementation (InstTracer)

The “Trace-based debugging” section above talked about how to use the Exec trace flag to print information about each instruction as it completes. That functionality is actually implemented by an InstTracer object which collects information about instructions as the execute. These objects can be swapped out, and different objects can do different things with the information they collect. For instance, the IntelTrace object prints out a trace in a different format which is compatible with an external tool. The objects can also do more than just print a trace. NativeTrace objects send information about architectural state over a socket to the statetrace tool (described below) instruction by instruction to validate execution. InstTracer objects are SimObjects which are assigned to the tracer parameter of each CPU. If you want to install a different tracer, just assign it to that parameter on the CPU of interest.

When writing your own InstTracer, you’ll write at least two different classes, one which inherits from InstTracer and one that inherits from InstRecord. The InstTracer class’s main responsibility is to generate InstRecord objects which are associated with a particular instruction. By subclassing InstTracer, you’ll be able to return your own specialized version of InstRecord which is the class that really does most of the work.

The InstRecord class have a number of fields which hold information about the history of an instruction. For instance, InstRecord records the instruction’s PC, what address it used if it accessed memory, a “data” value which it produced (multiple data values aren’t handled), etc. The InstRecord function also has a pointer to a ThreadContext which can be used to read out architectural state. When an instruction is finished executing, the InstRecord’s dump() virtual function is called to process the record. For the default InstTracer, this is where the instruction’s assembly language form, etc., is printed which is the output you see when you turn on Exec. For NativeTrace, this is where architectural state is gathered up to send to statetrace.

Comparing traces with a real machine

The statetrace tool runs alongside gem5 and compares execution of a workload on a real machine with execution in gem5. In the simulator and the real system, the workload is allowed to run one instruction at a time. After each instruction, architectural state is collected and compared and any differences are reported. It can be tricky to get it set up and producing useful results (described below), but it’s an extremely valuable tool for debugging because it tends to quickly pinpoint exactly where a problem is coming from, likely saving many hours of painful debugging per bug.

Native Trace

In gem5, a NativeTrace InstTracer object (described above) needs to be installed on the CPU that will run the workload of interest. When execution starts, the tracer will wait for the state trace utility to connect to it. Then, after each instruction executes, it uses the ThreadContext pointer in the InstRecord object to gather architectural state from the currently running process. It also reads in architectural state gather by state trace through the connection they established. The two versions of state are compared, and any meaningful differences are reported. The exact makeup of the state and how it should be compared is very ISA dependent on ISA, so each ISA defines its own version of NativeTrace. These specialized classes can handle things like expected differences when registers may become undefined, or situations where execution skips ahead for one reason or another.

statetrace utility

The statetrace utility is found in the util directory and is responsible for running the workload on the real machine. It uses the ptrace mechanism provided by the Linux kernel to single step the target process and to access its state. It uses scons, but is independent of scons as used by the rest of gem5. To build a version of statetrace suitable for a particular ISA, use the build/${ARCH}/statetrace target where ${ARCH} is replaced by the ISA of interest. Currently recognized values for ${ARCH} are amd64, arm, i686, and sparc. You can override the compiler used for any ISA using the CXX scons argument, and the compiler used for a particular ISA with ${ARCH}CXX. For instance, to build an arm version of statetrace, you could run:

cd util/statetrace
scons ARMCXX=arm-softfloat-linux-gnueabi-g++ build/arm/statetrace

statetrace accepts four flags, -h to print the help, --host to specify what ip and port gem5 is listening at, -i to print out what’s on the initial stack frame, and -nt to disable tracing. -nt is typically used with -i to get information about a processes initial stack without running it. The end of the command line options is marked with two dashes. Next, put the command line you want statetrace to run.

The exact text of the program name and arguments matters because these will be passed to the process on its stack. Longer values take up more room on the stack, that displaces other items to different addresses, and statetrace clog up with lots of unimportant differences. For instance, if you need to run a program found in your home directory in a gem5 subdirectory and you run this command:

statetrace -- ~/gem5/my_benchmark arg1 arg2

You must also override arg0 in gem5 to be ~/gem5/my_benchmark.

Tuning

statetrace is a very sensitive system, and any minor difference between simulated execution and real execution could produce lots and lots of spurious differences. In order to get useful information from statetrace you’ll need to adjust the real system and gem5 so that everything lines up perfectly. I normally create a patch which has all the modifications I’ve made to gem5 for statetrace. Then I can easily remove them or reapply them for as I find and fix problems. Mercurial queues is useful for managing that patch and patches for my fixes. The following is an incomplete list of the differences you may have to correct.

Address randomization: To improve security, Linux will randomize the address space of processes, moving around their stack and heap areas. This makes it harder for an attacker to predict what memory will look like, but it also thoroughly defeats statetrace. To disable it, echo 0 into /proc/sys/kernel/randomize_va_space. You’ll almost certainly need root permissions to do that.

argv values: Be sure to use exactly the same text for each argument to your program in gem5 and on the real system. This includes arg0, the program name.

File block size: Glibc uses the block size associated with a file to decide how to buffer it. Different behavior will throw off execution and prevent statetrace from working. You can change the block size gem5 reports in the convertStatBuf and convertStat64Buf functions in src/sim/syscall_emul.hh.

Initial stack contents: Depending on your version of Linux, the contents of the initial stack may be different. You can use the -i and -nt options to print out the content of the initial stack on the real machine. statetrace attempts to interpret the initial stack so you can more easily see what’s on it. You’ll need to adjust how gem5 sets up the stack to match your real system. This code is typically in a file called process.cc in the appropriate arch directory. gem5’s code has been painstakingly constructed so that it sets up a stack as identically to Linux as possible, but the underlying mechanism would change. Also, Linux puts a collection of auxiliary vectors on the initial stack. These are type, value pairs which let the kernel provide extra information to the process as it starts. From time to time Linux introduces a new type of auxiliary vector and adds it to the stack. You may need to dig into the Linux source and emulate any new entries.

Caveats

Because statetrace is very sensitive to any changes in execution, it can’t be used with programs that don’t behave in very predictable ways. For instance, if a program reads in a random value from /dev/random and uses that in a calculation (or worse in control flow) then that program can’t be used. Less obviously, if the program relies on the system time which is unpredictable, it also can’t be used. Generally speaking, many benchmarks try to be very deterministic so that they can be used to generate reproducible data. That makes them work well with statetrace.

Statetrace can’t be used at the operating system level for at least two main reasons. First, no system is implemented or will be implemented in the foreseeable future for single stepping an operating system. Second, real operating systems are not determinstic. Interrupts from hardware devices will almost certainly come in at unpredictable times, some devices will return unpredictable data, and gem5 is much less likely to exactly match the behavior of a system at that level where firmware and other implementation details are non longer abstracted away. Second the amount of state that’s relevant at the system level is typically larger than at the user level, especially in complex ISAs like x86. Gathering, comparing, and transporting all that extra state would significantly impact performance.

Not all implementations of ptrace actually work properly. For instance when I last used statetrace with ARM, certain functions called into a region of memory set up by the kernel which had kernel specific implementations of for various operations. Ptrace relied on software breakpoints which work by replacing the next instruction in the program with one that will trap. Because the region of memory really belonged to the kernel, ptrace couldn’t modify it to install a breakpoint. The process “escaped” single stepped execution and quickly ran to completion, leaving gem5 waiting for an update that never came.

statetrace isn’t able to track changes to memory. Because memory is very large and there isn’t a convenient way to detect modifications to it, statetrace only tracks register based architectural state. If an instruction changes registers correctly but stores the wrong value to memory and/or to the wrong address, that problem may not be detected for many instructions. Fortunately, those sorts of errors are the exception.

To compare execution to a real machine, you ideally need to have a real machine at your disposal. It’s still quite possible, however, to run statetrace inside an emulator like qemu. That’s likely a little slower and compares execution against the emulator and not real hardware, but it can still help identify bugs.

ISA support

Currently SPARC, ARM, and x86 support state. ARM’s support is currently the most sophisticated, only sending differences in state across the connection which improves performance, and only printing when differences start or stop which reduces output and improves readability. Those features are planned to be ported to the other ISAs. Hopefully that code can be factored out and put into the base NativeTrace class so that all ISAs can use it easily.