The per-PSB packet decoding logic was wrong because it was assuming that pt_insn_get_sync_offset was being udpated after every PSB. Silly me, that is not true. It returns the offset of the PSB packet after invoking pt_insn_sync_forward regardless of how many PSBs are visited later. Instead, I'm now following the approach described in https://github.com/intel/libipt/blob/master/doc/howto_libipt.md#parallel-decode for parallel decoding, which is basically what we need.
A nasty error that happened because of this is that when we had two PSBs (A and B), the following was happening
1. PSB A was processed all the way up to the end of the trace, which includes PSB B.
2. PSB B was then processed until the end of the trace.
The instructions emitted by step 2. were also emitted as part of step 1. so our trace had duplicated chunks. This problem becomes worse when you many PSBs.
As part of making sure this diff is correct, I added some other features that are very useful.
- Added a "synchronization point" event to the TraceCursor, so we can inspect when PSBs are emitted.
- Removed the single-thread decoder. Now the per-cpu decoder and single-thread decoder use the same code paths.
- Use the query decoder to fetch PSBs and timestamps. It turns out that the pt_insn_sync_forward of the instruction decoder can move past several PSBs (this means that we could skip some TSCs). On the other hand, the pt_query_sync_forward method doesn't skip PSBs, so we can get more accurate sync events and timing information.
- Turned LibiptDecoder into PSBBlockDecoder, which decodes single PSB blocks. It is the fundamental processing unit for decoding.
- Added many comments, asserts and improved error handling for clarity.
- Improved DecodeSystemWideTraceForThread so that a TSC is emitted always before a cpu change event. This was a bug that was annoying me before.
- SplitTraceInContinuousExecutions and FindLowestTSCInTrace are now using the query decoder, which can identify precisely each PSB along with their TSCs.
- Added an "only-events" option to the trace dumper to inspect only events.
I did extensive testing and I think we should have an in-house testing CI. The LLVM buildbots are not capable of supporting testing post-mortem traces of hundreds of megabytes. I'll leave that for later, but at least for now the current tests were able to catch most of the issues I encountered when doing this task.
A sample output of a program that I was single stepping is the following. You can see that only one PSB is emitted even though stepping happened!
```
thread #1: tid = 3578223
0: (event) trace synchronization point [offset = 0x0xef0]
a.out`main + 20 at main.cpp:29:20
1: 0x0000000000402479 leaq -0x1210(%rbp), %rax
2: (event) software disabled tracing
3: 0x0000000000402480 movq %rax, %rdi
4: (event) software disabled tracing
5: (event) software disabled tracing
6: 0x0000000000402483 callq 0x403bd4 ; std::vector<int, std::allocator<int>>::vector at stl_vector.h:391:7
7: (event) software disabled tracing
a.out`std::vector<int, std::allocator<int>>::vector() at stl_vector.h:391:7
8: 0x0000000000403bd4 pushq %rbp
9: (event) software disabled tracing
10: 0x0000000000403bd5 movq %rsp, %rbp
11: (event) software disabled tracing
```
This is another trace of a long program with a few PSBs.
```
(lldb) thread trace dump instructions -E -f thread #1: tid = 3603082
0: (event) trace synchronization point [offset = 0x0x80]
47417: (event) software disabled tracing
129231: (event) trace synchronization point [offset = 0x0x800]
146747: (event) software disabled tracing
246076: (event) software disabled tracing
259068: (event) trace synchronization point [offset = 0x0xf78]
259276: (event) software disabled tracing
259278: (event) software disabled tracing
no more data
```
Differential Revision: https://reviews.llvm.org/D131630
Add bindings for the `TraceCursor` to allow for programatic traversal of
traces.
This diff adds bindings for all public `TraceCursor` methods except
`GetHwClock` and also adds `SBTrace::CreateNewCursor`. A new unittest
has been added to TestTraceLoad.py that uses the new `SBTraceCursor` API
to test that the sequential and random access APIs of the `TraceCursor`
are equivalent.
This diff depends on D130925.
Test Plan:
`ninja lldb-dotest && ./bin/lldb-dotest -p TestTraceLoad`
Differential Revision: https://reviews.llvm.org/D130930
The use of `std::unique_ptr` with `TraceCursor` adds unnecessary complexity to adding `SBTraceCursor` bindings
Specifically, since `TraceCursor` is an abstract class there's no clean
way to provide "deep clone" semantics for `TraceCursorUP` short of
creating a pure virtual `clone()` method (afaict).
After discussing with @wallace, we decided there is no strong reason to
favor wrapping `TraceCursor` with `std::unique_ptr` over `std::shared_ptr`, thus this diff
replaces all usages of `std::unique_ptr<TraceCursor>` with `std::shared_ptr<TraceCursor>`.
This sets the stage for future diffs to introduce `SBTraceCursor`
bindings in a more clean fashion.
Test Plan:
Differential Revision: https://reviews.llvm.org/D130925
Thanks to ymeng@fb.com for coming up with this change.
`thread trace dump info` can dump some metrics that can be useful for
analyzing the performance and quality of a trace. This diff adds a --json
option for dumping this information in json format that can be easily
understood my machines.
Differential Revision: https://reviews.llvm.org/D129332
Thanks to fredzhou@fb.com for coming up with this feature.
When tracing in per-cpu mode, we have information of in which cpu we are execution each instruction, which comes from the context switch trace. This diff makes this information available as a `cpu changed event`, which an additional accessor in the cursor `GetCPU()`. As cpu changes are very infrequent, any consumer should listen to cpu change events instead of querying the actual cpu of a trace item. Once a cpu change event is seen, the consumer can invoke GetCPU() to get that information. Also, it's possible to invoke GetCPU() on an arbitrary instruction item, which will return the last cpu seen. However, this call is O(logn) and should be used sparingly.
Manually tested with a sample program that starts on cpu 52, then goes to 18, and then goes back to 52.
Differential Revision: https://reviews.llvm.org/D129340
In rare situations, disassemblying would fail that produce an invalid
InstructionSP object. We need to check that it's valid before using.
With this change, now the dumper doesn't crash with dumping instructions of
ioctl. In fact, it now dumps this output
{
"id": 6135,
"loadAddress": "0x7f4bfe5c7515",
"module": "libc.so.6",
"symbol": "ioctl",
"source": "glibc/2.34/src/glibc-2.34/sysdeps/unix/syscall-template.S",
"line": 120,
"column": 0
}
Anyway, we need to investigate why the diassembler failed disassembling that
instruction. From over 2B instructions I was disassembling today, just this
one failed, so this could be a bug in LLVM's core disassembler.
Differential Revision: https://reviews.llvm.org/D129588
We want to include events with metadata, like context switches, and this
requires the API to handle events with payloads (e.g. information about
such context switches). Besides this, we want to support multiple
similar events between two consecutive instructions, like multiple
context switches. However, the current implementation is not good for this because
we are defining events as bitmask enums associated with specific
instructions. Thus, we need to decouple instructions from events and
make events actual items in the trace, just like instructions and
errors.
- Add accessors in the TraceCursor to know if an item is an event or not
- Modify from the TraceDumper all the way to DecodedThread to support
- Renamed the paused event to disabled.
- Improved the tsc handling logic. I was using an API for getting the tsc from libipt, but that was an overkill that should be used when not processing events manually, but as we are already processing events, we can more easily get the tscs.
event items. Fortunately this simplified many things
- As part of this refactor, I also fixed and long stating issue, which is that some non decoding errors were being inserted in the decoded thread. I changed this so that TraceIntelPT::Decode returns an error if the decoder couldn't be set up proplerly. Then, errors within a trace are actual anomalies found in between instrutions.
All test pass
Differential Revision: https://reviews.llvm.org/D128576
Walter Erquinigo