[trace][intelpt] Support system-wide tracing [8] - Improve the single buffer perf_event configuration

[wallace]

Summary

We were setting some events to be written in the data buffer of the
perf_event, but we don't need that.

Besides that, we don't need the data buffer to be larger than 1, so we
can reduce its size.

[jj10306]

lldb/source/Plugins/Process/Linux/IntelPTSingleBufferTrace.cpp

186

won't we need this in order to get timestamps in the context switching events? I agree we don't need it for the time being so maybe in the diff where you add context switch collection you will reintroduce it 🙂

316

In the future if we need to collect itrace we will need a small data buffer.
Do you plan to collect context switch info as part of the same perf event used for intel pt or will you open a separate event?

[wallace]

lldb/source/Plugins/Process/Linux/IntelPTSingleBufferTrace.cpp

186

in a later diff I define separate settings for context switches, so this one should be fine

316

context switches are done separately
besides, that let's see in the future if itrace is useful

[wallace]

This was fixed in https://reviews.llvm.org/D127456

lldb/source/Plugins/Process/Linux/IntelPTSingleBufferTrace.cpp

 ///   or an \a llvm::Error otherwise.
 static Expected<perf_event_attr>
 CreateIntelPTPerfEventConfiguration(bool enable_tsc,
                                     llvm::Optional<uint64_t> psb_period) {
   perf_event_attr attr;
   memset(&attr, 0, sizeof(attr));
   attr.size = sizeof(attr);
   attr.exclude_kernel = 1;
-  attr.sample_type = PERF_SAMPLE_TIME;
-  attr.sample_id_all = 1;
   attr.exclude_hv = 1;
   attr.exclude_idle = 1;
-  attr.mmap = 1;
 
   if (Expected<uint64_t> config_value =
           GeneratePerfEventConfigValue(enable_tsc, psb_period))
     attr.config = *config_value;
   else
     return config_value.takeError();
 
   if (Expected<uint32_t> intel_pt_type = GetIntelPTOSEventType())
if (__builtin_popcount(request.trace_buffer_size) != 1 ||
       request.trace_buffer_size < 4096) {
     return createStringError(
         inconvertibleErrorCode(),
         "The trace buffer size must be a power of 2 greater than or equal to "
         "4096 (2^12) bytes. It was %" PRIu64 ".",
         request.trace_buffer_size);
   }
   uint64_t page_size = getpagesize();
-  uint64_t buffer_numpages = static_cast<uint64_t>(llvm::PowerOf2Floor(
+  uint64_t aux_buffer_numpages = static_cast<uint64_t>(llvm::PowerOf2Floor(
       (request.trace_buffer_size + page_size - 1) / page_size));
 
   Expected<perf_event_attr> attr = CreateIntelPTPerfEventConfiguration(
       request.enable_tsc, request.psb_period.map([](int value) {
         return static_cast<uint64_t>(value);
       }));
   if (!attr)
     return attr.takeError();
   attr->disabled = initial_state == TraceCollectionState::Paused;
 
   LLDB_LOG(log, "Will create trace buffer of size {0}",
            request.trace_buffer_size);
 
   if (Expected<PerfEvent> perf_event = PerfEvent::Init(*attr, tid, core_id)) {
-    if (Error mmap_err = perf_event->MmapMetadataAndBuffers(buffer_numpages,
-                                                            buffer_numpages)) {
+    if (Error mmap_err = perf_event->MmapMetadataAndBuffers(
+            /*num_data_pages=*/0, aux_buffer_numpages)) {
       return std::move(mmap_err);
     }
     IntelPTSingleBufferTraceUP trace_up(
         new IntelPTSingleBufferTrace(std::move(*perf_event), initial_state));
     return trace_up;
   } else {
     return perf_event.takeError();
   }
 #endif
 }