memtrace/pt: make pid/tid frontend state per-core

[iconstan01]

• Fixes multicore frontend state handling where pid/tid context was effectively shared across cores.
• Makes pid/tid handling per-core in the memtrace/PT frontend path.
• Also handles empty initial trace read safely during frontend initialization
• Validated on multicore workloads where the issue previously reproduced.

[iconstan01]

I rebased this branch on the latest upstream main and pushed because the change is straightforward and isolated to this PR’s scope.
After rebasing, I ran multicore memtrace and observed an unrelated assert:

No forward progress for 1000000 cycles
src/decoupled_frontend.cc:359: ASSERT FAILED ...

[yql5510718]

Nice catch. Thank you so much.

[Copilot]

Pull request overview

Makes pid/tid tracking state per-core in the PT and memtrace frontends to fix multicore context mixing, and adjusts frontend initialization behavior.

Changes:

• Convert prior pid/tid tracking from global scalars to per-proc arrays for PT and memtrace frontends.
• Update PT setup/read paths to use per-proc pid/tid state.
• Change initial frontend trace read to assert successful read during initialization.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/frontend/pt_memtrace/trace_fe.cc	Changes initialization to assert initial per-core trace_read() succeeds.
src/frontend/pt_memtrace/pt_fe.cc	Makes PT prior pid/tid state per-proc and updates read/setup logic accordingly.
src/frontend/pt_memtrace/memtrace_fe.cc	Makes memtrace prior pid/tid state per-proc and updates read logic accordingly.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

This ASSERT will crash when the initial trace read legitimately returns 0 (end-of-trace / empty trace), which conflicts with the PR description stating empty initial trace reads should be handled safely. Instead of asserting, handle a 0 return explicitly (e.g., mark the core as having no trace / set an appropriate sentinel state / skip initialization for that core) and continue initialization without aborting.

Suggested change

	ASSERT(proc_id, trace_read(proc_id, &next_onpath_pi[proc_id]));
	auto trace_count = trace_read(proc_id, &next_onpath_pi[proc_id]);
	if (trace_count == 0) {
	// Legitimate end-of-trace or empty trace for this core; skip initialization.
	DEBUG(proc_id, "No initial trace data for core %u", proc_id);
	continue;
	}
	ASSERT(proc_id, trace_count > 0);

[Copilot]

These arrays are already value-initialized to zero at declaration (= {0}), so the additional memset is redundant. Consider removing these lines (or, if re-initialization at runtime is required, add a brief comment explaining why pt_init() needs to forcibly reset them).

Suggested change

	memset(pt_prior_tid, 0, sizeof(pt_prior_tid));
	memset(pt_prior_pid, 0, sizeof(pt_prior_pid));