Add BPFStackTable::free_symcache() to free the symbol cache for an PID

[yzhao1012]

This is useful when BPFStackTable is used by a long running program to
limit the memory usage, by removing the cached symbols of an exited
process.

[yonghong-song]

[buildbot, test this please]

[yonghong-song]

@yzhao1012 The change makes sense for long running processes. Once a process exited, you may want to remove it from the symbol cache for reducing memory or avoid reusing the stale pid in the future.

The patch missed a declaration so it has compilation errors.

--- a/src/cc/api/BPFTable.h
+++ b/src/cc/api/BPFTable.h
@@ -381,6 +381,7 @@ class BPFStackTable : public BPFTableBase<int, stacktrace_t> {
   void clear_table_non_atomic();
   std::vector<uintptr_t> get_stack_addr(int stack_id);
   std::vector<std::string> get_stack_symbol(int stack_id, int pid);
+  void free_symcache(int pid);
 
  private:

Please fix and resubmit.

[yzhao1012]

@yzhao1012 The change makes sense for long running processes. Once a process exited, you may want to remove it from the symbol cache for reducing memory or avoid reusing the stale pid in the future.

The patch missed a declaration so it has compilation errors.

--- a/src/cc/api/BPFTable.h
+++ b/src/cc/api/BPFTable.h
@@ -381,6 +381,7 @@ class BPFStackTable : public BPFTableBase<int, stacktrace_t> {
   void clear_table_non_atomic();
   std::vector<uintptr_t> get_stack_addr(int stack_id);
   std::vector<std::string> get_stack_symbol(int stack_id, int pid);
+  void free_symcache(int pid);
 
  private:

Please fix and resubmit.

Moved the declaration to BPFStackTable. I mistakenly put it into another class.

[yonghong-song]

[buildbot, test this please]