trace2:gvfs:experiment: read_cache: annotate thread usage in read-cache

Add trace2_thread_start() and trace2_thread_exit() events to the worker
threads used to read the index.  This gives per-thread perf data.

These workers were introduced in:
abb4bb8384 read-cache: load cache extensions on a worker thread
77ff1127a4 read-cache: load cache entries on worker threads

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>

read-cache.c

@@ -2121,6 +2121,17 @@ static void *load_index_extensions(void *_data)
 	return NULL;
 }
 
+static void *load_index_extensions_threadproc(void *_data)
+{
+	void *result;
+
+	trace2_thread_start("load_index_extensions");
+	result = load_index_extensions(_data);
+	trace2_thread_exit();
+
+	return result;
+}
+
 /*
  * A helper function that will load the specified range of cache entries
  * from the memory mapped file and add them to the given index.
@@ -2197,12 +2208,17 @@ static void *load_cache_entries_thread(void *_data)
 	struct load_cache_entries_thread_data *p = _data;
 	int i;
 
+	trace2_thread_start("load_cache_entries");
+
 	/* iterate across all ieot blocks assigned to this thread */
 	for (i = p->ieot_start; i < p->ieot_start + p->ieot_blocks; i++) {
 		p->consumed += load_cache_entry_block(p->istate, p->ce_mem_pool,
 			p->offset, p->ieot->entries[i].nr, p->mmap, p->ieot->entries[i].offset, NULL);
 		p->offset += p->ieot->entries[i].nr;
 	}
+
+	trace2_thread_exit();
+
 	return NULL;
 }
 
@@ -2373,7 +2389,7 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 			int err;
 
 			p.src_offset = extension_offset;
-			err = pthread_create(&p.pthread, NULL, load_index_extensions, &p);
+			err = pthread_create(&p.pthread, NULL, load_index_extensions_threadproc, &p);
 			if (err)
 				die(_("unable to create load_index_extensions thread: %s"), strerror(err));