perf/core: Try to allocate task_ctx_data quickly

The attach_global_ctx_data() has O(N^2) algorithm to allocate the
context data for each thread.  This caused perfomance problems on large
systems with O(100k) threads.

Because kmalloc(GFP_KERNEL) can go sleep it cannot be called under the
RCU lock.  So let's try with GFP_NOWAIT first so that it can proceed in
normal cases.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260211223222.3119790-3-namhyung@kernel.org

kernel/events/core.c

@@ -5489,6 +5489,12 @@ again:
 					cd = NULL;
 			}
 			if (!cd) {
+				/*
+				 * Try to allocate context quickly before
+				 * traversing the whole thread list again.
+				 */
+				if (!attach_task_ctx_data(p, ctx_cache, true, GFP_NOWAIT))
+					continue;
 				get_task_struct(p);
 				goto alloc;
 			}