On Wed, Jul 18, 2018 at 02:03:18PM +0200, Peter Zijlstra wrote:

On Thu, Jul 12, 2018 at 01:29:40PM -0400, Johannes Weiner wrote:

quoted

+	/* Update task counts according to the set/clear bitmasks */
+	for (to = 0; (bo = ffs(clear)); to += bo, clear >>= bo) {
+		int idx = to + (bo - 1);
+
+		if (tasks[idx] == 0 && !psi_bug) {
+			printk_deferred(KERN_ERR "psi: task underflow! cpu=%d idx=%d tasks=[%u %u %u] clear=%x set=%x\n",
+					cpu, idx, tasks[0], tasks[1], tasks[2],
+					clear, set);
+			psi_bug = 1;
+		}

		WARN_ONCE(!tasks[idx], ...);

It's just open-coded because of the printk_deferred, since this is
inside the scheduler.

It actually used to be a straight-up WARN_ONCE() in older
versions. Recursive scheduling bugs are no fun to debug ;)