On Tue, Sep 21, 2021 at 07:43:16PM +0200, Peter Zijlstra wrote:

On Wed, Sep 15, 2021 at 04:18:01PM +0200, Vasily Gorbik wrote:

quoted

diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
index 291b857a6e20..2846a879f2dc 100644
--- a/kernel/livepatch/transition.c
+++ b/kernel/livepatch/transition.c

@@ -278,6 +278,8 @@ static int klp_check_stack(struct task_struct *task, char *err_buf)
  * Try to safely switch a task to the target patch state.  If it's currently
  * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or
  * if the stack is unreliable, return false.
+ *
+ * Idle tasks are switched in the main loop when running.
  */
 static bool klp_try_switch_task(struct task_struct *task)
 {

@@ -308,6 +310,12 @@ static bool klp_try_switch_task(struct task_struct *task)
 	rq = task_rq_lock(task, &flags);
 
 	if (task_running(rq, task) && task != current) {
+		/*
+		 * Idle task might stay running for a long time. Switch them
+		 * in the main loop.
+		 */
+		if (is_idle_task(task))
+			resched_curr(rq);
 		snprintf(err_buf, STACK_ERR_BUF_SIZE,
 			 "%s: %s:%d is running\n", __func__, task->comm,
 			 task->pid);

So 'recently' we grew try_invoke_on_locked_down_task() (yes, that's a
crap name), and I'm thinking this code is very similar to that. So
perhaps we re-use that.

Perhaps have func() > 0 imply resched.

I'll have a play...

https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/log/?h=sched/livepatch

I'll write proper changelogs and post tomorrow,... (and fix robot fail,
if any).

(also, notes to later self:

 - task_try_func() might need to re-check on_rq after acquiring rq_lock
 - klp_send_signal() might want wake_up_process()/TASK_NORMAL due to TASK_IDLE
)