[PATCH v2] livepatch: Fix idle cpu's tasks transition

[Vasily Gorbik]

On an idle system with large amount of cpus it might happen that
klp_update_patch_state() is not reached in do_idle() for a long periods
of time. With debug messages enabled log is filled with:
[  499.442643] livepatch: klp_try_switch_task: swapper/63:0 is running

without any signs of progress. Ending up with "failed to complete
transition".

On s390 LPAR with 128 cpus not a single transition is able to complete
and livepatch kselftests fail. Tests on idling x86 kvm instance with 128
cpus demonstrate similar symptoms with and without CONFIG_NO_HZ.

To deal with that, since runqueue is already locked in
klp_try_switch_task() identify idling cpus and trigger rescheduling
potentially waking them up and making sure idle tasks break out of
do_idle() inner loop and reach klp_update_patch_state(). This helps to
speed up transition time while avoiding unnecessary extra system load.

Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
Acked-by: Miroslav Benes <mbenes@xxxxxxx>
Signed-off-by: Vasily Gorbik <gor@xxxxxxxxxxxxx>
---
Ingo/Peter, as Josh mentioned, could you please ack if you are ok with
livepatch calling this private scheduler interface?

Changes since v1:
- added comments suggested by Petr
  lkml.kernel.org/r/patch.git-a4aad6b1540d.your-ad-here.call-01631177886-ext-3083@work.hours

Previous discussion and RFC PATCH:
  lkml.kernel.org/r/patch.git-b76842ceb035.your-ad-here.call-01625661932-ext-1304@work.hours

 kernel/livepatch/transition.c | 8 ++++++++
 1 file changed, 8 insertions(+)

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);
-- 
2.25.4