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Date:   Wed, 7 Jul 2021 14:49:38 +0200
From:   Vasily Gorbik <gor@linux.ibm.com>
To:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Jiri Kosina <jikos@kernel.org>,
        Miroslav Benes <mbenes@suse.cz>, Petr Mladek <pmladek@suse.com>
Cc:     Joe Lawrence <joe.lawrence@redhat.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        Sumanth Korikkar <sumanthk@linux.ibm.com>,
        live-patching@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [RFC PATCH] livepatch: Kick idle cpu's tasks to perform transition
Message-ID: <patch.git-b76842ceb035.your-ad-here.call-01625661932-ext-1304@work.hours>
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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.

To deal with that, make sure we break out of do_idle() inner loop to
reach klp_update_patch_state() by marking idle tasks as NEED_RESCHED
as well as kick cpus out of idle state.

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
---
 kernel/livepatch/transition.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
index 3a4beb9395c4..793eba46e970 100644
--- a/kernel/livepatch/transition.c
+++ b/kernel/livepatch/transition.c
@@ -415,8 +415,11 @@ void klp_try_complete_transition(void)
 	for_each_possible_cpu(cpu) {
 		task = idle_task(cpu);
 		if (cpu_online(cpu)) {
-			if (!klp_try_switch_task(task))
+			if (!klp_try_switch_task(task)) {
 				complete = false;
+				set_tsk_need_resched(task);
+				kick_process(task);
+			}
 		} else if (task->patch_state != klp_target_state) {
 			/* offline idle tasks can be switched immediately */
 			clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
-- 
2.25.4