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Date:   Fri, 27 Aug 2021 14:54:39 +0200
From:   Petr Mladek <pmladek@suse.com>
To:     Vasily Gorbik <gor@linux.ibm.com>
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Jiri Kosina <jikos@kernel.org>,
        Miroslav Benes <mbenes@suse.cz>,
        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: Re: [RFC PATCH] livepatch: Kick idle cpu's tasks to perform
 transition
Message-ID: <YSjgj+ZzOutFxevl@alley>
References: <patch.git-b76842ceb035.your-ad-here.call-01625661932-ext-1304@work.hours>
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On Wed 2021-07-07 14:49:38, Vasily Gorbik wrote:
> 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.

I see.

> 
> 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);

Is this really needed?

> +				kick_process(task);

This would probably do the job. Well, I wonder if the following is
a bit cleaner.

		wake_up_if_idle(cpu);


Also, please do this in klp_send_signals(). We kick there all other
tasks that block the transition for too long.

Best Regards,
Petr