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Date:   Tue, 21 Sep 2021 19:43:16 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Vasily Gorbik <gor@linux.ibm.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jiri Kosina <jikos@kernel.org>,
        Miroslav Benes <mbenes@suse.cz>,
        Petr Mladek <pmladek@suse.com>,
        Joe Lawrence <joe.lawrence@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        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: [PATCH v2] livepatch: Fix idle cpu's tasks transition
Message-ID: <YUoZtF1qPyTx07pU@hirez.programming.kicks-ass.net>
References: <patch.git-94c1daf66a9c.your-ad-here.call-01631714463-ext-3692@work.hours>
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On Wed, Sep 15, 2021 at 04:18:01PM +0200, Vasily Gorbik wrote:

> 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...