linux-kernel - Re: [PATCH v2] livepatch: Fix idle cpu's tasks transition

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Date:   Mon, 20 Sep 2021 10:27:57 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Vasily Gorbik <gor@...ux.ibm.com>, Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Sumanth Korikkar <sumanthk@...ux.ibm.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] livepatch: Fix idle cpu's tasks transition

On Wed 2021-09-15 16:18:01, 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. 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@...e.com>
> Acked-by: Miroslav Benes <mbenes@...e.cz>
> Signed-off-by: Vasily Gorbik <gor@...ux.ibm.com>
> ---
> Ingo/Peter, as Josh mentioned, could you please ack if you are ok with
> livepatch calling this private scheduler interface?

Ingo, Peter, Josh, could anyone please ack that it is acceptable
to call resched_curr(rq) from the livepatch code?

Or is there a better way to make an idle task go through
the main cycle?

Best Regards,
Petr

> 
> Changes since v1:
> - added comments suggested by Petr
>   lkml.kernel.org/r/patch.git-a4aad6b1540d.your-ad-here.call-01631177886-ext-3083@...k.hours
> 
> Previous discussion and RFC PATCH:
>   lkml.kernel.org/r/patch.git-b76842ceb035.your-ad-here.call-01625661932-ext-1304@...k.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