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@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?
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@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
