Re: [PATCH 08/12] task_isolation: don't interrupt CPUs with tick_nohz_full_kick_cpu()

[Frederic Weisbecker]

On Wed, Mar 04, 2020 at 04:12:40PM +0000, Alex Belits wrote:
> From: Yuri Norov <ynorov@xxxxxxxxxxx>
> 
> For nohz_full CPUs the desirable behavior is to receive interrupts
> generated by tick_nohz_full_kick_cpu(). But for hard isolation it's
> obviously not desirable because it breaks isolation.
> 
> This patch adds check for it.
> 
> Signed-off-by: Alex Belits <abelits@xxxxxxxxxxx>
> ---
>  kernel/time/tick-sched.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 1d4dec9d3ee7..fe4503ba1316 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -20,6 +20,7 @@
>  #include <linux/sched/clock.h>
>  #include <linux/sched/stat.h>
>  #include <linux/sched/nohz.h>
> +#include <linux/isolation.h>
>  #include <linux/module.h>
>  #include <linux/irq_work.h>
>  #include <linux/posix-timers.h>
> @@ -262,7 +263,7 @@ static void tick_nohz_full_kick(void)
>   */
>  void tick_nohz_full_kick_cpu(int cpu)
>  {
> -	if (!tick_nohz_full_cpu(cpu))
> +	if (!tick_nohz_full_cpu(cpu) || task_isolation_on_cpu(cpu))
>  		return;

I fear you can't do that. A nohz full CPU is kicked for a reason.
As for the other cases, you need to fix the callers.

In the general case, randomly ignoring an interrupt is a correctness
issue.

Thanks.