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Date:   Fri, 6 Mar 2020 17:03:42 +0100
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Alex Belits <abelits@marvell.com>
Cc:     "rostedt@goodmis.org" <rostedt@goodmis.org>,
        "mingo@kernel.org" <mingo@kernel.org>,
        "peterz@infradead.org" <peterz@infradead.org>,
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        Prasun Kapoor <pkapoor@marvell.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "linux-api@vger.kernel.org" <linux-api@vger.kernel.org>,
        "linux-mm@vger.kernel.org" <linux-mm@vger.kernel.org>,
        "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>
Subject: Re: [PATCH 08/12] task_isolation: don't interrupt CPUs with
 tick_nohz_full_kick_cpu()
Message-ID: <20200306160341.GE8590@lenoir>
References: <4473787e1b6bc3cc226067e8d122092a678b63de.camel@marvell.com>
 <d7ce01e57d4a35b126e1cb96a63109eaa9781cb6.camel@marvell.com>
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On Wed, Mar 04, 2020 at 04:12:40PM +0000, Alex Belits wrote:
> From: Yuri Norov <ynorov@marvell.com>
> 
> 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@marvell.com>
> ---
>  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.