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Date:   Thu, 1 Oct 2020 16:44:54 +0200
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Alex Belits <abelits@marvell.com>
Cc:     "rostedt@goodmis.org" <rostedt@goodmis.org>,
        Prasun Kapoor <pkapoor@marvell.com>,
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        "peterz@infradead.org" <peterz@infradead.org>,
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        "will@kernel.org" <will@kernel.org>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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Subject: Re: [PATCH v4 10/13] task_isolation: don't interrupt CPUs with
 tick_nohz_full_kick_cpu()
Message-ID: <20201001144454.GB6595@lothringen>
References: <04be044c1bcd76b7438b7563edc35383417f12c8.camel@marvell.com>
 <5acf7502c071c0d1365ba5e5940e003a7da6521f.camel@marvell.com>
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On Wed, Jul 22, 2020 at 02:57:33PM +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: Yuri Norov <ynorov@marvell.com>
> [abelits@marvell.com: updated, only exclude CPUs running isolated tasks]
> Signed-off-by: Alex Belits <abelits@marvell.com>
> ---
>  kernel/time/tick-sched.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 6e4cd8459f05..2f82a6daf8fc 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>
> @@ -268,7 +269,8 @@ static void tick_nohz_full_kick(void)
>   */
>  void tick_nohz_full_kick_cpu(int cpu)
>  {
> -	if (!tick_nohz_full_cpu(cpu))
> +	smp_rmb();

What is it ordering?

> +	if (!tick_nohz_full_cpu(cpu) || task_isolation_on_cpu(cpu))
>  		return;

You can't simply ignore an IPI. There is always a reason for a nohz_full CPU
to be kicked. Something triggered a tick dependency. It can be posix cpu timers
for example, or anything.


>  
>  	irq_work_queue_on(&per_cpu(nohz_full_kick_work, cpu), cpu);
> -- 
> 2.26.2
>