netdev - Re: [PATCH v4 10/13] task_isolation: don't interrupt CPUs with tick_nohz_full_kick

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Message-ID: <20201001144454.GB6595@lothringen>
Date:   Thu, 1 Oct 2020 16:44:54 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Alex Belits <abelits@...vell.com>
Cc:     "rostedt@...dmis.org" <rostedt@...dmis.org>,
        Prasun Kapoor <pkapoor@...vell.com>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "will@...nel.org" <will@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH v4 10/13] task_isolation: don't interrupt CPUs with
 tick_nohz_full_kick_cpu()

On Wed, Jul 22, 2020 at 02:57:33PM +0000, Alex Belits wrote:
> From: Yuri Norov <ynorov@...vell.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@...vell.com>
> [abelits@...vell.com: updated, only exclude CPUs running isolated tasks]
> Signed-off-by: Alex Belits <abelits@...vell.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
>