linux-kernel - Re: [PATCH RFC tip/core/rcu 14/14] rcu/nohz: Make multi_cpu

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Date:   Mon, 12 Aug 2019 23:02:33 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH RFC tip/core/rcu 14/14] rcu/nohz: Make multi_cpu_stop()
 enable tick on all online CPUs

On Fri, Aug 02, 2019 at 08:15:01AM -0700, Paul E. McKenney wrote:
> The multi_cpu_stop() function relies on the scheduler to gain control from
> whatever is running on the various online CPUs, including any nohz_full
> CPUs running long loops in kernel-mode code.  Lack of the scheduler-clock
> interrupt on such CPUs can delay multi_cpu_stop() for several minutes
> and can also result in RCU CPU stall warnings.  This commit therefore
> causes multi_cpu_stop() to enable the scheduler-clock interrupt on all
> online CPUs.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
> ---
>  kernel/stop_machine.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index b4f83f7bdf86..a2659f61ed92 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -20,6 +20,7 @@
>  #include <linux/smpboot.h>
>  #include <linux/atomic.h>
>  #include <linux/nmi.h>
> +#include <linux/tick.h>
>  #include <linux/sched/wake_q.h>
>  
>  /*
> @@ -187,15 +188,19 @@ static int multi_cpu_stop(void *data)
>  {
>  	struct multi_stop_data *msdata = data;
>  	enum multi_stop_state curstate = MULTI_STOP_NONE;
> -	int cpu = smp_processor_id(), err = 0;
> +	int cpu, err = 0;
>  	const struct cpumask *cpumask;
>  	unsigned long flags;
>  	bool is_active;
>  
> +	for_each_online_cpu(cpu)
> +		tick_nohz_dep_set_cpu(cpu, TICK_DEP_MASK_RCU);

Looks like it's not the right fix but, should you ever need to set an
all-CPUs (system wide) tick dependency in the future, you can use tick_set_dep().

Thanks.