linux-kernel - Re: [RFC 1/2] rcu: Do prepare and cleanup idle depending on in

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Date:   Wed, 20 Jun 2018 09:33:09 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     jiangshanlai@...il.com, paulmck@...ux.vnet.ibm.com,
        josh@...htriplett.org, mathieu.desnoyers@...icios.com,
        linux-kernel@...r.kernel.org, kernel-team@....com,
        joel@...lfernandes.org
Subject: Re: [RFC 1/2] rcu: Do prepare and cleanup idle depending on
 in_nmi()

On Wed, 20 Jun 2018 17:47:19 +0900
Byungchul Park <byungchul.park@....com> wrote:

> Get rid of dependency on ->dynticks_nmi_nesting.

This is not a trivial change. Can you please explain the rational and
background in the change log. Add enough context here to know why this
change is needed.

Thanks!

-- Steve

> 
> Signed-off-by: Byungchul Park <byungchul.park@....com>
> ---
>  kernel/rcu/tree.c | 22 ++++++++++------------
>  1 file changed, 10 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index deb2508..59ae94e 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -797,6 +797,11 @@ void rcu_nmi_exit(void)
>  		return;
>  	}
>  
> +	if (!in_nmi()) {
> +		rcu_prepare_for_idle();
> +		rcu_dynticks_task_enter();
> +	}
> +
>  	/* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
>  	trace_rcu_dyntick(TPS("Startirq"), rdtp->dynticks_nmi_nesting, 0, rdtp->dynticks);
>  	WRITE_ONCE(rdtp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
> @@ -824,14 +829,8 @@ void rcu_nmi_exit(void)
>   */
>  void rcu_irq_exit(void)
>  {
> -	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> -
>  	lockdep_assert_irqs_disabled();
> -	if (rdtp->dynticks_nmi_nesting == 1)
> -		rcu_prepare_for_idle();
>  	rcu_nmi_exit();
> -	if (rdtp->dynticks_nmi_nesting == 0)
> -		rcu_dynticks_task_enter();
>  }
>  
>  /*
> @@ -944,6 +943,11 @@ void rcu_nmi_enter(void)
>  	if (rcu_dynticks_curr_cpu_in_eqs()) {
>  		rcu_dynticks_eqs_exit();
>  		incby = 1;
> +
> +		if (!in_nmi()) {
> +			rcu_dynticks_task_exit();
> +			rcu_cleanup_after_idle();
> +		}
>  	}
>  	trace_rcu_dyntick(incby == 1 ? TPS("Endirq") : TPS("++="),
>  			  rdtp->dynticks_nmi_nesting,
> @@ -977,14 +981,8 @@ void rcu_nmi_enter(void)
>   */
>  void rcu_irq_enter(void)
>  {
> -	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> -
>  	lockdep_assert_irqs_disabled();
> -	if (rdtp->dynticks_nmi_nesting == 0)
> -		rcu_dynticks_task_exit();
>  	rcu_nmi_enter();
> -	if (rdtp->dynticks_nmi_nesting == 1)
> -		rcu_cleanup_after_idle();
>  }
>  
>  /*