linux-kernel - Re: [PATCH 07/10] rcu: Temporarily assume that nohz full CPUs might not be NOCB

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Message-ID: <20200513182527.GU2869@paulmck-ThinkPad-P72>
Date:   Wed, 13 May 2020 11:25:27 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH 07/10] rcu: Temporarily assume that nohz full CPUs might
 not be NOCB

On Wed, May 13, 2020 at 06:47:11PM +0200, Frederic Weisbecker wrote:
> So far nohz_full CPUs had to be nocb. This requirement may change
> temporarily as we are working on preparing RCU to be able to toggle the
> nocb state of a CPU. Once that is done and nohz_full can be toggled as
> well dynamically, we'll restore that initial requirement.

Would it simplify anything to make the CPU exit nohz_full first and
then exit rcu_nocb and vice versa in the other direction?  That way the
assumption about nohz_full CPUs always being rcu_nocb could remain while
still allowing runtime changes to both states.

Of course, given that setup, it would not be possible to cause a CPU to
exit rcu_nocb state if it was still in nohz_full state.

My fear is that allowing a CPU to be in nohz_full state without also
being in rcu_nocb state will cause needless confusion and bug reports.

							Thanx, Paul

> Thus for now as a temporary state, make rcu_nohz_full_cpu() aware of
> nohz_full CPUs that are not nocb so that they can handle the callbacks
> locally.
> 
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Josh Triplett <josh@...htriplett.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Lai Jiangshan <jiangshanlai@...il.com>
> Cc: Joel Fernandes <joel@...lfernandes.org>
> ---
>  kernel/rcu/tree.c        | 2 +-
>  kernel/rcu/tree.h        | 2 +-
>  kernel/rcu/tree_plugin.h | 7 ++++---
>  3 files changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index cc95419f6491..74b6798309ef 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3223,7 +3223,7 @@ static int rcu_pending(int user)
>  		return 1;
>  
>  	/* Is this a nohz_full CPU in userspace or idle?  (Ignore RCU if so.) */
> -	if ((user || rcu_is_cpu_rrupt_from_idle()) && rcu_nohz_full_cpu())
> +	if ((user || rcu_is_cpu_rrupt_from_idle()) && rcu_nohz_full_cpu(rdp))
>  		return 0;
>  
>  	/* Is the RCU core waiting for a quiescent state from this CPU? */
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index 9dc2ec021da5..4b9643d9f5e0 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -451,7 +451,7 @@ do {									\
>  #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
>  
>  static void rcu_bind_gp_kthread(void);
> -static bool rcu_nohz_full_cpu(void);
> +static bool rcu_nohz_full_cpu(struct rcu_data *rdp);
>  static void rcu_dynticks_task_enter(void);
>  static void rcu_dynticks_task_exit(void);
>  
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 43ecc047af26..f19e81e0c691 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2532,13 +2532,14 @@ static void show_rcu_nocb_state(struct rcu_data *rdp)
>   * The idea is to avoid waking up RCU core processing on such a
>   * CPU unless the grace period has extended for too long.
>   *
> - * This code relies on the fact that all NO_HZ_FULL CPUs are also
> - * CONFIG_RCU_NOCB_CPU CPUs.
> + * This code relies on the fact that NO_HZ_FULL CPUs might not
> + * be CONFIG_RCU_NOCB_CPU CPUs (temporary development state).
>   */
> -static bool rcu_nohz_full_cpu(void)
> +static bool rcu_nohz_full_cpu(struct rcu_data *rdp)
>  {
>  #ifdef CONFIG_NO_HZ_FULL
>  	if (tick_nohz_full_cpu(smp_processor_id()) &&
> +	    rcu_segcblist_is_offloaded(&rdp->cblist) &&
>  	    (!rcu_gp_in_progress() ||
>  	     ULONG_CMP_LT(jiffies, READ_ONCE(rcu_state.gp_start) + HZ)))
>  		return true;
> -- 
> 2.25.0
>