linux-kernel - Re: [PATCH 09/10] rcu: Allow to re-offload a CPU that used to be nocb

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Message-ID: <20200513184117.GW2869@paulmck-ThinkPad-P72>
Date:   Wed, 13 May 2020 11:41:18 -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 09/10] rcu: Allow to re-offload a CPU that used to be nocb

On Wed, May 13, 2020 at 06:47:13PM +0200, Frederic Weisbecker wrote:
> This is essentially the reverse operation of de-offloading. For now it's
> only supported on CPUs that used to be offloaded and therefore still have
> the relevant nocb_cb/nocb_gp kthreads around.

And I still believe that this is a good restriction to put in place,
especially to start with.  ;-)

> Inspired-by: Paul E. McKenney <paulmck@...nel.org>
> 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>
> ---
>  include/linux/rcupdate.h |  2 ++
>  kernel/rcu/tree_plugin.h | 44 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 46 insertions(+)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 1d3a4c37c3c1..ee95e49d675f 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -96,9 +96,11 @@ static inline void rcu_user_exit(void) { }
>  
>  #ifdef CONFIG_RCU_NOCB_CPU
>  void rcu_init_nohz(void);
> +void rcu_nocb_cpu_offload(int cpu);
>  void rcu_nocb_cpu_deoffload(int cpu);
>  #else /* #ifdef CONFIG_RCU_NOCB_CPU */
>  static inline void rcu_init_nohz(void) { }
> +static inline void rcu_nocb_cpu_offload(int cpu) { }
>  static inline void rcu_nocb_cpu_deoffload(int cpu) { }
>  #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
>  
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index c74a4df8d5f2..ae4b5e9f2fc5 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2224,6 +2224,50 @@ void rcu_nocb_cpu_deoffload(int cpu)
>  	mutex_unlock(&rcu_state.barrier_mutex);
>  }
>  
> +static void __rcu_nocb_rdp_offload(struct rcu_data *rdp)
> +{
> +	unsigned long flags;
> +	struct rcu_node *rnp = rdp->mynode;
> +
> +	printk("Offloading %d\n", rdp->cpu);
> +
> +	raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
> +	raw_spin_lock_rcu_node(rnp);
> +	rcu_segcblist_offload(&rdp->cblist, true);
> +	raw_spin_unlock_rcu_node(rnp);
> +	raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
> +
> +	kthread_unpark(rdp->nocb_cb_kthread);

And of course I have a similar set of questions here regarding
interruption of callback invocation.  ;-)

							Thanx, Paul

> +}
> +
> +static long rcu_nocb_rdp_offload(void *arg)
> +{
> +	struct rcu_data *rdp = arg;
> +
> +	WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id());
> +	__rcu_nocb_rdp_offload(rdp);
> +
> +	return 0;
> +}
> +
> +void rcu_nocb_cpu_offload(int cpu)
> +{
> +	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> +
> +	mutex_lock(&rcu_state.barrier_mutex);
> +	cpus_read_lock();
> +	if (!rcu_segcblist_is_offloaded(&rdp->cblist) && rdp->nocb_cb_kthread) {
> +		if (cpu_online(cpu)) {
> +			work_on_cpu(cpu, rcu_nocb_rdp_offload, rdp);
> +		} else {
> +			__rcu_nocb_rdp_offload(rdp);
> +		}
> +		cpumask_set_cpu(cpu, rcu_nocb_mask);
> +	}
> +	cpus_read_unlock();
> +	mutex_unlock(&rcu_state.barrier_mutex);
> +}
> +
>  void __init rcu_init_nohz(void)
>  {
>  	int cpu;
> -- 
> 2.25.0
>