linux-kernel - Re: [PATCH 4/6] rcu/nocb: Create kthreads on all CPUs if "rcu_nocb=" or "nohz

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Date:   Wed, 1 Dec 2021 14:57:18 +0530
From:   Neeraj Upadhyay <quic_neeraju@...cinc.com>
To:     Frederic Weisbecker <frederic@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
CC:     LKML <linux-kernel@...r.kernel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Joel Fernandes <joel@...lfernandes.org>, <rcu@...r.kernel.org>
Subject: Re: [PATCH 4/6] rcu/nocb: Create kthreads on all CPUs if "rcu_nocb="
 or "nohz_full=" are passed



On 11/23/2021 6:07 AM, Frederic Weisbecker wrote:
> In order to be able to (de-)offload any CPU using cpuset in the future,
> create a NOCB kthread for all possible CPUs. For now this is done only
> as long as the "rcu_nocb=" or "nohz_full=" kernel parameters are passed
> to avoid the unnecessary overhead for most users.
> 
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Cc: Neeraj Upadhyay <quic_neeraju@...cinc.com>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Uladzislau Rezki <urezki@...il.com>
> Cc: Josh Triplett <josh@...htriplett.org>
> Cc: Joel Fernandes <joel@...lfernandes.org>
> ---

Reviewed-by: Neeraj Upadhyay <quic_neeraju@...cinc.com>


>   kernel/rcu/tree_nocb.h | 14 ++++++--------
>   1 file changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index d8ed3ee47a67..9d37916278d4 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1229,11 +1229,8 @@ static void rcu_spawn_one_nocb_kthread(int cpu)
>   	struct rcu_data *rdp_gp;
>   	struct task_struct *t;
>   
> -	/*
> -	 * If this isn't a no-CBs CPU or if it already has an rcuo kthread,
> -	 * then nothing to do.
> -	 */
> -	if (!rcu_is_nocb_cpu(cpu) || rdp->nocb_cb_kthread)

As rcu_is_nocb_cpu() does not have a user, we can probably remove it?


Thanks
Neeraj

> +	/* If it already has an rcuo kthread, then nothing to do. */
> +	if (rdp->nocb_cb_kthread)
>   		return;
>   
>   	/* If we didn't spawn the GP kthread first, reorganize! */
> @@ -1261,7 +1258,7 @@ static void rcu_spawn_one_nocb_kthread(int cpu)
>    */
>   static void rcu_spawn_cpu_nocb_kthread(int cpu)
>   {
> -	if (rcu_scheduler_fully_active)
> +	if (rcu_scheduler_fully_active && rcu_nocb_is_setup)
>   		rcu_spawn_one_nocb_kthread(cpu);
>   }
>   
> @@ -1311,7 +1308,7 @@ static void __init rcu_organize_nocb_kthreads(void)
>   	 * Should the corresponding CPU come online in the future, then
>   	 * we will spawn the needed set of rcu_nocb_kthread() kthreads.
>   	 */
> -	for_each_cpu(cpu, rcu_nocb_mask) {
> +	for_each_possible_cpu(cpu) {
>   		rdp = per_cpu_ptr(&rcu_data, cpu);
>   		if (rdp->cpu >= nl) {
>   			/* New GP kthread, set up for CBs & next GP. */
> @@ -1335,7 +1332,8 @@ static void __init rcu_organize_nocb_kthreads(void)
>   				pr_cont(" %d", cpu);
>   		}
>   		rdp->nocb_gp_rdp = rdp_gp;
> -		list_add_tail(&rdp->nocb_entry_rdp, &rdp_gp->nocb_head_rdp);
> +		if (cpumask_test_cpu(cpu, rcu_nocb_mask))
> +			list_add_tail(&rdp->nocb_entry_rdp, &rdp_gp->nocb_head_rdp);
>   	}
>   	if (gotnocbs && dump_tree)
>   		pr_cont("%s\n", gotnocbscbs ? "" : " (self only)");
>