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SubjectRe: [PATCH 4/6] rcu/nocb: Create kthreads on all CPUs if "rcu_nocb=" or "nohz_full=" are passed
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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@kernel.org>
> Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Uladzislau Rezki <urezki@gmail.com>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Joel Fernandes <joel@joelfernandes.org>
> ---

Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.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)");
>

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