Re: [PATCH 4/8] rcu/exp: Handle RCU expedited grace period kworker allocation failure

[Kalesh Singh]

On Fri, Dec 8, 2023 at 5:06 PM Frederic Weisbecker <frederic@xxxxxxxxxx> wrote:
>
> Just like is done for the kworker performing nodes initialization,
> gracefully handle the possible allocation failure of the RCU expedited
> grace period main kworker.
>
> While at it perform a rename of the related checking functions to better
> reflect the expedited specifics.
>
> Fixes: 9621fbee44df ("rcu: Move expedited grace period (GP) work to RT kthread_worker")
> Cc: Kalesh Singh <kaleshsingh@xxxxxxxxxx>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>

Reviewed-by: Kalesh Singh <kaleshsingh@xxxxxxxxxx>

Thanks,
Kalesh

> ---
>  kernel/rcu/tree.c     |  2 ++
>  kernel/rcu/tree_exp.h | 25 +++++++++++++++++++------
>  2 files changed, 21 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 055f4817bc70..39679cf66c3a 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4717,6 +4717,7 @@ static void __init rcu_start_exp_gp_kworkers(void)
>         rcu_exp_gp_kworker = kthread_create_worker(0, gp_kworker_name);
>         if (IS_ERR_OR_NULL(rcu_exp_gp_kworker)) {
>                 pr_err("Failed to create %s!\n", gp_kworker_name);
> +               rcu_exp_gp_kworker = NULL;
>                 return;
>         }
>
> @@ -4725,6 +4726,7 @@ static void __init rcu_start_exp_gp_kworkers(void)
>                 pr_err("Failed to create %s!\n", par_gp_kworker_name);
>                 rcu_exp_par_gp_kworker = NULL;
>                 kthread_destroy_worker(rcu_exp_gp_kworker);
> +               rcu_exp_gp_kworker = NULL;
>                 return;
>         }
>
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 6d7cea5d591f..cb31f4fb4b36 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -429,7 +429,12 @@ static void sync_rcu_exp_select_node_cpus(struct kthread_work *wp)
>         __sync_rcu_exp_select_node_cpus(rewp);
>  }
>
> -static inline bool rcu_gp_par_worker_started(void)
> +static inline bool rcu_exp_worker_started(void)
> +{
> +       return !!READ_ONCE(rcu_exp_gp_kworker);
> +}
> +
> +static inline bool rcu_exp_par_worker_started(void)
>  {
>         return !!READ_ONCE(rcu_exp_par_gp_kworker);
>  }
> @@ -479,7 +484,12 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
>         __sync_rcu_exp_select_node_cpus(rewp);
>  }
>
> -static inline bool rcu_gp_par_worker_started(void)
> +static inline bool rcu_exp_worker_started(void)
> +{
> +       return !!READ_ONCE(rcu_gp_wq);
> +}
> +
> +static inline bool rcu_exp_par_worker_started(void)
>  {
>         return !!READ_ONCE(rcu_par_gp_wq);
>  }
> @@ -542,7 +552,7 @@ static void sync_rcu_exp_select_cpus(void)
>                 rnp->exp_need_flush = false;
>                 if (!READ_ONCE(rnp->expmask))
>                         continue; /* Avoid early boot non-existent wq. */
> -               if (!rcu_gp_par_worker_started() ||
> +               if (!rcu_exp_par_worker_started() ||
>                     rcu_scheduler_active != RCU_SCHEDULER_RUNNING ||
>                     rcu_is_last_leaf_node(rnp)) {
>                         /* No worker started yet or last leaf, do direct call. */
> @@ -957,7 +967,7 @@ static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp)
>   */
>  void synchronize_rcu_expedited(void)
>  {
> -       bool boottime = (rcu_scheduler_active == RCU_SCHEDULER_INIT);
> +       bool can_queue;
>         unsigned long flags;
>         struct rcu_exp_work rew;
>         struct rcu_node *rnp;
> @@ -968,6 +978,9 @@ void synchronize_rcu_expedited(void)
>                          lock_is_held(&rcu_sched_lock_map),
>                          "Illegal synchronize_rcu_expedited() in RCU read-side critical section");
>
> +       can_queue = (rcu_scheduler_active != RCU_SCHEDULER_INIT) &&
> +                   rcu_exp_worker_started();
> +
>         /* Is the state is such that the call is a grace period? */
>         if (rcu_blocking_is_gp()) {
>                 // Note well that this code runs with !PREEMPT && !SMP.
> @@ -997,7 +1010,7 @@ void synchronize_rcu_expedited(void)
>                 return;  /* Someone else did our work for us. */
>
>         /* Ensure that load happens before action based on it. */
> -       if (unlikely(boottime)) {
> +       if (unlikely(!can_queue)) {
>                 /* Direct call during scheduler init and early_initcalls(). */
>                 rcu_exp_sel_wait_wake(s);
>         } else {
> @@ -1015,7 +1028,7 @@ void synchronize_rcu_expedited(void)
>         /* Let the next expedited grace period start. */
>         mutex_unlock(&rcu_state.exp_mutex);
>
> -       if (likely(!boottime))
> +       if (likely(can_queue))
>                 synchronize_rcu_expedited_destroy_work(&rew);
>  }
>  EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
> --
> 2.42.1
>