Re: [PATCH v4 4/4] rcu: Support direct wake-up of synchronize_rcu() users

[Z qiang]

>
> This patch introduces a small enhancement which allows to do a
> direct wake-up of synchronize_rcu() callers. It occurs after a
> completion of grace period, thus by the gp-kthread.
>
> Number of clients is limited by the hard-coded maximum allowed
> threshold. The remaining part, if still exists is deferred to
> a main worker.
>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> ---
>  kernel/rcu/tree.c | 31 +++++++++++++++++++++++++++++--
>  kernel/rcu/tree.h |  6 ++++++
>  2 files changed, 35 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 88a47a6a658e..96fe9cc122ca 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1642,7 +1642,8 @@ static DECLARE_WORK(sr_normal_gp_cleanup, rcu_sr_normal_gp_cleanup_work);
>   */
>  static void rcu_sr_normal_gp_cleanup(void)
>  {
> -       struct llist_node *wait_tail;
> +       struct llist_node *wait_tail, *next, *rcu;
> +       int done = 0;
>
>         wait_tail = rcu_state.srs_wait_tail;
>         if (wait_tail == NULL)
> @@ -1650,12 +1651,38 @@ static void rcu_sr_normal_gp_cleanup(void)
>
>         rcu_state.srs_wait_tail = NULL;
>         ASSERT_EXCLUSIVE_WRITER(rcu_state.srs_wait_tail);
> +       WARN_ON_ONCE(!rcu_sr_is_wait_head(wait_tail));
> +
> +       /*
> +        * Process (a) and (d) cases. See an illustration. Apart of
> +        * that it handles the scenario when all clients are done,
> +        * wait-head is released if last. The worker is not kicked.
> +        */
> +       llist_for_each_safe(rcu, next, wait_tail->next) {
> +               if (rcu_sr_is_wait_head(rcu)) {
> +                       if (!rcu->next) {
> +                               rcu_sr_put_wait_head(rcu);
> +                               wait_tail->next = NULL;
> +                       } else {
> +                               wait_tail->next = rcu;
> +                       }
> +
> +                       break;
> +               }
> +
> +               rcu_sr_normal_complete(rcu);
> +               // It can be last, update a next on this step.
> +               wait_tail->next = next;
> +
> +               if (++done == SR_MAX_USERS_WAKE_FROM_GP)
> +                       break;
> +       }
>
>         // concurrent sr_normal_gp_cleanup work might observe this update.
>         smp_store_release(&rcu_state.srs_done_tail, wait_tail);
>         ASSERT_EXCLUSIVE_WRITER(rcu_state.srs_done_tail);
>
> -       if (wait_tail)
> +       if (wait_tail->next)
>                 queue_work(system_highpri_wq, &sr_normal_gp_cleanup);
>

I'm testing these patches :) , one question is as follows:
Can we use (WQ_MEM_RECLAIM | WQ_HIGHPR)type of workqueue to perform
wake-up actions? avoid kworker creation failure under memory pressure, causing
the wake-up action to be delayed.

Thanks
Zqiang



>  }
>
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index 4c35d7d37647..5d8b71a1caec 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -316,6 +316,12 @@ do {                                                                       \
>         __set_current_state(TASK_RUNNING);                              \
>  } while (0)
>
> +/*
> + * A max threshold for synchronize_rcu() users which are
> + * awaken directly by the rcu_gp_kthread(). Left part is
> + * deferred to the main worker.
> + */
> +#define SR_MAX_USERS_WAKE_FROM_GP 5
>  #define SR_NORMAL_GP_WAIT_HEAD_MAX 5
>
>  struct sr_wait_node {
> --
> 2.39.2
>
>