linux-kernel - Re: [PATCH RFC] rcu/kfree: Do not request RCU when not needed

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Date:   Wed, 2 Nov 2022 12:11:24 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, paulmck@...nel.org, urezki@...il.com
Subject: Re: [PATCH RFC] rcu/kfree: Do not request RCU when not needed

On Sat, Oct 29, 2022 at 9:29 AM Joel Fernandes (Google)
<joel@...lfernandes.org> wrote:
>
> On ChromeOS, I am (almost) always seeing the optimization trigger.
> Tested boot up and trace_printk'ing how often it triggers.
>
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> ---
>  kernel/rcu/tree.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 591187b6352e..3e4c50b9fd33 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2935,6 +2935,7 @@ struct kfree_rcu_cpu_work {
>
>  /**
>   * struct kfree_rcu_cpu - batch up kfree_rcu() requests for RCU grace period
> + * @rdp: The rdp of the CPU that this kfree_rcu corresponds to.
>   * @head: List of kfree_rcu() objects not yet waiting for a grace period
>   * @bkvhead: Bulk-List of kvfree_rcu() objects not yet waiting for a grace period
>   * @krw_arr: Array of batches of kfree_rcu() objects waiting for a grace period
> @@ -2964,6 +2965,8 @@ struct kfree_rcu_cpu {
>         struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES];
>         raw_spinlock_t lock;
>         struct delayed_work monitor_work;
> +       struct rcu_data *rdp;
> +       unsigned long last_gp_seq;
>         bool initialized;
>         int count;
>
> @@ -3167,6 +3170,7 @@ schedule_delayed_monitor_work(struct kfree_rcu_cpu *krcp)
>                         mod_delayed_work(system_wq, &krcp->monitor_work, delay);
>                 return;
>         }
> +       krcp->last_gp_seq = krcp->rdp->gp_seq;

TODO: As Paul pointed out today in a meeting, we can't just sample
rdp->gp_seq directly as it can jump forward quickly from out-of-date
to non-out-of-date. Such GP can happen before a grace period actually
elapses. Boom!

He did make me feel better though that it is something easy to miss,
and to audit the current code about.

 - Joel



 - Joel



>         queue_delayed_work(system_wq, &krcp->monitor_work, delay);
>  }
>
> @@ -3217,7 +3221,17 @@ static void kfree_rcu_monitor(struct work_struct *work)
>                         // be that the work is in the pending state when
>                         // channels have been detached following by each
>                         // other.
> -                       queue_rcu_work(system_wq, &krwp->rcu_work);
> +                       //
> +                       // NOTE about gp_seq wrap: In case of gp_seq overflow,
> +                       // it is possible for rdp->gp_seq to be less than
> +                       // krcp->last_gp_seq even though a GP might be over. In
> +                       // this rare case, we would just have one extra GP.
> +                       if (krcp->last_gp_seq &&
> +                           rcu_seq_completed_gp(krcp->last_gp_seq, krcp->rdp->gp_seq)) {
> +                               queue_work(system_wq, &krwp->rcu_work.work);
> +                       } else {
> +                               queue_rcu_work(system_wq, &krwp->rcu_work);
> +                       }
>                 }
>         }
>
> @@ -4802,6 +4816,8 @@ static void __init kfree_rcu_batch_init(void)
>         for_each_possible_cpu(cpu) {
>                 struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
>
> +               krcp->rdp = per_cpu_ptr(&rcu_data, cpu);
> +               krcp->last_gp_seq = 0;
>                 for (i = 0; i < KFREE_N_BATCHES; i++) {
>                         INIT_RCU_WORK(&krcp->krw_arr[i].rcu_work, kfree_rcu_work);
>                         krcp->krw_arr[i].krcp = krcp;
> --
> 2.38.1.273.g43a17bfeac-goog
>