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

[Uladzislau Rezki]

On Sat, Oct 29, 2022 at 01:28:56PM +0000, Joel Fernandes (Google) 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@xxxxxxxxxxxxxxxxx>
> ---
>  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;
>  	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 &&
>
This check can be eliminated i think. A kfree_rcu_cpu is defined as
static so by default the last_gp_set is set to zero.

>  
> @@ -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;
>
Yep. This one can be just dropped.

But all the rest looks good :) I will give it a try from test point of
view. It is interested from the memory footprint point of view.

--
Uladzislau Rezki