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Date:   Thu, 2 Jun 2022 23:43:39 +0000
From:   Joel Fernandes <joel@joelfernandes.org>
To:     "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <neeraj.iitr10@gmail.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@sony.com>
Subject: Re: [PATCH 1/2] rcu/kvfree: Remove useless monitor_todo flag
Message-ID: <YplLK2BcTn2oM0hr@google.com>
References: <20220602080644.432156-1-urezki@gmail.com>
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On Thu, Jun 02, 2022 at 10:06:43AM +0200, Uladzislau Rezki (Sony) wrote:
> From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
> 
> monitor_todo is not needed as the work struct already tracks
> if work is pending. Just use that to know if work is pending
> using schedule_delayed_work() helper.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  kernel/rcu/tree.c | 33 ++++++++++++++++-----------------
>  1 file changed, 16 insertions(+), 17 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 222d59299a2a..fd16c0b46d9e 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3295,7 +3295,6 @@ struct kfree_rcu_cpu_work {
>   * @krw_arr: Array of batches of kfree_rcu() objects waiting for a grace period
>   * @lock: Synchronize access to this structure
>   * @monitor_work: Promote @head to @head_free after KFREE_DRAIN_JIFFIES
> - * @monitor_todo: Tracks whether a @monitor_work delayed work is pending
>   * @initialized: The @rcu_work fields have been initialized
>   * @count: Number of objects for which GP not started
>   * @bkvcache:
> @@ -3320,7 +3319,6 @@ struct kfree_rcu_cpu {
>  	struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES];
>  	raw_spinlock_t lock;
>  	struct delayed_work monitor_work;
> -	bool monitor_todo;
>  	bool initialized;
>  	int count;
>  
> @@ -3500,6 +3498,18 @@ static void kfree_rcu_work(struct work_struct *work)
>  	}
>  }
>  
> +static bool
> +need_offload_krc(struct kfree_rcu_cpu *krcp)
> +{
> +	int i;
> +
> +	for (i = 0; i < FREE_N_CHANNELS; i++)
> +		if (krcp->bkvhead[i])
> +			return true;
> +
> +	return !!krcp->head;
> +}

Thanks for modifying my original patch to do this, and thanks for giving me
the attribution for the patch. This function is a nice addition.

For the patch in its entirety:
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

thanks,

 - Joel


> +
>  /*
>   * This function is invoked after the KFREE_DRAIN_JIFFIES timeout.
>   */
> @@ -3556,9 +3566,7 @@ static void kfree_rcu_monitor(struct work_struct *work)
>  	// of the channels that is still busy we should rearm the
>  	// work to repeat an attempt. Because previous batches are
>  	// still in progress.
> -	if (!krcp->bkvhead[0] && !krcp->bkvhead[1] && !krcp->head)
> -		krcp->monitor_todo = false;
> -	else
> +	if (need_offload_krc(krcp))
>  		schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
>  
>  	raw_spin_unlock_irqrestore(&krcp->lock, flags);
> @@ -3746,11 +3754,8 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
>  	WRITE_ONCE(krcp->count, krcp->count + 1);
>  
>  	// Set timer to drain after KFREE_DRAIN_JIFFIES.
> -	if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
> -	    !krcp->monitor_todo) {
> -		krcp->monitor_todo = true;
> +	if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING)
>  		schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
> -	}
>  
>  unlock_return:
>  	krc_this_cpu_unlock(krcp, flags);
> @@ -3825,14 +3830,8 @@ void __init kfree_rcu_scheduler_running(void)
>  		struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
>  
>  		raw_spin_lock_irqsave(&krcp->lock, flags);
> -		if ((!krcp->bkvhead[0] && !krcp->bkvhead[1] && !krcp->head) ||
> -				krcp->monitor_todo) {
> -			raw_spin_unlock_irqrestore(&krcp->lock, flags);
> -			continue;
> -		}
> -		krcp->monitor_todo = true;
> -		schedule_delayed_work_on(cpu, &krcp->monitor_work,
> -					 KFREE_DRAIN_JIFFIES);
> +		if (need_offload_krc(krcp))
> +			schedule_delayed_work_on(cpu, &krcp->monitor_work, KFREE_DRAIN_JIFFIES);
>  		raw_spin_unlock_irqrestore(&krcp->lock, flags);
>  	}
>  }
> -- 
> 2.30.2
>