Received-SPF: softfail (google.com: domain of transitioning linux-kernel-owner@vger.kernel.org does not designate 23.128.96.19 as permitted sender) client-ip=23.128.96.19;
Date:   Sat, 4 Jun 2022 08:51:08 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <neeraj.iitr10@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@sony.com>
Subject: Re: [PATCH 2/2] rcu/kvfree: Introduce KFREE_DRAIN_JIFFIES_[MAX/MIN]
 interval
Message-ID: <20220604155108.GU1790663@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <20220602080644.432156-1-urezki@gmail.com>
 <20220602080644.432156-2-urezki@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20220602080644.432156-2-urezki@gmail.com>
Precedence: bulk

On Thu, Jun 02, 2022 at 10:06:44AM +0200, Uladzislau Rezki (Sony) wrote:
> Currently the monitor work is scheduled with a fixed interval that
> is HZ/20 or each 50 milliseconds. The drawback of such approach is
> a low utilization of page slot in some scenarios. The page can store
> up to 512 records. For example on Android system it can look like:
> 
> <snip>
>   kworker/3:0-13872   [003] .... 11286.007048: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000026522604 nr_records=1
>   kworker/3:0-13872   [003] .... 11286.015638: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000095ed6fca nr_records=2
>   kworker/1:2-20434   [001] .... 11286.051230: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000044872ffd nr_records=1
>   kworker/1:2-20434   [001] .... 11286.059322: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000026522604 nr_records=2
>   kworker/0:1-20052   [000] .... 11286.095295: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000044872ffd nr_records=2
>   kworker/0:1-20052   [000] .... 11286.103418: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000cbcf05db nr_records=1
>   kworker/2:3-14372   [002] .... 11286.135155: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000095ed6fca nr_records=2
>   kworker/2:3-14372   [002] .... 11286.135198: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000044872ffd nr_records=1
>   kworker/1:2-20434   [001] .... 11286.155377: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000cbcf05db nr_records=5
>   kworker/2:3-14372   [002] .... 11286.167181: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000026522604 nr_records=5
>   kworker/1:2-20434   [001] .... 11286.179202: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x000000008ef95e14 nr_records=1
>   kworker/2:3-14372   [002] .... 11286.187398: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000c597d297 nr_records=6
>   kworker/3:0-13872   [003] .... 11286.187445: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000050bf92e2 nr_records=3
>   kworker/1:2-20434   [001] .... 11286.198975: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000cbcf05db nr_records=4
>   kworker/1:2-20434   [001] .... 11286.207203: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000095ed6fca nr_records=4
> <snip>
> 
> where a page only carries few records to reclaim a memory. In order to
> improve batching and make utilization more efficient the patch introduces
> a drain interval that can be set either to KFREE_DRAIN_JIFFIES_MAX or
> KFREE_DRAIN_JIFFIES_MIN. It is adjusted if a flood is detected, in this
> case a memory reclaim occurs more often whereas in mostly idle cases the
> interval is set to its maximum timeout that improves the utilization of
> page slots.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

That does look like a problem well worth solving!

But I am missing one thing.  If we are having a callback flood, why do we
need a shorter timeout?  Wouldn't a check on the number of blocks queued
be simpler, more direct, and provide faster response to the start of a
callback flood?

							Thanx, Paul

> ---
>  kernel/rcu/tree.c | 29 +++++++++++++++++++++++++----
>  1 file changed, 25 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index fd16c0b46d9e..c02a64995b85 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3249,7 +3249,8 @@ EXPORT_SYMBOL_GPL(call_rcu);
>  
>  
>  /* Maximum number of jiffies to wait before draining a batch. */
> -#define KFREE_DRAIN_JIFFIES (HZ / 50)
> +#define KFREE_DRAIN_JIFFIES_MAX (HZ)
> +#define KFREE_DRAIN_JIFFIES_MIN (HZ / 50)
>  #define KFREE_N_BATCHES 2
>  #define FREE_N_CHANNELS 2
>  
> @@ -3510,6 +3511,26 @@ need_offload_krc(struct kfree_rcu_cpu *krcp)
>  	return !!krcp->head;
>  }
>  
> +static void
> +schedule_delayed_monitor_work(struct kfree_rcu_cpu *krcp)
> +{
> +	long delay, delay_left;
> +
> +	delay = READ_ONCE(krcp->count) >= KVFREE_BULK_MAX_ENTR ?
> +		KFREE_DRAIN_JIFFIES_MIN:KFREE_DRAIN_JIFFIES_MAX;
> +
> +	if (delayed_work_pending(&krcp->monitor_work)) {
> +		delay_left = krcp->monitor_work.timer.expires - jiffies;
> +
> +		if (delay < delay_left)
> +			mod_delayed_work(system_wq, &krcp->monitor_work, delay);
> +
> +		return;
> +	}
> +
> +	queue_delayed_work(system_wq, &krcp->monitor_work, delay);
> +}
> +
>  /*
>   * This function is invoked after the KFREE_DRAIN_JIFFIES timeout.
>   */
> @@ -3567,7 +3588,7 @@ static void kfree_rcu_monitor(struct work_struct *work)
>  	// work to repeat an attempt. Because previous batches are
>  	// still in progress.
>  	if (need_offload_krc(krcp))
> -		schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
> +		schedule_delayed_monitor_work(krcp);
>  
>  	raw_spin_unlock_irqrestore(&krcp->lock, flags);
>  }
> @@ -3755,7 +3776,7 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
>  
>  	// Set timer to drain after KFREE_DRAIN_JIFFIES.
>  	if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING)
> -		schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
> +		schedule_delayed_monitor_work(krcp);
>  
>  unlock_return:
>  	krc_this_cpu_unlock(krcp, flags);
> @@ -3831,7 +3852,7 @@ void __init kfree_rcu_scheduler_running(void)
>  
>  		raw_spin_lock_irqsave(&krcp->lock, flags);
>  		if (need_offload_krc(krcp))
> -			schedule_delayed_work_on(cpu, &krcp->monitor_work, KFREE_DRAIN_JIFFIES);
> +			schedule_delayed_monitor_work(krcp);
>  		raw_spin_unlock_irqrestore(&krcp->lock, flags);
>  	}
>  }
> -- 
> 2.30.2
>