linux-kernel - Re: [PATCH v4] kvfree_rcu: Release page cache under memory pressure

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Date:   Mon, 8 Feb 2021 14:02:48 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     qiang.zhang@...driver.com
Cc:     urezki@...il.com, paulmck@...nel.org, joel@...lfernandes.org,
        rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] kvfree_rcu: Release page cache under memory pressure

Hello, Zqiang.

Thank you for your v4!

Some small nits see below:

> From: Zqiang <qiang.zhang@...driver.com>
> 
> Add free per-cpu existing krcp's page cache operation, when
> the system is under memory pressure.
> 
> Signed-off-by: Zqiang <qiang.zhang@...driver.com>
> Co-developed-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> ---
>  v1->v2->v3->v4:
>  During the test a page shrinker is pretty active, because of low memory
>  condition. callback drains it whereas kvfree_rcu() part refill it right
>  away making kind of vicious circle.
>  Through Vlad Rezki suggestion, to avoid this, schedule a periodic delayed
>  work with HZ, and it's easy to do that.
> 
I think the commit message should be improved. Please add a clear
description how it works, i mean its connection with shrinker, etc.

>  kernel/rcu/tree.c | 50 +++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 42 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index c1ae1e52f638..f3b772eef468 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3139,7 +3139,7 @@ struct kfree_rcu_cpu {
>  	bool initialized;
>  	int count;
>  
> -	struct work_struct page_cache_work;
> +	struct delayed_work page_cache_work;
>  	atomic_t work_in_progress;
>  	struct hrtimer hrtimer;
>  
> @@ -3395,7 +3395,7 @@ schedule_page_work_fn(struct hrtimer *t)
>  	struct kfree_rcu_cpu *krcp =
>  		container_of(t, struct kfree_rcu_cpu, hrtimer);
>  
> -	queue_work(system_highpri_wq, &krcp->page_cache_work);
> +	queue_delayed_work(system_highpri_wq, &krcp->page_cache_work, 0);
>  	return HRTIMER_NORESTART;
>  }
>  
> @@ -3404,7 +3404,7 @@ static void fill_page_cache_func(struct work_struct *work)
>  	struct kvfree_rcu_bulk_data *bnode;
>  	struct kfree_rcu_cpu *krcp =
>  		container_of(work, struct kfree_rcu_cpu,
> -			page_cache_work);
> +			page_cache_work.work);
>  	unsigned long flags;
>  	bool pushed;
>  	int i;
> @@ -3428,15 +3428,22 @@ static void fill_page_cache_func(struct work_struct *work)
>  	atomic_set(&krcp->work_in_progress, 0);
>  }
>  
> +static bool backoff_page_cache_fill;
> +
>  static void
>  run_page_cache_worker(struct kfree_rcu_cpu *krcp)
>  {
>  	if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
>  			!atomic_xchg(&krcp->work_in_progress, 1)) {
> -		hrtimer_init(&krcp->hrtimer, CLOCK_MONOTONIC,
> -			HRTIMER_MODE_REL);
> -		krcp->hrtimer.function = schedule_page_work_fn;
> -		hrtimer_start(&krcp->hrtimer, 0, HRTIMER_MODE_REL);
> +		if (READ_ONCE(backoff_page_cache_fill)) {
Can we just use xchg directly inside "if" statement? So we can
get rid of below WRITE_ONCE(). It is not considered as a "hot"
path, so it should not be an issue.

> +			queue_delayed_work(system_highpri_wq, &krcp->page_cache_work, HZ);
> +			WRITE_ONCE(backoff_page_cache_fill, false);
> +		} else {
> +			hrtimer_init(&krcp->hrtimer, CLOCK_MONOTONIC,
> +				HRTIMER_MODE_REL);
> +			krcp->hrtimer.function = schedule_page_work_fn;
> +			hrtimer_start(&krcp->hrtimer, 0, HRTIMER_MODE_REL);
> +		}
>  	}
>  }

Thank you!

--
Vlad Rezki