linux-kernel - Re: [PATCHi v4] rcu/kvfree: Prevents cache growing when the backoff_page_cache

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Date:   Tue, 11 Apr 2023 09:43:51 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Zqiang <qiang1.zhang@...el.com>
Cc:     urezki@...il.com, frederic@...nel.org, joel@...lfernandes.org,
        qiang.zhang1211@...il.com, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHi v4] rcu/kvfree: Prevents cache growing when the
 backoff_page_cache_fill is set

On Tue, Apr 11, 2023 at 10:27:33PM +0800, Zqiang wrote:
> Currently, in kfree_rcu_shrink_scan(), the drain_page_cache() is
> executed before kfree_rcu_monitor() to drain page cache, if the bnode
> structure's->gp_snap has done, the kvfree_rcu_bulk() will fill the
> page cache again in kfree_rcu_monitor(), this commit add a check
> for krcp structure's->backoff_page_cache_fill in put_cached_bnode(),
> if the krcp structure's->backoff_page_cache_fill is set, prevent page
> cache growing and start fill page from krcp->nr_bkv_objs, if the
> krcp->nr_bkv_objs is greater or equal to nr_pages, not alloc page,
> avoid unnecessary __get_free_page() calls.
> 
> Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> ---
>  kernel/rcu/tree.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 41daae3239b5..d5639025bd1a 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2908,6 +2908,8 @@ static inline bool
>  put_cached_bnode(struct kfree_rcu_cpu *krcp,
>  	struct kvfree_rcu_bulk_data *bnode)
>  {
> +	if (atomic_read(&krcp->backoff_page_cache_fill))
> +		return false;

This one is still missing the logic that allows that cache to hold
one page during low memory?  Or is that being handled somewhere else?

							Thanx, Paul

>  	// Check the limit.
>  	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
>  		return false;
> @@ -3223,7 +3225,7 @@ static void fill_page_cache_func(struct work_struct *work)
>  	nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
>  		1 : rcu_min_cached_objs;
>  
> -	for (i = 0; i < nr_pages; i++) {
> +	for (i = krcp->nr_bkv_objs; i < nr_pages; i++) {
>  		bnode = (struct kvfree_rcu_bulk_data *)
>  			__get_free_page(GFP_KERNEL | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
>  
> -- 
> 2.32.0
>