Re: [PATCH v2 4/9] mm: vmalloc: Remove global vmap_area_root rb-tree

From: Baoquan He <bhe@redhat.com>
To: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Lorenzo Stoakes <lstoakes@gmail.com>,
	Christoph Hellwig <hch@infradead.org>,
	Matthew Wilcox <willy@infradead.org>,
	"Liam R . Howlett" <Liam.Howlett@oracle.com>,
	Dave Chinner <david@fromorbit.com>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	Joel Fernandes <joel@joelfernandes.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@sony.com>
Subject: Re: [PATCH v2 4/9] mm: vmalloc: Remove global vmap_area_root rb-tree
Date: Mon, 11 Sep 2023 10:38:29 +0800	[thread overview]
Message-ID: <ZP59pbh9SKROtjlr@MiWiFi-R3L-srv> (raw)
In-Reply-To: <20230829081142.3619-5-urezki@gmail.com>

On 08/29/23 at 10:11am, Uladzislau Rezki (Sony) wrote:
> Store allocated objects in a separate nodes. A va->va_start
> address is converted into a correct node where it should
> be placed and resided. An addr_to_node() function is used
> to do a proper address conversion to determine a node that
> contains a VA.
> 
> Such approach balances VAs across nodes as a result an access
> becomes scalable. Number of nodes in a system depends on number
> of CPUs divided by two. The density factor in this case is 1/2.
> 
> Please note:
> 
> 1. As of now allocated VAs are bound to a node-0. It means the
>    patch does not give any difference comparing with a current
>    behavior;
> 
> 2. The global vmap_area_lock, vmap_area_root are removed as there
>    is no need in it anymore. The vmap_area_list is still kept and
>    is _empty_. It is exported for a kexec only;
> 
> 3. The vmallocinfo and vread() have to be reworked to be able to
>    handle multiple nodes.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> ---
>  mm/vmalloc.c | 209 +++++++++++++++++++++++++++++++++++++++------------
>  1 file changed, 161 insertions(+), 48 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index b7deacca1483..ae0368c314ff 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -728,11 +728,9 @@ EXPORT_SYMBOL(vmalloc_to_pfn);
>  #define DEBUG_AUGMENT_LOWEST_MATCH_CHECK 0
>  
>  
> -static DEFINE_SPINLOCK(vmap_area_lock);
>  static DEFINE_SPINLOCK(free_vmap_area_lock);
>  /* Export for kexec only */
>  LIST_HEAD(vmap_area_list);
> -static struct rb_root vmap_area_root = RB_ROOT;
>  static bool vmap_initialized __read_mostly;
>  
>  static struct rb_root purge_vmap_area_root = RB_ROOT;
> @@ -772,6 +770,38 @@ static struct rb_root free_vmap_area_root = RB_ROOT;
>   */
>  static DEFINE_PER_CPU(struct vmap_area *, ne_fit_preload_node);
>  
> +/*
> + * An effective vmap-node logic. Users make use of nodes instead
> + * of a global heap. It allows to balance an access and mitigate
> + * contention.
> + */
> +struct rb_list {
> +	struct rb_root root;
> +	struct list_head head;
> +	spinlock_t lock;
> +};
> +
> +struct vmap_node {
> +	/* Bookkeeping data of this node. */
> +	struct rb_list busy;
> +};
> +
> +static struct vmap_node *nodes, snode;
> +static __read_mostly unsigned int nr_nodes = 1;
> +static __read_mostly unsigned int node_size = 1;

It could be better if calling these global variables a meaningful name,
e.g vmap_nodes, static_vmap_nodes, nr_vmap_nodes. When I use vim+cscope
to reference them, it gives me a super long list. Aside from that, a
simple name often makes me mistake it as a local virable. A weak
opinion.

> +
> +static inline unsigned int
> +addr_to_node_id(unsigned long addr)
> +{
> +	return (addr / node_size) % nr_nodes;
> +}
> +
> +static inline struct vmap_node *
> +addr_to_node(unsigned long addr)
> +{
> +	return &nodes[addr_to_node_id(addr)];
> +}
> +
>  static __always_inline unsigned long
>  va_size(struct vmap_area *va)
>  {