Re: [PATCH v2 5/9] mm: vmalloc: Remove global purge_vmap_area_root rb-tree

[Baoquan He <bhe@redhat.com>]

On 08/29/23 at 10:11am, Uladzislau Rezki (Sony) wrote:
> Similar to busy VA, lazily-freed area is stored to a node
> it belongs to. Such approach does not require any global
> locking primitive, instead an access becomes scalable what
> mitigates a contention.
> 
> This patch removes a global purge-lock, global purge-tree
> and global purge list.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> ---
>  mm/vmalloc.c | 135 +++++++++++++++++++++++++++++++--------------------
>  1 file changed, 82 insertions(+), 53 deletions(-)

LGTM,

Reviewed-by: Baoquan He <bhe@redhat.com>

> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index ae0368c314ff..5a8a9c1370b6 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -733,10 +733,6 @@ static DEFINE_SPINLOCK(free_vmap_area_lock);
>  LIST_HEAD(vmap_area_list);
>  static bool vmap_initialized __read_mostly;
>  
> -static struct rb_root purge_vmap_area_root = RB_ROOT;
> -static LIST_HEAD(purge_vmap_area_list);
> -static DEFINE_SPINLOCK(purge_vmap_area_lock);
> -
>  /*
>   * This kmem_cache is used for vmap_area objects. Instead of
>   * allocating from slab we reuse an object from this cache to
> @@ -784,6 +780,12 @@ struct rb_list {
>  struct vmap_node {
>  	/* Bookkeeping data of this node. */
>  	struct rb_list busy;
> +	struct rb_list lazy;
> +
> +	/*
> +	 * Ready-to-free areas.
> +	 */
> +	struct list_head purge_list;
>  };
>  
>  static struct vmap_node *nodes, snode;
> @@ -1768,40 +1770,22 @@ static DEFINE_MUTEX(vmap_purge_lock);
>  
>  /* for per-CPU blocks */
>  static void purge_fragmented_blocks_allcpus(void);
> +static cpumask_t purge_nodes;
>  
>  /*
>   * Purges all lazily-freed vmap areas.
>   */
> -static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
> +static unsigned long
> +purge_vmap_node(struct vmap_node *vn)
>  {
> -	unsigned long resched_threshold;
> -	unsigned int num_purged_areas = 0;
> -	struct list_head local_purge_list;
> +	unsigned long num_purged_areas = 0;
>  	struct vmap_area *va, *n_va;
>  
> -	lockdep_assert_held(&vmap_purge_lock);
> -
> -	spin_lock(&purge_vmap_area_lock);
> -	purge_vmap_area_root = RB_ROOT;
> -	list_replace_init(&purge_vmap_area_list, &local_purge_list);
> -	spin_unlock(&purge_vmap_area_lock);
> -
> -	if (unlikely(list_empty(&local_purge_list)))
> -		goto out;
> -
> -	start = min(start,
> -		list_first_entry(&local_purge_list,
> -			struct vmap_area, list)->va_start);
> -
> -	end = max(end,
> -		list_last_entry(&local_purge_list,
> -			struct vmap_area, list)->va_end);
> -
> -	flush_tlb_kernel_range(start, end);
> -	resched_threshold = lazy_max_pages() << 1;
> +	if (list_empty(&vn->purge_list))
> +		return 0;
>  
>  	spin_lock(&free_vmap_area_lock);
> -	list_for_each_entry_safe(va, n_va, &local_purge_list, list) {
> +	list_for_each_entry_safe(va, n_va, &vn->purge_list, list) {
>  		unsigned long nr = (va->va_end - va->va_start) >> PAGE_SHIFT;
>  		unsigned long orig_start = va->va_start;
>  		unsigned long orig_end = va->va_end;
> @@ -1823,13 +1807,55 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
>  
>  		atomic_long_sub(nr, &vmap_lazy_nr);
>  		num_purged_areas++;
> -
> -		if (atomic_long_read(&vmap_lazy_nr) < resched_threshold)
> -			cond_resched_lock(&free_vmap_area_lock);
>  	}
>  	spin_unlock(&free_vmap_area_lock);
>  
> -out:
> +	return num_purged_areas;
> +}
> +
> +/*
> + * Purges all lazily-freed vmap areas.
> + */
> +static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
> +{
> +	unsigned long num_purged_areas = 0;
> +	struct vmap_node *vn;
> +	int i;
> +
> +	lockdep_assert_held(&vmap_purge_lock);
> +	purge_nodes = CPU_MASK_NONE;
> +
> +	for (i = 0; i < nr_nodes; i++) {
> +		vn = &nodes[i];
> +
> +		INIT_LIST_HEAD(&vn->purge_list);
> +
> +		if (RB_EMPTY_ROOT(&vn->lazy.root))
> +			continue;
> +
> +		spin_lock(&vn->lazy.lock);
> +		WRITE_ONCE(vn->lazy.root.rb_node, NULL);
> +		list_replace_init(&vn->lazy.head, &vn->purge_list);
> +		spin_unlock(&vn->lazy.lock);
> +
> +		start = min(start, list_first_entry(&vn->purge_list,
> +			struct vmap_area, list)->va_start);
> +
> +		end = max(end, list_last_entry(&vn->purge_list,
> +			struct vmap_area, list)->va_end);
> +
> +		cpumask_set_cpu(i, &purge_nodes);
> +	}
> +
> +	if (cpumask_weight(&purge_nodes) > 0) {
> +		flush_tlb_kernel_range(start, end);
> +
> +		for_each_cpu(i, &purge_nodes) {
> +			vn = &nodes[i];
> +			num_purged_areas += purge_vmap_node(vn);
> +		}
> +	}
> +
>  	trace_purge_vmap_area_lazy(start, end, num_purged_areas);
>  	return num_purged_areas > 0;
>  }
> @@ -1848,16 +1874,9 @@ static void reclaim_and_purge_vmap_areas(void)
>  
>  static void drain_vmap_area_work(struct work_struct *work)
>  {
> -	unsigned long nr_lazy;
> -
> -	do {
> -		mutex_lock(&vmap_purge_lock);
> -		__purge_vmap_area_lazy(ULONG_MAX, 0);
> -		mutex_unlock(&vmap_purge_lock);
> -
> -		/* Recheck if further work is required. */
> -		nr_lazy = atomic_long_read(&vmap_lazy_nr);
> -	} while (nr_lazy > lazy_max_pages());
> +	mutex_lock(&vmap_purge_lock);
> +	__purge_vmap_area_lazy(ULONG_MAX, 0);
> +	mutex_unlock(&vmap_purge_lock);
>  }
>  
>  /*
> @@ -1867,6 +1886,7 @@ static void drain_vmap_area_work(struct work_struct *work)
>   */
>  static void free_vmap_area_noflush(struct vmap_area *va)
>  {
> +	struct vmap_node *vn = addr_to_node(va->va_start);
>  	unsigned long nr_lazy_max = lazy_max_pages();
>  	unsigned long va_start = va->va_start;
>  	unsigned long nr_lazy;
> @@ -1880,10 +1900,9 @@ static void free_vmap_area_noflush(struct vmap_area *va)
>  	/*
>  	 * Merge or place it to the purge tree/list.
>  	 */
> -	spin_lock(&purge_vmap_area_lock);
> -	merge_or_add_vmap_area(va,
> -		&purge_vmap_area_root, &purge_vmap_area_list);
> -	spin_unlock(&purge_vmap_area_lock);
> +	spin_lock(&vn->lazy.lock);
> +	merge_or_add_vmap_area(va, &vn->lazy.root, &vn->lazy.head);
> +	spin_unlock(&vn->lazy.lock);
>  
>  	trace_free_vmap_area_noflush(va_start, nr_lazy, nr_lazy_max);
>  
> @@ -4390,15 +4409,21 @@ static void show_numa_info(struct seq_file *m, struct vm_struct *v)
>  
>  static void show_purge_info(struct seq_file *m)
>  {
> +	struct vmap_node *vn;
>  	struct vmap_area *va;
> +	int i;
>  
> -	spin_lock(&purge_vmap_area_lock);
> -	list_for_each_entry(va, &purge_vmap_area_list, list) {
> -		seq_printf(m, "0x%pK-0x%pK %7ld unpurged vm_area\n",
> -			(void *)va->va_start, (void *)va->va_end,
> -			va->va_end - va->va_start);
> +	for (i = 0; i < nr_nodes; i++) {
> +		vn = &nodes[i];
> +
> +		spin_lock(&vn->lazy.lock);
> +		list_for_each_entry(va, &vn->lazy.head, list) {
> +			seq_printf(m, "0x%pK-0x%pK %7ld unpurged vm_area\n",
> +				(void *)va->va_start, (void *)va->va_end,
> +				va->va_end - va->va_start);
> +		}
> +		spin_unlock(&vn->lazy.lock);
>  	}
> -	spin_unlock(&purge_vmap_area_lock);
>  }
>  
>  static int s_show(struct seq_file *m, void *p)
> @@ -4545,6 +4570,10 @@ static void vmap_init_nodes(void)
>  		vn->busy.root = RB_ROOT;
>  		INIT_LIST_HEAD(&vn->busy.head);
>  		spin_lock_init(&vn->busy.lock);
> +
> +		vn->lazy.root = RB_ROOT;
> +		INIT_LIST_HEAD(&vn->lazy.head);
> +		spin_lock_init(&vn->lazy.lock);
>  	}
>  }
>  
> -- 
> 2.30.2
>