Re: [PATCH] mm/vmalloc: use rb_tree instead of list for vread() lookups

[Uladzislau Rezki <urezki@gmail.com>]

On Mon, Feb 08, 2021 at 03:53:03PM +0000, Serapheim Dimitropoulos wrote:
> vread() has been linearly searching vmap_area_list for looking up
> vmalloc areas to read from. These same areas are also tracked by
> a rb_tree (vmap_area_root) which offers logarithmic lookup.
> 
> This patch modifies vread() to use the rb_tree structure instead
> of the list and the speedup for heavy /proc/kcore readers can
> be pretty significant. Below are the wall clock measurements of
> a Python application that leverages the drgn debugging library
> to read and interpret data read from /proc/kcore.
> 
> Before the patch:
> -----
> $ time sudo sdb -e 'dbuf | head 2500 | wc'
> (unsigned long)2500
> 
> real	0m21.128s
> user	0m2.321s
> sys	0m19.227s
> -----
> 
> With the patch:
> -----
> $ time sudo sdb -e 'dbuf | head 2500 | wc'
> (unsigned long)2500
> 
> real	0m1.870s
> user	0m1.628s
> sys	0m0.660s
> -----
> 
> Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
> ---
>  mm/vmalloc.c | 19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 49ab9b6c001d..86343b879938 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2851,6 +2851,7 @@ long vread(char *buf, char *addr, unsigned long count)
>  {
>  	struct vmap_area *va;
>  	struct vm_struct *vm;
> +	struct rb_node *node;
>  	char *vaddr, *buf_start = buf;
>  	unsigned long buflen = count;
>  	unsigned long n;
> @@ -2860,17 +2861,15 @@ long vread(char *buf, char *addr, unsigned long count)
>  		count = -(unsigned long) addr;
>  
>  	spin_lock(&vmap_area_lock);
> -	list_for_each_entry(va, &vmap_area_list, list) {
> -		if (!count)
> -			break;
> -
> +	va = __find_vmap_area((unsigned long)addr);
> +	if (!va)
> +		goto finished;
> +	while (count) {
>  		if (!va->vm)
> -			continue;
> +			goto next_node;
>  
>  		vm = va->vm;
>  		vaddr = (char *) vm->addr;
> -		if (addr >= vaddr + get_vm_area_size(vm))
> -			continue;
>  		while (addr < vaddr) {
>  			if (count == 0)
>  				goto finished;
> @@ -2889,6 +2888,12 @@ long vread(char *buf, char *addr, unsigned long count)
>  		buf += n;
>  		addr += n;
>  		count -= n;
> +
> +next_node:
> +		node = rb_next(&va->rb_node);
> +		if (!node)
> +			break;
> +		va = rb_entry(node, struct vmap_area, rb_node);
>
You can also improve it. Instead of rb_next() you can directly access
to a "next" element via "va->list" making it O(1) complexity.

--
Vlad Rezki