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SubjectRe: [PATCH] dm-writecache: avoid unnecessary lookups in writecache_find_entry


On Sun, 21 Apr 2019, Huaisheng Ye wrote:

> From: Huaisheng Ye <yehs1@lenovo.com>
>
> Only when entry has been found, that would only be necessary to check the
> lowest or highest seq-count.
>
> Add local variable "found" in writecache_find_entry, if no entry has been
> found, it is meaningless that having a useless rb_prev or rb_next.


Hi

I don't quite see what is this patch trying to fix.
Don't fix something that isn't broken.

Mikulas


> Signed-off-by: Huaisheng Ye <yehs1@lenovo.com>
> ---
> drivers/md/dm-writecache.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/md/dm-writecache.c b/drivers/md/dm-writecache.c
> index ddf1732..047ae09 100644
> --- a/drivers/md/dm-writecache.c
> +++ b/drivers/md/dm-writecache.c
> @@ -537,14 +537,18 @@ static struct wc_entry *writecache_find_entry(struct dm_writecache *wc,
> {
> struct wc_entry *e;
> struct rb_node *node = wc->tree.rb_node;
> + bool found = false;
>
> if (unlikely(!node))
> return NULL;
>
> while (1) {
> e = container_of(node, struct wc_entry, rb_node);
> - if (read_original_sector(wc, e) == block)
> + if (read_original_sector(wc, e) == block) {
> + found = true;
> break;
> + }
> +
> node = (read_original_sector(wc, e) >= block ?
> e->rb_node.rb_left : e->rb_node.rb_right);
> if (unlikely(!node)) {
> @@ -564,7 +568,8 @@ static struct wc_entry *writecache_find_entry(struct dm_writecache *wc,
> }
> }
>
> - while (1) {
> + /* only need to check lowest or highest seq-count when entry has been found */
> + while (found) {
> struct wc_entry *e2;
> if (flags & WFE_LOWEST_SEQ)
> node = rb_prev(&e->rb_node);
> @@ -577,6 +582,9 @@ static struct wc_entry *writecache_find_entry(struct dm_writecache *wc,
> return e;
> e = e2;
> }
> +
> + /* no entry has been found, return the following entry */
> + return e;
> }
>
> static void writecache_insert_entry(struct dm_writecache *wc, struct wc_entry *ins)
> --
> 1.8.3.1
>
>

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