From: Huaisheng Ye <[email protected]> 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. This patch is not designed for fixing logical error. That is used for optimizing the behavior of writecache_find_entry. Give an example to illustrate the point below. Suppose that is the case, here is a normal READ bio comes to writecache_map. And because of bio's direction is READ, writecache_find_entry would be called with flags WFE_RETURN_FOLLOWING. Now there are two scenarios, 1. writecache_find_entry successfully get an existing entry by searching rb_tree, we could call it HIT. Then the first 'while' will be finished by 'break'. Next it will move to second 'while' loop, because of the flags hasn't been marked as WFE_LOWEST_SEQ. writecache_find_entry will try to return an entry with HIGHEST_SEQ, if there are other entries which has same original_sector in rb_tree. For this situation, the current code is okay to deal with that. 2. writecache_find_entry couldn't get an existing entry from rb_tree, we could call it MISS. Because of same flags WFE_RETURN_FOLLOWING, writecache_find_entry will get other entry, which's original_sector will slightly larger than input parameter block, with big probability. For this scenario, function writecache_find_entry doesn't need to enter second 'while' loop. But current code would still try to check there were other entry with same original_sector. So the additional rb_next or rb_prev is unnecessary by this case, also the code doesn't need to compare the original_sector of 'e2' with parameter 'block'. This patch is designed to optimize the second case. so it could skip the second 'while' loop when the block is missed from rb_tree. Signed-off-by: Huaisheng Ye <[email protected]> --- 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 -- dm-devel mailing list [email protected] https://www.redhat.com/mailman/listinfo/dm-devel
