On 10/07/2016 09:16 PM, Josef Bacik wrote:
On 09/21/2016 02:59 AM, Wang Xiaoguang wrote:
In flush_space()->shrink_delalloc(), if can_overcommit() returns true,
though no bytes added to space_info, we still may satisfy some requests.

Signed-off-by: Wang Xiaoguang <wangxg.f...@cn.fujitsu.com>
 fs/btrfs/extent-tree.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 38c2df8..fdfc97f 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -4960,6 +4960,43 @@ static void wake_all_tickets(struct list_head *head)

+ * This function must be protected by btrfs_space_info's lock.
+ */
+static void try_to_wake_tickets(struct btrfs_root *root,
+                struct btrfs_space_info *space_info)
+    struct reserve_ticket *ticket;
+    struct list_head *head = &space_info->priority_tickets;
+    enum btrfs_reserve_flush_enum flush = BTRFS_RESERVE_NO_FLUSH;
+    u64 used;
+    while (!list_empty(head)) {
+        ticket = list_first_entry(head, struct reserve_ticket,
+                      list);
+        used = space_info->bytes_used +
+            space_info->bytes_reserved + space_info->bytes_pinned +
+            space_info->bytes_readonly + space_info->bytes_may_use;
+        if (used + ticket->bytes <= space_info->total_bytes ||
+            can_overcommit(root, space_info, ticket->bytes, flush)) {
+            space_info->bytes_may_use += ticket->bytes;
+            list_del_init(&ticket->list);
+            ticket->bytes = 0;
+            space_info->tickets_id++;
+            wake_up(&ticket->wait);
+        } else
+            return;
+    }
+    if (head == &space_info->priority_tickets) {
+        head = &space_info->tickets;
+        flush = BTRFS_RESERVE_FLUSH_ALL;
+        goto again;
+    }
* This is for normal flushers, we can wait all goddamned day if we want to. We * will loop and continuously try to flush as long as we are making progress.
  * We count progress as clearing off tickets each time we have to loop.
@@ -4995,6 +5032,8 @@ static void btrfs_async_reclaim_metadata_space(struct work_struct *work)
         ret = flush_space(fs_info->fs_root, space_info, to_reclaim,
                 to_reclaim, flush_state);
+        if (!ret)
+            try_to_wake_tickets(fs_info->fs_root, space_info);
         if (list_empty(&space_info->tickets)) {
             space_info->flush = 0;

So first off I have no problems with this patch, it seems reasonable to me.

However I'm curious to see where it helped. The only time can_overcommit() would suddenly start returning true where it wouldn't have before without actually adding space to the space_info would be when we've dropped an empty block group (or added a new drive) and suddenly fs_info->free_chunk_space is larger than it was. Is that what you were observing? Thanks,
I think you're right. I don't add new drive when doing my big files create and delete test, so it maybe "remove useless chunks" causing can_overcommit() returns true, but I don't know how to observe this in codes, sorry. And this patch can really help to fix my enospc issue :)

Meanwhile, in shrink_delalloc(), if can_overcommit() returns true, shrink_delalloc() will not shrink delalloc bytes any more, in this case we should check whether some tickcts' requests can overcommit, if some can, we can satisfy them timely and directly.

I notice original btrfs codes before your patch "Btrfs: introduce ticketed enospc infrastructure" will have over_commit try when every flush_space() returns, so here we may also need to do it, thanks.

Xiaoguang Wang


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