Btrfs: send, apply asynchronous page cache readahead to enhance page read

By analyzing the perf on btrfs send, we found it take large amount of
cpu time on page_cache_sync_readahead. This effort can be reduced after
switching to asynchronous one. Overall performance gain on HDD and SSD
were 9 and 15 percent if simply send a large file.

Signed-off-by: Kuanling Huang <peterh@synology.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
index 07445be..0746eda 100644 (file)
--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -4720,16 +4720,27 @@ static ssize_t fill_read_buf(struct send_ctx *sctx, u64 offset, u32 len)
        /* initial readahead */
        memset(&sctx->ra, 0, sizeof(struct file_ra_state));
        file_ra_state_init(&sctx->ra, inode->i_mapping);
-       page_cache_sync_readahead(inode->i_mapping, &sctx->ra, NULL, index,
-                      last_index - index + 1);
 
        while (index <= last_index) {
                unsigned cur_len = min_t(unsigned, len,
                                         PAGE_SIZE - pg_offset);
-               page = find_or_create_page(inode->i_mapping, index, GFP_KERNEL);
+
+               page = find_lock_page(inode->i_mapping, index);
                if (!page) {
-                       ret = -ENOMEM;
-                       break;
+                       page_cache_sync_readahead(inode->i_mapping, &sctx->ra,
+                               NULL, index, last_index + 1 - index);
+
+                       page = find_or_create_page(inode->i_mapping, index,
+                                       GFP_KERNEL);
+                       if (!page) {
+                               ret = -ENOMEM;
+                               break;
+                       }
+               }
+
+               if (PageReadahead(page)) {
+                       page_cache_async_readahead(inode->i_mapping, &sctx->ra,
+                               NULL, page, index, last_index + 1 - index);
                }
 
                if (!PageUptodate(page)) {