ext4: add mb_debug logging when there are lost chunks

Lost chunks are when some other process raced with the current thread
to grab a particular block allocation.  Add mb_debug log for
developers who wants to see how often this is happening for a
particular workload.

Signed-off-by: Chunguang Xu <brookxu@tencent.com>
Link: https://lore.kernel.org/r/0a165ac0-1912-aebd-8a0d-b42e7cd1aea1@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 51e2fe6..0993044 100644 (file)
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2309,6 +2309,7 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
        struct ext4_sb_info *sbi;
        struct super_block *sb;
        struct ext4_buddy e4b;
+       unsigned int lost;
 
        sb = ac->ac_sb;
        sbi = EXT4_SB(sb);
@@ -2456,22 +2457,24 @@ repeat:
                 * We've been searching too long. Let's try to allocate
                 * the best chunk we've found so far
                 */
-
                ext4_mb_try_best_found(ac, &e4b);
                if (ac->ac_status != AC_STATUS_FOUND) {
                        /*
                         * Someone more lucky has already allocated it.
                         * The only thing we can do is just take first
                         * found block(s)
-                       printk(KERN_DEBUG "EXT4-fs: someone won our chunk\n");
                         */
+                       lost = (unsigned int)atomic_inc_return(&sbi->s_mb_lost_chunks);
+                       mb_debug(sb, "lost chunk, group: %u, start: %d, len: %d, lost: %u\n",
+                                ac->ac_b_ex.fe_group, ac->ac_b_ex.fe_start,
+                                ac->ac_b_ex.fe_len, lost);
+
                        ac->ac_b_ex.fe_group = 0;
                        ac->ac_b_ex.fe_start = 0;
                        ac->ac_b_ex.fe_len = 0;
                        ac->ac_status = AC_STATUS_CONTINUE;
                        ac->ac_flags |= EXT4_MB_HINT_FIRST;
                        cr = 3;
-                       atomic_inc(&sbi->s_mb_lost_chunks);
                        goto repeat;
                }
        }