ext4: drop s_mb_bal_lock and convert protected fields to atomic

s_mb_buddies_generated gets used later in this patch series to
determine if the cr 0 and cr 1 optimziations should be performed or
not. Currently, s_mb_buddies_generated is protected under a
spin_lock. In the allocation path, it is better if we don't depend on
the lock and instead read the value atomically. In order to do that,
we drop s_bal_lock altogether and we convert the only two protected
fields by it s_mb_buddies_generated and s_mb_generation_time to atomic
type.
Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Reviewed-by: Ritesh Harjani <ritesh.list@gmail.com>
Link: https://lore.kernel.org/r/20210401172129.189766-2-harshadshirwadkar@gmail.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/ext4.h

@@ -1553,9 +1553,8 @@ struct ext4_sb_info {
 	atomic_t s_bal_goals;	/* goal hits */
 	atomic_t s_bal_breaks;	/* too long searches */
 	atomic_t s_bal_2orders;	/* 2^order hits */
-	spinlock_t s_bal_lock;
-	unsigned long s_mb_buddies_generated;
-	unsigned long long s_mb_generation_time;
+	atomic_t s_mb_buddies_generated;	/* number of buddies generated */
+	atomic64_t s_mb_generation_time;
 	atomic_t s_mb_lost_chunks;
 	atomic_t s_mb_preallocated;
 	atomic_t s_mb_discarded;

fs/ext4/mballoc.c

@@ -816,10 +816,8 @@ void ext4_mb_generate_buddy(struct super_block *sb,
 	clear_bit(EXT4_GROUP_INFO_NEED_INIT_BIT, &(grp->bb_state));
 
 	period = get_cycles() - period;
-	spin_lock(&sbi->s_bal_lock);
-	sbi->s_mb_buddies_generated++;
-	sbi->s_mb_generation_time += period;
-	spin_unlock(&sbi->s_bal_lock);
+	atomic_inc(&sbi->s_mb_buddies_generated);
+	atomic64_add(period, &sbi->s_mb_generation_time);
 }
 
 /* The buddy information is attached the buddy cache inode
@@ -2850,7 +2848,6 @@ int ext4_mb_init(struct super_block *sb)
 	} while (i <= sb->s_blocksize_bits + 1);
 
 	spin_lock_init(&sbi->s_md_lock);
-	spin_lock_init(&sbi->s_bal_lock);
 	sbi->s_mb_free_pending = 0;
 	INIT_LIST_HEAD(&sbi->s_freed_data_list);
 
@@ -2986,9 +2983,9 @@ int ext4_mb_release(struct super_block *sb)
 				atomic_read(&sbi->s_bal_breaks),
 				atomic_read(&sbi->s_mb_lost_chunks));
 		ext4_msg(sb, KERN_INFO,
-		       "mballoc: %lu generated and it took %Lu",
-				sbi->s_mb_buddies_generated,
-				sbi->s_mb_generation_time);
+		       "mballoc: %u generated and it took %llu",
+				atomic_read(&sbi->s_mb_buddies_generated),
+				atomic64_read(&sbi->s_mb_generation_time));
 		ext4_msg(sb, KERN_INFO,
 		       "mballoc: %u preallocated, %u discarded",
 				atomic_read(&sbi->s_mb_preallocated),