ext4: Give symbolic names to mballoc criterias

mballoc criterias have historically been called by numbers
like CR0, CR1... however this makes it confusing to understand
what each criteria is about.

Change these criterias from numbers to symbolic names and add
relevant comments. While we are at it, also reformat and add some
comments to ext4_seq_mb_stats_show() for better readability.

Additionally, define CR_FAST which signifies the criteria
below which we can make quicker decisions like:
  * quitting early if (free block < requested len)
  * avoiding to scan free extents smaller than required len.
  * avoiding to initialize buddy cache and work with existing cache
  * limiting prefetches

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://lore.kernel.org/r/a2dc6ec5aea5e5e68cf8e788c2a964ffead9c8b0.1685449706.git.ojaswin@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 42 insertions, 13 deletions

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index fb9d2e2a9e42..6a1f013d23f7 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -135,16 +135,45 @@ enum SHIFT_DIRECTION {
  */
 #define EXT4_MB_NUM_CRS 5
 /*
- * All possible allocation criterias for mballoc
+ * All possible allocation criterias for mballoc. Lower are faster.
  */
 enum criteria {
-	CR0,
-	CR1,
-	CR1_5,
-	CR2,
-	CR3,
+	/*
+	 * Used when number of blocks needed is a power of 2. This doesn't
+	 * trigger any disk IO except prefetch and is the fastest criteria.
+	 */
+	CR_POWER2_ALIGNED,
+
+	/*
+	 * Tries to lookup in-memory data structures to find the most suitable
+	 * group that satisfies goal request. No disk IO except block prefetch.
+	 */
+	CR_GOAL_LEN_FAST,
+
+        /*
+	 * Same as CR_GOAL_LEN_FAST but is allowed to reduce the goal length to
+         * the best available length for faster allocation.
+	 */
+	CR_BEST_AVAIL_LEN,
+
+	/*
+	 * Reads each block group sequentially, performing disk IO if necessary, to
+	 * find find_suitable block group. Tries to allocate goal length but might trim
+	 * the request if nothing is found after enough tries.
+	 */
+	CR_GOAL_LEN_SLOW,
+
+	/*
+	 * Finds the first free set of blocks and allocates those. This is only
+	 * used in rare cases when CR_GOAL_LEN_SLOW also fails to allocate
+	 * anything.
+	 */
+	CR_ANY_FREE,
 };
 
+/* criteria below which we use fast block scanning and avoid unnecessary IO */
+#define CR_FAST CR_GOAL_LEN_SLOW
+
 /*
  * Flags used in mballoc's allocation_context flags field.
  *
@@ -183,11 +212,11 @@ enum criteria {
 /* Do strict check for free blocks while retrying block allocation */
 #define EXT4_MB_STRICT_CHECK		0x4000
 /* Large fragment size list lookup succeeded at least once for cr = 0 */
-#define EXT4_MB_CR0_OPTIMIZED		0x8000
+#define EXT4_MB_CR_POWER2_ALIGNED_OPTIMIZED		0x8000
 /* Avg fragment size rb tree lookup succeeded at least once for cr = 1 */
-#define EXT4_MB_CR1_OPTIMIZED		0x00010000
+#define EXT4_MB_CR_GOAL_LEN_FAST_OPTIMIZED		0x00010000
 /* Avg fragment size rb tree lookup succeeded at least once for cr = 1.5 */
-#define EXT4_MB_CR1_5_OPTIMIZED		0x00020000
+#define EXT4_MB_CR_BEST_AVAIL_LEN_OPTIMIZED		0x00020000
 
 struct ext4_allocation_request {
 	/* target inode for block we're allocating */
@@ -1553,7 +1582,7 @@ struct ext4_sb_info {
 	unsigned long s_mb_last_start;
 	unsigned int s_mb_prefetch;
 	unsigned int s_mb_prefetch_limit;
-	unsigned int s_mb_cr1_5_max_trim_order;
+	unsigned int s_mb_best_avail_max_trim_order;
 
 	/* stats for buddy allocator */
 	atomic_t s_bal_reqs;	/* number of reqs with len > 1 */
@@ -1566,9 +1595,9 @@ struct ext4_sb_info {
 	atomic_t s_bal_len_goals;	/* len goal hits */
 	atomic_t s_bal_breaks;	/* too long searches */
 	atomic_t s_bal_2orders;	/* 2^order hits */
-	atomic_t s_bal_cr0_bad_suggestions;
-	atomic_t s_bal_cr1_bad_suggestions;
-	atomic_t s_bal_cr1_5_bad_suggestions;
+	atomic_t s_bal_p2_aligned_bad_suggestions;
+	atomic_t s_bal_goal_fast_bad_suggestions;
+	atomic_t s_bal_best_avail_bad_suggestions;
 	atomic64_t s_bal_cX_groups_considered[EXT4_MB_NUM_CRS];
 	atomic64_t s_bal_cX_hits[EXT4_MB_NUM_CRS];
 	atomic64_t s_bal_cX_failed[EXT4_MB_NUM_CRS];		/* cX loop didn't find blocks */