Fix commit budget calculation with multiple holders

The try_drain_data_freed() path was generating errors about overrunning
its commit budget:

scoutfs f.2b8928.r.02689f error: 1 holders exceeded alloc budget av: bef 8185 now 8036, fr: bef 8185 now 7602

The budget overrun check was using the current number of commit holders
(in this case one) instead of the the maximum number of concurrent holders
(in this case two). So even well behaved paths like try_drain_data_freed()
can appear to exceed their commit budget if other holders dirty some blocks
and apply their commits before the try_drain_data_freed() thread does its
final budget reconciliation.

Signed-off-by: Chris Kirby <ckirby@versity.com>

kmod/src/alloc.c

@@ -86,18 +86,47 @@ static u64 smallest_order_length(u64 len)
 }
 
 /*
- * An extent modification dirties three distinct leaves of an allocator
- * btree as it adds and removes the blkno and size sorted items for the
- * old and new lengths of the extent.  Dirtying the paths to these
- * leaves can grow the tree and grow/shrink neighbours at each level.
- * We over-estimate the number of blocks allocated and freed (the paths
- * share a root, growth doesn't free) to err on the simpler and safer
- * side.  The overhead is minimal given the relatively large list blocks
- * and relatively short allocator trees.
+ * Moving an extent between trees can dirty blocks in several ways. This
+ * function calculates worst case number of blocks across these scenarions.
+ * We treat the alloc and free counts independently, so the values below are
+ * max(allocated, freed), not the sum.
+ *
+ * We track extents with two separate btree items: by block number and by size.
+ *
+ * If we're removing an extent from the btree (allocating), we can dirty
+ * two blocks if the keys are in different leaves. If we wind up merging
+ * leaves because we fall below the low water mark, we can wind up freeing
+ * three leaves.
+ *
+ * That sequence is as follows, assuming the original keys are removed from
+ * blocks A and B:
+ *
+ * Allocate new dirty A' and B'
+ * Free old stable A and B
+ * B' has fallen below the low water mark, so copy B' into A'
+ * Free B'
+ *
+ * An extent insertion (freeing an extent) can dirty up to five distinct items
+ * in the btree as it adds and removes the blkno and size sorted items for the
+ * old and new lengths of the extent:
+ *
+ * In the by-blkno portion of the btree, we can dirty (allocate for COW) up
+ * to two blocks- either by merging adjacent extents, which can cause us to
+ * join leaf blocks; or by an insertion that causes a split.
+ *
+ * In the by-size portion, we never merge extents, so normally we just dirty
+ * a single item with a size insertion. But if we merged adjacent extents in
+ * the by-blkno portion of the tree, we might be working with three by-sizex
+ * items: removing the two old ones that were combined in the merge; and
+ * adding the new one for the larger, merged size.
+ *
+ * Finally, dirtying the paths to these leaves can grow the tree and grow/shrink
+ * neighbours at each level, so we multiply by the height of the tree after
+ * accounting for a possible new level.
  */
 static u32 extent_mod_blocks(u32 height)
 {
-	return ((1 + height) * 2) * 3;
+	return ((1 + height) * 3) * 5;
 }
 
 /*

kmod/src/scoutfs_trace.h

@@ -1966,15 +1966,17 @@ DEFINE_EVENT(scoutfs_server_client_count_class, scoutfs_server_client_down,
 );
 
 DECLARE_EVENT_CLASS(scoutfs_server_commit_users_class,
-        TP_PROTO(struct super_block *sb, int holding, int applying, int nr_holders,
-		 u32 avail_before, u32 freed_before, int committing, int exceeded),
-        TP_ARGS(sb, holding, applying, nr_holders, avail_before, freed_before, committing,
-		exceeded),
+        TP_PROTO(struct super_block *sb, int holding, int applying,
+		 int nr_holders, u32 budget,
+		 u32 avail_before, u32 freed_before,
+		 int committing, int exceeded),
+        TP_ARGS(sb, holding, applying, nr_holders, budget, avail_before, freed_before, committing, exceeded),
         TP_STRUCT__entry(
 		SCSB_TRACE_FIELDS
 		__field(int, holding)
 		__field(int, applying)
 		__field(int, nr_holders)
+		__field(u32, budget)
 		__field(__u32, avail_before)
 		__field(__u32, freed_before)
 		__field(int, committing)
@@ -1985,35 +1987,45 @@ DECLARE_EVENT_CLASS(scoutfs_server_commit_users_class,
 		__entry->holding = !!holding;
 		__entry->applying = !!applying;
 		__entry->nr_holders = nr_holders;
+		__entry->budget = budget;
 		__entry->avail_before = avail_before;
 		__entry->freed_before = freed_before;
 		__entry->committing = !!committing;
 		__entry->exceeded = !!exceeded;
         ),
-	TP_printk(SCSBF" holding %u applying %u nr %u avail_before %u freed_before %u committing %u exceeded %u",
-		  SCSB_TRACE_ARGS, __entry->holding, __entry->applying, __entry->nr_holders,
-		  __entry->avail_before, __entry->freed_before, __entry->committing,
-		  __entry->exceeded)
+	TP_printk(SCSBF" holding %u applying %u nr %u budget %u avail_before %u freed_before %u committing %u exceeded %u",
+		  SCSB_TRACE_ARGS, __entry->holding, __entry->applying,
+		  __entry->nr_holders, __entry->budget,
+		  __entry->avail_before, __entry->freed_before,
+		  __entry->committing, __entry->exceeded)
 );
 DEFINE_EVENT(scoutfs_server_commit_users_class, scoutfs_server_commit_hold,
-        TP_PROTO(struct super_block *sb, int holding, int applying, int nr_holders,
-		 u32 avail_before, u32 freed_before, int committing, int exceeded),
-        TP_ARGS(sb, holding, applying, nr_holders, avail_before, freed_before, committing, exceeded)
+        TP_PROTO(struct super_block *sb, int holding, int applying,
+		 int nr_holders, u32 budget,
+		 u32 avail_before, u32 freed_before,
+		 int committing, int exceeded),
+        TP_ARGS(sb, holding, applying, nr_holders, budget, avail_before, freed_before, committing, exceeded)
 );
 DEFINE_EVENT(scoutfs_server_commit_users_class, scoutfs_server_commit_apply,
-        TP_PROTO(struct super_block *sb, int holding, int applying, int nr_holders,
-		 u32 avail_before, u32 freed_before, int committing, int exceeded),
-        TP_ARGS(sb, holding, applying, nr_holders, avail_before, freed_before, committing, exceeded)
+        TP_PROTO(struct super_block *sb, int holding, int applying,
+		 int nr_holders, u32 budget,
+		 u32 avail_before, u32 freed_before,
+		 int committing, int exceeded),
+        TP_ARGS(sb, holding, applying, nr_holders, budget, avail_before, freed_before, committing, exceeded)
 );
 DEFINE_EVENT(scoutfs_server_commit_users_class, scoutfs_server_commit_start,
-        TP_PROTO(struct super_block *sb, int holding, int applying, int nr_holders,
-		 u32 avail_before, u32 freed_before, int committing, int exceeded),
-        TP_ARGS(sb, holding, applying, nr_holders, avail_before, freed_before, committing, exceeded)
+        TP_PROTO(struct super_block *sb, int holding, int applying,
+		 int nr_holders, u32 budget,
+		 u32 avail_before, u32 freed_before,
+		 int committing, int exceeded),
+        TP_ARGS(sb, holding, applying, nr_holders, budget, avail_before, freed_before, committing, exceeded)
 );
 DEFINE_EVENT(scoutfs_server_commit_users_class, scoutfs_server_commit_end,
-        TP_PROTO(struct super_block *sb, int holding, int applying, int nr_holders,
-		 u32 avail_before, u32 freed_before, int committing, int exceeded),
-        TP_ARGS(sb, holding, applying, nr_holders, avail_before, freed_before, committing, exceeded)
+        TP_PROTO(struct super_block *sb, int holding, int applying,
+		 int nr_holders, u32 budget,
+		 u32 avail_before, u32 freed_before,
+		 int committing, int exceeded),
+        TP_ARGS(sb, holding, applying, nr_holders, budget, avail_before, freed_before, committing, exceeded)
 );
 
 #define slt_symbolic(mode)						\

kmod/src/server.c

@@ -65,6 +65,7 @@ struct commit_users {
 	struct list_head holding;
 	struct list_head applying;
 	unsigned int nr_holders;
+	u32 budget;
 	u32 avail_before;
 	u32 freed_before;
 	bool committing;
@@ -84,8 +85,9 @@ static void init_commit_users(struct commit_users *cusers)
 do {												\
 	__typeof__(cusers) _cusers = (cusers);							\
 	trace_scoutfs_server_commit_##which(sb, !list_empty(&_cusers->holding),			\
-		!list_empty(&_cusers->applying), _cusers->nr_holders, _cusers->avail_before,	\
-		_cusers->freed_before, _cusers->committing, _cusers->exceeded);			\
+		!list_empty(&_cusers->applying), _cusers->nr_holders, _cusers->budget,		\
+		_cusers->avail_before, _cusers->freed_before, _cusers->committing,		\
+		_cusers->exceeded);								\
 } while (0)
 
 struct server_info {
@@ -303,7 +305,6 @@ static void check_holder_budget(struct super_block *sb, struct server_info *serv
 	u32 freed_used;
 	u32 avail_now;
 	u32 freed_now;
-	u32 budget;
 
 	assert_spin_locked(&cusers->lock);
 
@@ -318,15 +319,14 @@ static void check_holder_budget(struct super_block *sb, struct server_info *serv
 	else
 		freed_used = SCOUTFS_ALLOC_LIST_MAX_BLOCKS - freed_now;
 
-	budget = cusers->nr_holders * COMMIT_HOLD_ALLOC_BUDGET;
-	if (avail_used <= budget && freed_used <= budget)
+	if (avail_used <= cusers->budget && freed_used <= cusers->budget)
 		return;
 
 	exceeded_once = true;
 	cusers->exceeded = cusers->nr_holders;
 
-	scoutfs_err(sb, "%u holders exceeded alloc budget av: bef %u now %u, fr: bef %u now %u",
-		    cusers->nr_holders, cusers->avail_before, avail_now,
+	scoutfs_err(sb, "holders exceeded alloc budget %u av: bef %u now %u, fr: bef %u now %u",
+		    cusers->budget, cusers->avail_before, avail_now,
 		    cusers->freed_before, freed_now);
 
 	list_for_each_entry(hold, &cusers->holding, entry) {
@@ -349,7 +349,7 @@ static bool hold_commit(struct super_block *sb, struct server_info *server,
 {
 	bool has_room;
 	bool held;
-	u32 budget;
+	u32 new_budget;
 	u32 av;
 	u32 fr;
 
@@ -367,8 +367,8 @@ static bool hold_commit(struct super_block *sb, struct server_info *server,
 	}
 
 	/* +2 for our additional hold and then for the final commit work the server does */
-	budget = (cusers->nr_holders + 2) * COMMIT_HOLD_ALLOC_BUDGET;
-	has_room = av >= budget && fr >= budget;
+	new_budget = max(cusers->budget, (cusers->nr_holders + 2) * COMMIT_HOLD_ALLOC_BUDGET);
+	has_room = av >= new_budget && fr >= new_budget;
 	/* checking applying so holders drain once an apply caller starts waiting */
 	held = !cusers->committing && has_room && list_empty(&cusers->applying);
 
@@ -388,6 +388,7 @@ static bool hold_commit(struct super_block *sb, struct server_info *server,
 		list_add_tail(&hold->entry, &cusers->holding);
 
 		cusers->nr_holders++;
+		cusers->budget = new_budget;
 
 	} else if (!has_room && cusers->nr_holders == 0 && !cusers->committing) {
 		cusers->committing = true;
@@ -516,6 +517,7 @@ static void commit_end(struct super_block *sb, struct commit_users *cusers, int
 	list_for_each_entry_safe(hold, tmp, &cusers->applying, entry)
 		list_del_init(&hold->entry);
 	cusers->committing = false;
+	cusers->budget = 0;
 	spin_unlock(&cusers->lock);
 
 	wake_up(&cusers->waitq);