[PATCH] aio: optimize free kioctx slot search

[Gu Zheng]

Add a 'next_free' field into kioctx_table to store the next id that
may be free, in order to reduce the search region when we insert
kioctx into kioctx table.
When we take a free slot from kioctx_table, update 'next_free' to
the next slot. Also when  we free one slot to kioctx_table, update 'next_free'
to the new free slot if it's smaller than 'next_free'. So that we can
ensure that the slots before 'next_free' are all used, and the search can start
from 'next_free' to reduce the search and improve the performance.

Signed-off-by: Gu Zheng <guz.fnst@xxxxxxxxxxxxxx>
---
 fs/aio.c |   10 ++++++++--
 1 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index 062a5f6..489fda6 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -69,6 +69,7 @@ struct aio_ring {
 struct kioctx_table {
 	struct rcu_head	rcu;
 	unsigned	nr;
+	unsigned	next_free;
 	struct kioctx	*table[];
 };
 
@@ -548,11 +549,12 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
 	table = rcu_dereference(mm->ioctx_table);
 
 	while (1) {
-		if (table)
-			for (i = 0; i < table->nr; i++)
+		if (table && table->next_free < table->nr)
+			for (i = table->next_free; i < table->nr; i++)
 				if (!table->table[i]) {
 					ctx->id = i;
 					table->table[i] = ctx;
+					table->next_free = i + 1;
 					rcu_read_unlock();
 					spin_unlock(&mm->ioctx_lock);
 
@@ -579,8 +581,10 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
 		old = rcu_dereference(mm->ioctx_table);
 
 		if (!old) {
+			table->next_free = 0;
 			rcu_assign_pointer(mm->ioctx_table, table);
 		} else if (table->nr > old->nr) {
+			table->next_free = old->nr;
 			memcpy(table->table, old->table,
 			       old->nr * sizeof(struct kioctx *));
 
@@ -716,6 +720,8 @@ static void kill_ioctx(struct mm_struct *mm, struct kioctx *ctx)
 
 		WARN_ON(ctx != table->table[ctx->id]);
 		table->table[ctx->id] = NULL;
+		if (ctx->id < table->next_free)
+			table->next_free = ctx->id;
 		rcu_read_unlock();
 		spin_unlock(&mm->ioctx_lock);
 
-- 
1.7.7

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