[RFC PATCH v1] mm: improve call_controls_lock

[Asier Gutierrez]

This is a minor patch set for a call_controls_lock synchronization improvement.

Spinlocks are faster than mutexes, even when the mutex takes the fast
path. Hence, this patch replaces the mutex call_controls_lock with a spinlock.

Initial benchmarking shows the following results


# bpftrace -e 'kprobe:kdamond_call { @start[tid] = nsecs; }
    kretprobe:kdamond_call /@start[tid]/
    { @ns[comm] = hist(nsecs - @start[tid]); delete(@start[tid]); }'

@ns[kdamond.0]:
[16K, 32K)            22 |@@@@@@@@@@@@                                        |
[32K, 64K)            90 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[64K, 128K)            0 |                                                    |
[128K, 256K)           1 |                                                    |

@ns[kdamond.0]:
[16K, 32K)            19 |@@@@@@@@                                            |
[32K, 64K)           118 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[64K, 128K)            0 |                                                    |
[128K, 256K)           1 |

Signed-off-by: Asier Gutierrez <gutierrez.asier@xxxxxxxxxxxxxxxxxxx>
---
 include/linux/damon.h |  2 +-
 mm/damon/core.c       | 17 ++++++++---------
 2 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index 3813373a9200..43665e63a498 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -801,7 +801,7 @@ struct damon_ctx {
 
 	/* lists of &struct damon_call_control */
 	struct list_head call_controls;
-	struct mutex call_controls_lock;
+	spinlock_t call_controls_lock;
 
 	struct damos_walk_control *walk_control;
 	struct mutex walk_control_lock;
diff --git a/mm/damon/core.c b/mm/damon/core.c
index f9fc0375890a..a929bdf3bc7b 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -539,7 +539,6 @@ struct damon_ctx *damon_new_ctx(void)
 
 	mutex_init(&ctx->kdamond_lock);
 	INIT_LIST_HEAD(&ctx->call_controls);
-	mutex_init(&ctx->call_controls_lock);
 	mutex_init(&ctx->walk_control_lock);
 
 	ctx->attrs.min_nr_regions = 10;
@@ -1457,9 +1456,9 @@ int damon_call(struct damon_ctx *ctx, struct damon_call_control *control)
 	control->canceled = false;
 	INIT_LIST_HEAD(&control->list);
 
-	mutex_lock(&ctx->call_controls_lock);
+	spin_lock(&ctx->call_controls_lock);
 	list_add_tail(&control->list, &ctx->call_controls);
-	mutex_unlock(&ctx->call_controls_lock);
+	spin_unlock(&ctx->call_controls_lock);
 	if (!damon_is_running(ctx))
 		return -EINVAL;
 	if (control->repeat)
@@ -2549,10 +2548,10 @@ static void kdamond_call(struct damon_ctx *ctx, bool cancel)
 	int ret = 0;
 
 	while (true) {
-		mutex_lock(&ctx->call_controls_lock);
+		spin_lock(&ctx->call_controls_lock);
 		control = list_first_entry_or_null(&ctx->call_controls,
 				struct damon_call_control, list);
-		mutex_unlock(&ctx->call_controls_lock);
+		spin_unlock(&ctx->call_controls_lock);
 		if (!control)
 			break;
 		if (cancel) {
@@ -2561,9 +2560,9 @@ static void kdamond_call(struct damon_ctx *ctx, bool cancel)
 			ret = control->fn(control->data);
 			control->return_code = ret;
 		}
-		mutex_lock(&ctx->call_controls_lock);
+		spin_lock(&ctx->call_controls_lock);
 		list_del(&control->list);
-		mutex_unlock(&ctx->call_controls_lock);
+		spin_unlock(&ctx->call_controls_lock);
 		if (!control->repeat) {
 			complete(&control->completion);
 		} else if (control->canceled && control->dealloc_on_cancel) {
@@ -2577,9 +2576,9 @@ static void kdamond_call(struct damon_ctx *ctx, bool cancel)
 			struct damon_call_control, list);
 	if (!control || cancel)
 		return;
-	mutex_lock(&ctx->call_controls_lock);
+	spin_lock(&ctx->call_controls_lock);
 	list_add_tail(&control->list, &ctx->call_controls);
-	mutex_unlock(&ctx->call_controls_lock);
+	spin_unlock(&ctx->call_controls_lock);
 }
 
 /* Returns negative error code if it's not activated but should return */
-- 
2.43.0