[PATCH] improve kcopyd latency

[Mikulas Patocka <mpatocka@xxxxxxxxxx>]

Hi

I've made this patch that improves kcopyd latency. When we set the bit 
DM_KCOPYD_EARLY_CALLBACK, the completion is called from the interrupt 
context instead of process context.

I'd like to ask if you can benchmark it and test if it improves 
performance of dm-thin and dm-cache.

Mikulas



Index: linux-2.6/drivers/md/dm-cache-target.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-cache-target.c
+++ linux-2.6/drivers/md/dm-cache-target.c
@@ -1171,9 +1171,9 @@ static void copy(struct dm_cache_migrati
 	c_region.count = cache->sectors_per_block;
 
 	if (promote)
-		dm_kcopyd_copy(cache->copier, &o_region, 1, &c_region, 0, copy_complete, &mg->k);
+		dm_kcopyd_copy(cache->copier, &o_region, 1, &c_region, 1UL << DM_KCOPYD_EARLY_CALLBACK, copy_complete, &mg->k);
 	else
-		dm_kcopyd_copy(cache->copier, &c_region, 1, &o_region, 0, copy_complete, &mg->k);
+		dm_kcopyd_copy(cache->copier, &c_region, 1, &o_region, 1UL << DM_KCOPYD_EARLY_CALLBACK, copy_complete, &mg->k);
 }
 
 static void bio_drop_shared_lock(struct cache *cache, struct bio *bio)
Index: linux-2.6/drivers/md/dm-kcopyd.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-kcopyd.c
+++ linux-2.6/drivers/md/dm-kcopyd.c
@@ -512,6 +512,10 @@ static int run_complete_job(struct kcopy
 	return 0;
 }
 
+static void null_completion(int read_err, unsigned long write_err, void *context)
+{
+}
+
 static void complete_io(unsigned long error, void *context)
 {
 	struct kcopyd_job *job = (struct kcopyd_job *) context;
@@ -532,10 +536,13 @@ static void complete_io(unsigned long er
 		}
 	}
 
-	if (op_is_write(job->rw))
+	if (op_is_write(job->rw)) {
+		if (test_bit(DM_KCOPYD_EARLY_CALLBACK, &job->flags)) {
+			job->fn(job->read_err, job->write_err, job->context);
+			job->fn = null_completion;
+		}
 		push(&kc->complete_jobs, job);
-
-	else {
+	} else {
 		job->rw = WRITE;
 		push(&kc->io_jobs, job);
 	}
@@ -735,6 +742,7 @@ static void segment_complete(int read_er
 			sub_job->dests[i].count = count;
 		}
 
+		sub_job->flags &= ~(1UL << DM_KCOPYD_EARLY_CALLBACK);
 		sub_job->fn = segment_complete;
 		sub_job->context = sub_job;
 		dispatch_job(sub_job);
Index: linux-2.6/drivers/md/dm-thin.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-thin.c
+++ linux-2.6/drivers/md/dm-thin.c
@@ -1359,7 +1359,7 @@ static void schedule_copy(struct thin_c
 		to.count = len;
 
 		dm_kcopyd_copy(pool->copier, &from, 1, &to,
-			       0, copy_complete, m);
+			       1UL << DM_KCOPYD_EARLY_CALLBACK, copy_complete, m);
 
 		/*
 		 * Do we need to zero a tail region?
Index: linux-2.6/include/linux/dm-kcopyd.h
===================================================================
--- linux-2.6.orig/include/linux/dm-kcopyd.h
+++ linux-2.6/include/linux/dm-kcopyd.h
@@ -19,8 +19,9 @@
 /* FIXME: make this configurable */
 #define DM_KCOPYD_MAX_REGIONS 8
 
-#define DM_KCOPYD_IGNORE_ERROR 1
-#define DM_KCOPYD_WRITE_SEQ    2
+#define DM_KCOPYD_IGNORE_ERROR		1
+#define DM_KCOPYD_WRITE_SEQ		2
+#define DM_KCOPYD_EARLY_CALLBACK	3
 
 struct dm_kcopyd_throttle {
 	unsigned throttle;

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