This is a new tempt to make raid5 handle stripes in multiple threads, as
suggested by Neil to have maxium flexibility and better numa binding. It
basically is a combination of my first and second generation patches. By
default, no multiple thread is enabled (all stripes are handled by raid5d).

An example to enable multiple threads:
#echo 3 > /sys/block/md0/md/auxthread_number
This will create 3 auxiliary threads to handle stripes. The threads can run
on any cpus and handle stripes produced by any cpus.

#echo 1-3 > /sys/block/md0/md/auxth0/cpulist
This will bind auxiliary thread 0 to cpu 1-3, and this thread will only handle
stripes produced by cpu 1-3. User tool can further change the thread's
affinity, but the thread can only handle stripes produced by cpu 1-3 till the
sysfs entry is changed again.

If stripes produced by a CPU aren't handled by any auxiliary thread, such
stripes will be handled by raid5d. Otherwise, raid5d doesn't handle any
stripes.

Signed-off-by: Shaohua Li <redacted>
---
 drivers/md/md.c    |    8 -
 drivers/md/md.h    |    8 +
 drivers/md/raid5.c |  406 ++++++++++++++++++++++++++++++++++++++++++++++++++---
 drivers/md/raid5.h |   19 ++
 4 files changed, 418 insertions(+), 23 deletions(-)

Index: linux/drivers/md/raid5.c
===================================================================

--- linux.orig/drivers/md/raid5.c	2012-08-09 10:43:04.800022626 +0800
+++ linux/drivers/md/raid5.c	2012-08-09 16:44:39.663278511 +0800

@@ -196,6 +196,21 @@ static int stripe_operations_active(stru
 	       test_bit(STRIPE_COMPUTE_RUN, &sh->state);
 }
 
+static void raid5_wakeup_stripe_thread(struct stripe_head *sh)
+{
+	struct r5conf *conf = sh->raid_conf;
+	struct raid5_percpu *percpu;
+	int i, orphaned = 1;
+
+	percpu = per_cpu_ptr(conf->percpu, sh->cpu);
+	for_each_cpu(i, &percpu->handle_threads) {
+		md_wakeup_thread(conf->aux_threads[i]->thread);
+		orphaned = 0;
+	}
+	if (orphaned)
+		md_wakeup_thread(conf->mddev->thread);
+}
+
 static void do_release_stripe(struct r5conf *conf, struct stripe_head *sh)
 {
 	BUG_ON(!list_empty(&sh->lru));

@@ -208,9 +223,19 @@ static void do_release_stripe(struct r5c
 			   sh->bm_seq - conf->seq_write > 0)
 			list_add_tail(&sh->lru, &conf->bitmap_list);
 		else {
+			int cpu = sh->cpu;
+			struct raid5_percpu *percpu;
+			if (!cpu_online(cpu)) {
+				cpu = cpumask_any(cpu_online_mask);
+				sh->cpu = cpu;
+			}
+			percpu = per_cpu_ptr(conf->percpu, cpu);
+
 			clear_bit(STRIPE_DELAYED, &sh->state);
 			clear_bit(STRIPE_BIT_DELAY, &sh->state);
-			list_add_tail(&sh->lru, &conf->handle_list);
+			list_add_tail(&sh->lru, &percpu->handle_list);
+			raid5_wakeup_stripe_thread(sh);
+			return;

 		}
 		md_wakeup_thread(conf->mddev->thread);
 	} else {

@@ -355,6 +380,7 @@ static void init_stripe(struct stripe_he
 		raid5_build_block(sh, i, previous);
 	}
 	insert_hash(conf, sh);
+	sh->cpu = smp_processor_id();
 }
 
 static struct stripe_head *__find_stripe(struct r5conf *conf, sector_t sector,

@@ -3689,12 +3715,19 @@ static void raid5_activate_delayed(struc
 		while (!list_empty(&conf->delayed_list)) {
 			struct list_head *l = conf->delayed_list.next;
 			struct stripe_head *sh;
+			int cpu;
 			sh = list_entry(l, struct stripe_head, lru);
 			list_del_init(l);
 			clear_bit(STRIPE_DELAYED, &sh->state);
 			if (!test_and_set_bit(STRIPE_PREREAD_ACTIVE, &sh->state))
 				atomic_inc(&conf->preread_active_stripes);
 			list_add_tail(&sh->lru, &conf->hold_list);
+			cpu = sh->cpu;
+			if (!cpu_online(cpu)) {
+				cpu = cpumask_any(cpu_online_mask);
+				sh->cpu = cpu;
+			}
+			raid5_wakeup_stripe_thread(sh);