md/raid456: distribute raid processing over multiple cores

Now that the resources to handle stripe_head operations are allocated
percpu it is possible for raid5d to distribute stripe handling over
multiple cores.  This conversion also adds a call to cond_resched() in
the non-multicore case to prevent one core from getting monopolized for
raid operations.

Cc: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/md/Kconfig

@@ -154,6 +154,17 @@ config MD_RAID456
 
 	  If unsure, say Y.
 
+config MULTICORE_RAID456
+	bool "RAID-4/RAID-5/RAID-6 Multicore processing (EXPERIMENTAL)"
+	depends on MD_RAID456
+	depends on SMP
+	depends on EXPERIMENTAL
+	---help---
+	  Enable the raid456 module to dispatch per-stripe raid operations to a
+	  thread pool.
+
+	  If unsure, say N.
+
 config MD_RAID6_PQ
 	tristate
 

drivers/md/raid5.c

@@ -47,6 +47,7 @@
 #include <linux/kthread.h>
 #include <linux/raid/pq.h>
 #include <linux/async_tx.h>
+#include <linux/async.h>
 #include <linux/seq_file.h>
 #include <linux/cpu.h>
 #include "md.h"
@@ -4314,6 +4315,36 @@ static int  retry_aligned_read(raid5_conf_t *conf, struct bio *raid_bio)
 	return handled;
 }
 
+#ifdef CONFIG_MULTICORE_RAID456
+static void __process_stripe(void *param, async_cookie_t cookie)
+{
+	struct stripe_head *sh = param;
+
+	handle_stripe(sh);
+	release_stripe(sh);
+}
+
+static void process_stripe(struct stripe_head *sh, struct list_head *domain)
+{
+	async_schedule_domain(__process_stripe, sh, domain);
+}
+
+static void synchronize_stripe_processing(struct list_head *domain)
+{
+	async_synchronize_full_domain(domain);
+}
+#else
+static void process_stripe(struct stripe_head *sh, struct list_head *domain)
+{
+	handle_stripe(sh);
+	release_stripe(sh);
+	cond_resched();
+}
+
+static void synchronize_stripe_processing(struct list_head *domain)
+{
+}
+#endif
 
 
 /*
@@ -4328,6 +4359,7 @@ static void raid5d(mddev_t *mddev)
 	struct stripe_head *sh;
 	raid5_conf_t *conf = mddev_to_conf(mddev);
 	int handled;
+	LIST_HEAD(raid_domain);
 
 	pr_debug("+++ raid5d active\n");
 
@@ -4364,15 +4396,15 @@ static void raid5d(mddev_t *mddev)
 		spin_unlock_irq(&conf->device_lock);
 
 		handled++;
-		handle_stripe(sh);
-		release_stripe(sh);
+		process_stripe(sh, &raid_domain);
 
 		spin_lock_irq(&conf->device_lock);
 	}
 	pr_debug("%d stripes handled\n", handled);
 
 	spin_unlock_irq(&conf->device_lock);
 
+	synchronize_stripe_processing(&raid_domain);
 	async_tx_issue_pending_all();
 	unplug_slaves(mddev);