raid5: only wakeup necessary threads

If there are not enough stripes to handle, we'd better not always
queue all available work_structs. If one worker can only handle small
or even none stripes, it will impact request merge and create lock
contention.

With this patch, the number of work_struct running will depend on
pending stripes number. Note: some statistics info used in the patch
are accessed without locking protection. This should doesn't matter,
we just try best to avoid queue unnecessary work_struct.

Signed-off-by: Shaohua Li <shli@fusionio.com>
Signed-off-by: NeilBrown <neilb@suse.de>

2 files changed, 38 insertions, 6 deletions

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 663a8e58d439..7ff4f252ca1a 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -77,6 +77,7 @@ static struct workqueue_struct *raid5_wq;
 #define BYPASS_THRESHOLD        1
 #define NR_HASH                 (PAGE_SIZE / sizeof(struct hlist_head))
 #define HASH_MASK               (NR_HASH - 1)
+#define MAX_STRIPE_BATCH        8
 
 static inline struct hlist_head *stripe_hash(struct r5conf *conf, sector_t sect)
 {
@@ -209,6 +210,7 @@ static void raid5_wakeup_stripe_thread(struct stripe_head *sh)
 {
         struct r5conf *conf = sh->raid_conf;
         struct r5worker_group *group;
+        int thread_cnt;
         int i, cpu = sh->cpu;
 
         if (!cpu_online(cpu)) {
@@ -220,6 +222,8 @@ static void raid5_wakeup_stripe_thread(struct stripe_head *sh)
                 struct r5worker_group *group;
                 group = conf->worker_groups + cpu_to_group(cpu);
                 list_add_tail(&sh->lru, &group->handle_list);
+                group->stripes_cnt++;
+                sh->group = group;
         }
 
         if (conf->worker_cnt_per_group == 0) {
@@ -229,8 +233,20 @@ static void raid5_wakeup_stripe_thread(struct stripe_head *sh)
 
         group = conf->worker_groups + cpu_to_group(sh->cpu);
 
-        for (i = 0; i < conf->worker_cnt_per_group; i++)
-                queue_work_on(sh->cpu, raid5_wq, &group->workers[i].work);
+        group->workers[0].working = true;
+        /* at least one worker should run to avoid race */
+        queue_work_on(sh->cpu, raid5_wq, &group->workers[0].work);
+
+        thread_cnt = group->stripes_cnt / MAX_STRIPE_BATCH - 1;
+        /* wakeup more workers */
+        for (i = 1; i < conf->worker_cnt_per_group && thread_cnt > 0; i++) {
+                if (group->workers[i].working == false) {
+                        group->workers[i].working = true;
+                        queue_work_on(sh->cpu, raid5_wq,
+                                      &group->workers[i].work);
+                        thread_cnt--;
+                }
+        }
 }
 
 static void do_release_stripe(struct r5conf *conf, struct stripe_head *sh)
@@ -589,6 +605,10 @@ get_active_stripe(struct r5conf *conf, sector_t sector,
                                     !test_bit(STRIPE_EXPANDING, &sh->state))
                                         BUG();
                                 list_del_init(&sh->lru);
+                                if (sh->group) {
+                                        sh->group->stripes_cnt--;
+                                        sh->group = NULL;
+                                }
                         }
                 }
         } while (sh == NULL);
@@ -4153,15 +4173,18 @@ static struct stripe_head *__get_priority_stripe(struct r5conf *conf, int group)
 {
         struct stripe_head *sh = NULL, *tmp;
         struct list_head *handle_list = NULL;
+        struct r5worker_group *wg = NULL;
 
         if (conf->worker_cnt_per_group == 0) {
                 handle_list = &conf->handle_list;
         } else if (group != ANY_GROUP) {
                 handle_list = &conf->worker_groups[group].handle_list;
+                wg = &conf->worker_groups[group];
         } else {
                 int i;
                 for (i = 0; i < conf->group_cnt; i++) {
                         handle_list = &conf->worker_groups[i].handle_list;
+                        wg = &conf->worker_groups[i];
                         if (!list_empty(handle_list))
                                 break;
                 }
@@ -4208,11 +4231,16 @@ static struct stripe_head *__get_priority_stripe(struct r5conf *conf, int group)
                         if (conf->bypass_count < 0)
                                 conf->bypass_count = 0;
                 }
+                wg = NULL;
         }
 
         if (!sh)
                 return NULL;
 
+        if (wg) {
+                wg->stripes_cnt--;
+                sh->group = NULL;
+        }
         list_del_init(&sh->lru);
         atomic_inc(&sh->count);
         BUG_ON(atomic_read(&sh->count) != 1);
@@ -4919,8 +4947,8 @@ static int  retry_aligned_read(struct r5conf *conf, struct bio *raid_bio)
         return handled;
 }
 
-#define MAX_STRIPE_BATCH 8
-static int handle_active_stripes(struct r5conf *conf, int group)
+static int handle_active_stripes(struct r5conf *conf, int group,
+                                 struct r5worker *worker)
 {
         struct stripe_head *batch[MAX_STRIPE_BATCH], *sh;
         int i, batch_size = 0;
@@ -4963,7 +4991,8 @@ static void raid5_do_work(struct work_struct *work)
 
                 released = release_stripe_list(conf);
 
-                batch_size = handle_active_stripes(conf, group_id);
+                batch_size = handle_active_stripes(conf, group_id, worker);
+                worker->working = false;
                 if (!batch_size && !released)
                         break;
                 handled += batch_size;
@@ -5025,7 +5054,7 @@ static void raid5d(struct md_thread *thread)
                         handled++;
                 }
 
-                batch_size = handle_active_stripes(conf, ANY_GROUP);
+                batch_size = handle_active_stripes(conf, ANY_GROUP, NULL);
                 if (!batch_size && !released)
                         break;
                 handled += batch_size;
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index 435b12d58165..2113ffa82c7a 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -213,6 +213,7 @@ struct stripe_head {
         enum reconstruct_states reconstruct_state;
         spinlock_t              stripe_lock;
         int                     cpu;
+        struct r5worker_group   *group;
         /**
          * struct stripe_operations
          * @target - STRIPE_OP_COMPUTE_BLK target
@@ -369,12 +370,14 @@ struct disk_info {
 struct r5worker {
         struct work_struct work;
         struct r5worker_group *group;
+        bool working;
 };
 
 struct r5worker_group {
         struct list_head handle_list;
         struct r5conf *conf;
         struct r5worker *workers;
+        int stripes_cnt;
 };
 
 struct r5conf {