blkcg: remove blkio_group->stats_lock

With recent plug merge updates, all non-percpu stat updates happen
under queue_lock making stats_lock unnecessary to synchronize stat
updates.  The only synchronization necessary is stat reading, which
can be done using u64_stats_sync instead.

This patch removes blkio_group->stats_lock and adds
blkio_group_stats->syncp for reader synchronization.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

2 files changed, 103 insertions, 109 deletions

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 80887bc3a049..b15a51711bca 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -156,7 +156,7 @@ static inline void blkio_update_group_iops(struct blkio_group *blkg,
 
 /*
  * Add to the appropriate stat variable depending on the request type.
- * This should be called with the blkg->stats_lock held.
+ * This should be called with queue_lock held.
  */
 static void blkio_add_stat(uint64_t *stat, uint64_t add, bool direction,
                                 bool sync)
@@ -174,7 +174,7 @@ static void blkio_add_stat(uint64_t *stat, uint64_t add, bool direction,
 /*
  * Decrements the appropriate stat variable if non-zero depending on the
  * request type. Panics on value being zero.
- * This should be called with the blkg->stats_lock held.
+ * This should be called with the queue_lock held.
  */
 static void blkio_check_and_dec_stat(uint64_t *stat, bool direction, bool sync)
 {
@@ -195,7 +195,7 @@ static void blkio_check_and_dec_stat(uint64_t *stat, bool direction, bool sync)
 }
 
 #ifdef CONFIG_DEBUG_BLK_CGROUP
-/* This should be called with the blkg->stats_lock held. */
+/* This should be called with the queue_lock held. */
 static void blkio_set_start_group_wait_time(struct blkio_group *blkg,
                                             struct blkio_policy_type *pol,
                                             struct blkio_group *curr_blkg)
@@ -210,7 +210,7 @@ static void blkio_set_start_group_wait_time(struct blkio_group *blkg,
         blkio_mark_blkg_waiting(&pd->stats);
 }
 
-/* This should be called with the blkg->stats_lock held. */
+/* This should be called with the queue_lock held. */
 static void blkio_update_group_wait_time(struct blkio_group_stats *stats)
 {
         unsigned long long now;
@@ -224,7 +224,7 @@ static void blkio_update_group_wait_time(struct blkio_group_stats *stats)
         blkio_clear_blkg_waiting(stats);
 }
 
-/* This should be called with the blkg->stats_lock held. */
+/* This should be called with the queue_lock held. */
 static void blkio_end_empty_time(struct blkio_group_stats *stats)
 {
         unsigned long long now;
@@ -241,84 +241,74 @@ static void blkio_end_empty_time(struct blkio_group_stats *stats)
 void blkiocg_update_set_idle_time_stats(struct blkio_group *blkg,
                                         struct blkio_policy_type *pol)
 {
-        struct blkg_policy_data *pd = blkg->pd[pol->plid];
-        unsigned long flags;
+        struct blkio_group_stats *stats = &blkg->pd[pol->plid]->stats;
 
-        spin_lock_irqsave(&blkg->stats_lock, flags);
-        BUG_ON(blkio_blkg_idling(&pd->stats));
-        pd->stats.start_idle_time = sched_clock();
-        blkio_mark_blkg_idling(&pd->stats);
-        spin_unlock_irqrestore(&blkg->stats_lock, flags);
+        lockdep_assert_held(blkg->q->queue_lock);
+        BUG_ON(blkio_blkg_idling(stats));
+
+        stats->start_idle_time = sched_clock();
+        blkio_mark_blkg_idling(stats);
 }
 EXPORT_SYMBOL_GPL(blkiocg_update_set_idle_time_stats);
 
 void blkiocg_update_idle_time_stats(struct blkio_group *blkg,
                                     struct blkio_policy_type *pol)
 {
-        struct blkg_policy_data *pd = blkg->pd[pol->plid];
-        unsigned long flags;
-        unsigned long long now;
-        struct blkio_group_stats *stats;
+        struct blkio_group_stats *stats = &blkg->pd[pol->plid]->stats;
+
+        lockdep_assert_held(blkg->q->queue_lock);
 
-        spin_lock_irqsave(&blkg->stats_lock, flags);
-        stats = &pd->stats;
         if (blkio_blkg_idling(stats)) {
-                now = sched_clock();
-                if (time_after64(now, stats->start_idle_time))
+                unsigned long long now = sched_clock();
+
+                if (time_after64(now, stats->start_idle_time)) {
+                        u64_stats_update_begin(&stats->syncp);
                         stats->idle_time += now - stats->start_idle_time;
+                        u64_stats_update_end(&stats->syncp);
+                }
                 blkio_clear_blkg_idling(stats);
         }
-        spin_unlock_irqrestore(&blkg->stats_lock, flags);
 }
 EXPORT_SYMBOL_GPL(blkiocg_update_idle_time_stats);
 
 void blkiocg_update_avg_queue_size_stats(struct blkio_group *blkg,
                                          struct blkio_policy_type *pol)
 {
-        struct blkg_policy_data *pd = blkg->pd[pol->plid];
-        unsigned long flags;
-        struct blkio_group_stats *stats;
+        struct blkio_group_stats *stats = &blkg->pd[pol->plid]->stats;
 
-        spin_lock_irqsave(&blkg->stats_lock, flags);
-        stats = &pd->stats;
+        lockdep_assert_held(blkg->q->queue_lock);
+
+        u64_stats_update_begin(&stats->syncp);
         stats->avg_queue_size_sum +=
                         stats->stat_arr[BLKIO_STAT_QUEUED][BLKIO_STAT_READ] +
                         stats->stat_arr[BLKIO_STAT_QUEUED][BLKIO_STAT_WRITE];
         stats->avg_queue_size_samples++;
         blkio_update_group_wait_time(stats);
-        spin_unlock_irqrestore(&blkg->stats_lock, flags);
+        u64_stats_update_end(&stats->syncp);
 }
 EXPORT_SYMBOL_GPL(blkiocg_update_avg_queue_size_stats);
 
 void blkiocg_set_start_empty_time(struct blkio_group *blkg,
                                   struct blkio_policy_type *pol)
 {
-        struct blkg_policy_data *pd = blkg->pd[pol->plid];
-        unsigned long flags;
-        struct blkio_group_stats *stats;
+        struct blkio_group_stats *stats = &blkg->pd[pol->plid]->stats;
 
-        spin_lock_irqsave(&blkg->stats_lock, flags);
-        stats = &pd->stats;
+        lockdep_assert_held(blkg->q->queue_lock);
 
         if (stats->stat_arr[BLKIO_STAT_QUEUED][BLKIO_STAT_READ] ||
-                        stats->stat_arr[BLKIO_STAT_QUEUED][BLKIO_STAT_WRITE]) {
-                spin_unlock_irqrestore(&blkg->stats_lock, flags);
+                        stats->stat_arr[BLKIO_STAT_QUEUED][BLKIO_STAT_WRITE])
                 return;
-        }
 
         /*
          * group is already marked empty. This can happen if cfqq got new
          * request in parent group and moved to this group while being added
          * to service tree. Just ignore the event and move on.
          */
-        if(blkio_blkg_empty(stats)) {
-                spin_unlock_irqrestore(&blkg->stats_lock, flags);
+        if (blkio_blkg_empty(stats))
                 return;
-        }
 
         stats->start_empty_time = sched_clock();
         blkio_mark_blkg_empty(stats);
-        spin_unlock_irqrestore(&blkg->stats_lock, flags);
 }
 EXPORT_SYMBOL_GPL(blkiocg_set_start_empty_time);
 
@@ -328,6 +318,8 @@ void blkiocg_update_dequeue_stats(struct blkio_group *blkg,
 {
         struct blkg_policy_data *pd = blkg->pd[pol->plid];
 
+        lockdep_assert_held(blkg->q->queue_lock);
+
         pd->stats.dequeue += dequeue;
 }
 EXPORT_SYMBOL_GPL(blkiocg_update_dequeue_stats);
@@ -343,15 +335,16 @@ void blkiocg_update_io_add_stats(struct blkio_group *blkg,
                                  struct blkio_group *curr_blkg, bool direction,
                                  bool sync)
 {
-        struct blkg_policy_data *pd = blkg->pd[pol->plid];
-        unsigned long flags;
+        struct blkio_group_stats *stats = &blkg->pd[pol->plid]->stats;
+
+        lockdep_assert_held(blkg->q->queue_lock);
+
+        u64_stats_update_begin(&stats->syncp);
+        blkio_add_stat(stats->stat_arr[BLKIO_STAT_QUEUED], 1, direction, sync);
+        blkio_end_empty_time(stats);
+        u64_stats_update_end(&stats->syncp);
 
-        spin_lock_irqsave(&blkg->stats_lock, flags);
-        blkio_add_stat(pd->stats.stat_arr[BLKIO_STAT_QUEUED], 1, direction,
-                        sync);
-        blkio_end_empty_time(&pd->stats);
         blkio_set_start_group_wait_time(blkg, pol, curr_blkg);
-        spin_unlock_irqrestore(&blkg->stats_lock, flags);
 }
 EXPORT_SYMBOL_GPL(blkiocg_update_io_add_stats);
 
@@ -359,13 +352,14 @@ void blkiocg_update_io_remove_stats(struct blkio_group *blkg,
                                     struct blkio_policy_type *pol,
                                     bool direction, bool sync)
 {
-        struct blkg_policy_data *pd = blkg->pd[pol->plid];
-        unsigned long flags;
+        struct blkio_group_stats *stats = &blkg->pd[pol->plid]->stats;
 
-        spin_lock_irqsave(&blkg->stats_lock, flags);
-        blkio_check_and_dec_stat(pd->stats.stat_arr[BLKIO_STAT_QUEUED],
-                                        direction, sync);
-        spin_unlock_irqrestore(&blkg->stats_lock, flags);
+        lockdep_assert_held(blkg->q->queue_lock);
+
+        u64_stats_update_begin(&stats->syncp);
+        blkio_check_and_dec_stat(stats->stat_arr[BLKIO_STAT_QUEUED], direction,
+                                 sync);
+        u64_stats_update_end(&stats->syncp);
 }
 EXPORT_SYMBOL_GPL(blkiocg_update_io_remove_stats);
 
@@ -374,15 +368,16 @@ void blkiocg_update_timeslice_used(struct blkio_group *blkg,
                                    unsigned long time,
                                    unsigned long unaccounted_time)
 {
-        struct blkg_policy_data *pd = blkg->pd[pol->plid];
-        unsigned long flags;
+        struct blkio_group_stats *stats = &blkg->pd[pol->plid]->stats;
+
+        lockdep_assert_held(blkg->q->queue_lock);
 
-        spin_lock_irqsave(&blkg->stats_lock, flags);
-        pd->stats.time += time;
+        u64_stats_update_begin(&stats->syncp);
+        stats->time += time;
 #ifdef CONFIG_DEBUG_BLK_CGROUP
-        pd->stats.unaccounted_time += unaccounted_time;
+        stats->unaccounted_time += unaccounted_time;
 #endif
-        spin_unlock_irqrestore(&blkg->stats_lock, flags);
+        u64_stats_update_end(&stats->syncp);
 }
 EXPORT_SYMBOL_GPL(blkiocg_update_timeslice_used);
 
@@ -428,20 +423,19 @@ void blkiocg_update_completion_stats(struct blkio_group *blkg,
                                      uint64_t io_start_time, bool direction,
                                      bool sync)
 {
-        struct blkg_policy_data *pd = blkg->pd[pol->plid];
-        struct blkio_group_stats *stats;
-        unsigned long flags;
+        struct blkio_group_stats *stats = &blkg->pd[pol->plid]->stats;
         unsigned long long now = sched_clock();
 
-        spin_lock_irqsave(&blkg->stats_lock, flags);
-        stats = &pd->stats;
+        lockdep_assert_held(blkg->q->queue_lock);
+
+        u64_stats_update_begin(&stats->syncp);
         if (time_after64(now, io_start_time))
                 blkio_add_stat(stats->stat_arr[BLKIO_STAT_SERVICE_TIME],
                                 now - io_start_time, direction, sync);
         if (time_after64(io_start_time, start_time))
                 blkio_add_stat(stats->stat_arr[BLKIO_STAT_WAIT_TIME],
                                 io_start_time - start_time, direction, sync);
-        spin_unlock_irqrestore(&blkg->stats_lock, flags);
+        u64_stats_update_end(&stats->syncp);
 }
 EXPORT_SYMBOL_GPL(blkiocg_update_completion_stats);
 
@@ -450,14 +444,13 @@ void blkiocg_update_io_merged_stats(struct blkio_group *blkg,
                                     struct blkio_policy_type *pol,
                                     bool direction, bool sync)
 {
-        struct blkg_policy_data *pd = blkg->pd[pol->plid];
-        struct blkio_group_stats *stats;
-        unsigned long flags;
+        struct blkio_group_stats *stats = &blkg->pd[pol->plid]->stats;
 
-        spin_lock_irqsave(&blkg->stats_lock, flags);
-        stats = &pd->stats;
+        lockdep_assert_held(blkg->q->queue_lock);
+
+        u64_stats_update_begin(&stats->syncp);
         blkio_add_stat(stats->stat_arr[BLKIO_STAT_MERGED], 1, direction, sync);
-        spin_unlock_irqrestore(&blkg->stats_lock, flags);
+        u64_stats_update_end(&stats->syncp);
 }
 EXPORT_SYMBOL_GPL(blkiocg_update_io_merged_stats);
 
@@ -558,7 +551,6 @@ static struct blkio_group *blkg_alloc(struct blkio_cgroup *blkcg,
         if (!blkg)
                 return NULL;
 
-        spin_lock_init(&blkg->stats_lock);
         blkg->q = q;
         INIT_LIST_HEAD(&blkg->q_node);
         INIT_LIST_HEAD(&blkg->alloc_node);
@@ -929,7 +921,6 @@ static uint64_t blkio_get_stat_cpu(struct blkio_group *blkg, int plid,
         return disk_total;
 }
 
-/* This should be called with blkg->stats_lock held */
 static uint64_t blkio_get_stat(struct blkio_group *blkg, int plid,
                                struct cgroup_map_cb *cb, const char *dname,
                                enum stat_type type)
@@ -937,42 +928,46 @@ static uint64_t blkio_get_stat(struct blkio_group *blkg, int plid,
         struct blkio_group_stats *stats = &blkg->pd[plid]->stats;
         uint64_t v = 0, disk_total = 0;
         char key_str[MAX_KEY_LEN];
+        unsigned int sync_start;
         int st;
 
         if (type >= BLKIO_STAT_ARR_NR) {
-                switch (type) {
-                case BLKIO_STAT_TIME:
-                        v = stats->time;
-                        break;
+                do {
+                        sync_start = u64_stats_fetch_begin(&stats->syncp);
+                        switch (type) {
+                        case BLKIO_STAT_TIME:
+                                v = stats->time;
+                                break;
 #ifdef CONFIG_DEBUG_BLK_CGROUP
-                case BLKIO_STAT_UNACCOUNTED_TIME:
-                        v = stats->unaccounted_time;
-                        break;
-                case BLKIO_STAT_AVG_QUEUE_SIZE: {
-                        uint64_t samples = stats->avg_queue_size_samples;
+                        case BLKIO_STAT_UNACCOUNTED_TIME:
+                                v = stats->unaccounted_time;
+                                break;
+                        case BLKIO_STAT_AVG_QUEUE_SIZE: {
+                                uint64_t samples = stats->avg_queue_size_samples;
 
-                        if (samples) {
-                                v = stats->avg_queue_size_sum;
-                                do_div(v, samples);
+                                if (samples) {
+                                        v = stats->avg_queue_size_sum;
+                                        do_div(v, samples);
+                                }
+                                break;
                         }
-                        break;
-                }
-                case BLKIO_STAT_IDLE_TIME:
-                        v = stats->idle_time;
-                        break;
-                case BLKIO_STAT_EMPTY_TIME:
-                        v = stats->empty_time;
-                        break;
-                case BLKIO_STAT_DEQUEUE:
-                        v = stats->dequeue;
-                        break;
-                case BLKIO_STAT_GROUP_WAIT_TIME:
-                        v = stats->group_wait_time;
-                        break;
+                        case BLKIO_STAT_IDLE_TIME:
+                                v = stats->idle_time;
+                                break;
+                        case BLKIO_STAT_EMPTY_TIME:
+                                v = stats->empty_time;
+                                break;
+                        case BLKIO_STAT_DEQUEUE:
+                                v = stats->dequeue;
+                                break;
+                        case BLKIO_STAT_GROUP_WAIT_TIME:
+                                v = stats->group_wait_time;
+                                break;
 #endif
-                default:
-                        WARN_ON_ONCE(1);
-                }
+                        default:
+                                WARN_ON_ONCE(1);
+                        }
+                } while (u64_stats_fetch_retry(&stats->syncp, sync_start));
 
                 blkio_get_key_name(0, dname, key_str, MAX_KEY_LEN, true);
                 cb->fill(cb, key_str, v);
@@ -980,7 +975,10 @@ static uint64_t blkio_get_stat(struct blkio_group *blkg, int plid,
         }
 
         for (st = BLKIO_STAT_READ; st < BLKIO_STAT_TOTAL; st++) {
-                v = stats->stat_arr[type][st];
+                do {
+                        sync_start = u64_stats_fetch_begin(&stats->syncp);
+                        v = stats->stat_arr[type][st];
+                } while (u64_stats_fetch_retry(&stats->syncp, sync_start));
 
                 blkio_get_key_name(st, dname, key_str, MAX_KEY_LEN, false);
                 cb->fill(cb, key_str, v);
@@ -1250,15 +1248,12 @@ static int blkio_read_blkg_stats(struct blkio_cgroup *blkcg,
 
                 if (!dname)
                         continue;
-                if (pcpu) {
+                if (pcpu)
                         cgroup_total += blkio_get_stat_cpu(blkg, plid,
                                                            cb, dname, type);
-                } else {
-                        spin_lock(&blkg->stats_lock);
+                else
                         cgroup_total += blkio_get_stat(blkg, plid,
                                                        cb, dname, type);
-                        spin_unlock(&blkg->stats_lock);
-                }
         }
         if (show_total)
                 cb->fill(cb, "Total", cgroup_total);
diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index 8bdcf504f94b..9df5ab04b6d6 100644
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -119,6 +119,7 @@ struct blkio_cgroup {
 };
 
 struct blkio_group_stats {
+        struct u64_stats_sync syncp;
         /* total disk time and nr sectors dispatched by this group */
         uint64_t time;
         uint64_t stat_arr[BLKIO_STAT_ARR_NR][BLKIO_STAT_TOTAL];
@@ -200,8 +201,6 @@ struct blkio_group {
         /* reference count */
         int refcnt;
 
-        /* Need to serialize the stats in the case of reset/update */
-        spinlock_t stats_lock;
         struct blkg_policy_data *pd[BLKIO_NR_POLICIES];
 
         /* List of blkg waiting for per cpu stats memory to be allocated */