blkcg: make blkcg_[rw]stat per-cpu

blkcg_[rw]stat are used as stat counters for blkcg policies.  It isn't
per-cpu by itself and blk-throttle makes it per-cpu by wrapping around
it.  This patch makes blkcg_[rw]stat per-cpu and drop the ad-hoc
per-cpu wrapping in blk-throttle.

* blkg_[rw]stat->cnt is replaced with cpu_cnt which is struct
  percpu_counter.  This makes syncp unnecessary as remote accesses are
  handled by percpu_counter itself.

* blkg_[rw]stat_init() can now fail due to percpu allocation failure
  and thus are updated to return int.

* percpu_counters need explicit freeing.  blkg_[rw]stat_exit() added.

* As blkg_rwstat->cpu_cnt[] can't be read directly anymore, reading
  and summing results are stored in ->aux_cnt[] instead.

* Custom per-cpu stat implementation in blk-throttle is removed.

This makes all blkcg stat counters per-cpu without complicating policy
implmentations.

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

1 files changed, 28 insertions, 61 deletions

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 29c22ed4b073..c0b2263a222a 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -83,14 +83,6 @@ enum tg_state_flags {
 
 #define rb_entry_tg(node)       rb_entry((node), struct throtl_grp, rb_node)
 
-/* Per-cpu group stats */
-struct tg_stats_cpu {
-        /* total bytes transferred */
-        struct blkg_rwstat              service_bytes;
-        /* total IOs serviced, post merge */
-        struct blkg_rwstat              serviced;
-};
-
 struct throtl_grp {
         /* must be the first member */
         struct blkg_policy_data pd;
@@ -142,8 +134,10 @@ struct throtl_grp {
         unsigned long slice_start[2];
         unsigned long slice_end[2];
 
-        /* Per cpu stats pointer */
-        struct tg_stats_cpu __percpu *stats_cpu;
+        /* total bytes transferred */
+        struct blkg_rwstat              service_bytes;
+        /* total IOs serviced, post merge */
+        struct blkg_rwstat              serviced;
 };
 
 struct throtl_data
@@ -337,17 +331,15 @@ static void throtl_service_queue_init(struct throtl_service_queue *sq)
 static struct blkg_policy_data *throtl_pd_alloc(gfp_t gfp, int node)
 {
         struct throtl_grp *tg;
-        int rw, cpu;
+        int rw;
 
         tg = kzalloc_node(sizeof(*tg), gfp, node);
         if (!tg)
-                return NULL;
+                goto err;
 
-        tg->stats_cpu = alloc_percpu_gfp(struct tg_stats_cpu, gfp);
-        if (!tg->stats_cpu) {
-                kfree(tg);
-                return NULL;
-        }
+        if (blkg_rwstat_init(&tg->service_bytes, gfp) ||
+            blkg_rwstat_init(&tg->serviced, gfp))
+                goto err_free_tg;
 
         throtl_service_queue_init(&tg->service_queue);
 
@@ -362,14 +354,14 @@ static struct blkg_policy_data *throtl_pd_alloc(gfp_t gfp, int node)
         tg->iops[READ] = -1;
         tg->iops[WRITE] = -1;
 
-        for_each_possible_cpu(cpu) {
-                struct tg_stats_cpu *stats_cpu = per_cpu_ptr(tg->stats_cpu, cpu);
-
-                blkg_rwstat_init(&stats_cpu->service_bytes);
-                blkg_rwstat_init(&stats_cpu->serviced);
-        }
-
         return &tg->pd;
+
+err_free_tg:
+        blkg_rwstat_exit(&tg->serviced);
+        blkg_rwstat_exit(&tg->service_bytes);
+        kfree(tg);
+err:
+        return NULL;
 }
 
 static void throtl_pd_init(struct blkg_policy_data *pd)
@@ -427,21 +419,17 @@ static void throtl_pd_free(struct blkg_policy_data *pd)
         struct throtl_grp *tg = pd_to_tg(pd);
 
         del_timer_sync(&tg->service_queue.pending_timer);
-        free_percpu(tg->stats_cpu);
+        blkg_rwstat_exit(&tg->serviced);
+        blkg_rwstat_exit(&tg->service_bytes);
         kfree(tg);
 }
 
 static void throtl_pd_reset_stats(struct blkg_policy_data *pd)
 {
         struct throtl_grp *tg = pd_to_tg(pd);
-        int cpu;
 
-        for_each_possible_cpu(cpu) {
-                struct tg_stats_cpu *sc = per_cpu_ptr(tg->stats_cpu, cpu);
-
-                blkg_rwstat_reset(&sc->service_bytes);
-                blkg_rwstat_reset(&sc->serviced);
-        }
+        blkg_rwstat_reset(&tg->service_bytes);
+        blkg_rwstat_reset(&tg->serviced);
 }
 
 static struct throtl_grp *
@@ -855,7 +843,6 @@ static void throtl_update_dispatch_stats(struct blkcg_gq *blkg, u64 bytes,
                                          int rw)
 {
         struct throtl_grp *tg = blkg_to_tg(blkg);
-        struct tg_stats_cpu *stats_cpu;
         unsigned long flags;
 
         /*
@@ -865,10 +852,8 @@ static void throtl_update_dispatch_stats(struct blkcg_gq *blkg, u64 bytes,
          */
         local_irq_save(flags);
 
-        stats_cpu = this_cpu_ptr(tg->stats_cpu);
-
-        blkg_rwstat_add(&stats_cpu->serviced, rw, 1);
-        blkg_rwstat_add(&stats_cpu->service_bytes, rw, bytes);
+        blkg_rwstat_add(&tg->serviced, rw, 1);
+        blkg_rwstat_add(&tg->service_bytes, rw, bytes);
 
         local_irq_restore(flags);
 }
@@ -1176,27 +1161,9 @@ static void blk_throtl_dispatch_work_fn(struct work_struct *work)
         }
 }
 
-static u64 tg_prfill_cpu_rwstat(struct seq_file *sf,
-                                struct blkg_policy_data *pd, int off)
-{
-        struct throtl_grp *tg = pd_to_tg(pd);
-        struct blkg_rwstat rwstat = { }, tmp;
-        int i, cpu;
-
-        for_each_possible_cpu(cpu) {
-                struct tg_stats_cpu *sc = per_cpu_ptr(tg->stats_cpu, cpu);
-
-                tmp = blkg_rwstat_read((void *)sc + off);
-                for (i = 0; i < BLKG_RWSTAT_NR; i++)
-                        rwstat.cnt[i] += tmp.cnt[i];
-        }
-
-        return __blkg_prfill_rwstat(sf, pd, &rwstat);
-}
-
-static int tg_print_cpu_rwstat(struct seq_file *sf, void *v)
+static int tg_print_rwstat(struct seq_file *sf, void *v)
 {
-        blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)), tg_prfill_cpu_rwstat,
+        blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)), blkg_prfill_rwstat,
                           &blkcg_policy_throtl, seq_cft(sf)->private, true);
         return 0;
 }
@@ -1337,13 +1304,13 @@ static struct cftype throtl_files[] = {
         },
         {
                 .name = "throttle.io_service_bytes",
-                .private = offsetof(struct tg_stats_cpu, service_bytes),
-                .seq_show = tg_print_cpu_rwstat,
+                .private = offsetof(struct throtl_grp, service_bytes),
+                .seq_show = tg_print_rwstat,
         },
         {
                 .name = "throttle.io_serviced",
-                .private = offsetof(struct tg_stats_cpu, serviced),
-                .seq_show = tg_print_cpu_rwstat,
+                .private = offsetof(struct throtl_grp, serviced),
+                .seq_show = tg_print_rwstat,
         },
         { }     /* terminate */
 };