dm kcopyd: introduce configurable throttling

This patch allows the administrator to reduce the rate at which kcopyd
issues I/O.

Each module that uses kcopyd acquires a throttle parameter that can be
set in /sys/module/*/parameters.

We maintain a history of kcopyd usage by each module in the variables
io_period and total_period in struct dm_kcopyd_throttle. The actual
kcopyd activity is calculated as a percentage of time equal to
"(100 * io_period / total_period)".  This is compared with the user-defined
throttle percentage threshold and if it is exceeded, we sleep.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

4 files changed, 132 insertions, 4 deletions

diff --git a/drivers/md/dm-kcopyd.c b/drivers/md/dm-kcopyd.c
index 68c02673263b..d581fe5d2faf 100644
--- a/drivers/md/dm-kcopyd.c
+++ b/drivers/md/dm-kcopyd.c
@@ -22,6 +22,7 @@
 #include <linux/vmalloc.h>
 #include <linux/workqueue.h>
 #include <linux/mutex.h>
+#include <linux/delay.h>
 #include <linux/device-mapper.h>
 #include <linux/dm-kcopyd.h>
 
@@ -51,6 +52,8 @@ struct dm_kcopyd_client {
         struct workqueue_struct *kcopyd_wq;
         struct work_struct kcopyd_work;
 
+        struct dm_kcopyd_throttle *throttle;
+
 /*
  * We maintain three lists of jobs:
  *
@@ -68,6 +71,117 @@ struct dm_kcopyd_client {
 
 static struct page_list zero_page_list;
 
+static DEFINE_SPINLOCK(throttle_spinlock);
+
+/*
+ * IO/IDLE accounting slowly decays after (1 << ACCOUNT_INTERVAL_SHIFT) period.
+ * When total_period >= (1 << ACCOUNT_INTERVAL_SHIFT) the counters are divided
+ * by 2.
+ */
+#define ACCOUNT_INTERVAL_SHIFT          SHIFT_HZ
+
+/*
+ * Sleep this number of milliseconds.
+ *
+ * The value was decided experimentally.
+ * Smaller values seem to cause an increased copy rate above the limit.
+ * The reason for this is unknown but possibly due to jiffies rounding errors
+ * or read/write cache inside the disk.
+ */
+#define SLEEP_MSEC                      100
+
+/*
+ * Maximum number of sleep events. There is a theoretical livelock if more
+ * kcopyd clients do work simultaneously which this limit avoids.
+ */
+#define MAX_SLEEPS                      10
+
+static void io_job_start(struct dm_kcopyd_throttle *t)
+{
+        unsigned throttle, now, difference;
+        int slept = 0, skew;
+
+        if (unlikely(!t))
+                return;
+
+try_again:
+        spin_lock_irq(&throttle_spinlock);
+
+        throttle = ACCESS_ONCE(t->throttle);
+
+        if (likely(throttle >= 100))
+                goto skip_limit;
+
+        now = jiffies;
+        difference = now - t->last_jiffies;
+        t->last_jiffies = now;
+        if (t->num_io_jobs)
+                t->io_period += difference;
+        t->total_period += difference;
+
+        /*
+         * Maintain sane values if we got a temporary overflow.
+         */
+        if (unlikely(t->io_period > t->total_period))
+                t->io_period = t->total_period;
+
+        if (unlikely(t->total_period >= (1 << ACCOUNT_INTERVAL_SHIFT))) {
+                int shift = fls(t->total_period >> ACCOUNT_INTERVAL_SHIFT);
+                t->total_period >>= shift;
+                t->io_period >>= shift;
+        }
+
+        skew = t->io_period - throttle * t->total_period / 100;
+
+        if (unlikely(skew > 0) && slept < MAX_SLEEPS) {
+                slept++;
+                spin_unlock_irq(&throttle_spinlock);
+                msleep(SLEEP_MSEC);
+                goto try_again;
+        }
+
+skip_limit:
+        t->num_io_jobs++;
+
+        spin_unlock_irq(&throttle_spinlock);
+}
+
+static void io_job_finish(struct dm_kcopyd_throttle *t)
+{
+        unsigned long flags;
+
+        if (unlikely(!t))
+                return;
+
+        spin_lock_irqsave(&throttle_spinlock, flags);
+
+        t->num_io_jobs--;
+
+        if (likely(ACCESS_ONCE(t->throttle) >= 100))
+                goto skip_limit;
+
+        if (!t->num_io_jobs) {
+                unsigned now, difference;
+
+                now = jiffies;
+                difference = now - t->last_jiffies;
+                t->last_jiffies = now;
+
+                t->io_period += difference;
+                t->total_period += difference;
+
+                /*
+                 * Maintain sane values if we got a temporary overflow.
+                 */
+                if (unlikely(t->io_period > t->total_period))
+                        t->io_period = t->total_period;
+        }
+
+skip_limit:
+        spin_unlock_irqrestore(&throttle_spinlock, flags);
+}
+
+
 static void wake(struct dm_kcopyd_client *kc)
 {
         queue_work(kc->kcopyd_wq, &kc->kcopyd_work);
@@ -348,6 +462,8 @@ static void complete_io(unsigned long error, void *context)
         struct kcopyd_job *job = (struct kcopyd_job *) context;
         struct dm_kcopyd_client *kc = job->kc;
 
+        io_job_finish(kc->throttle);
+
         if (error) {
                 if (job->rw & WRITE)
                         job->write_err |= error;
@@ -389,6 +505,8 @@ static int run_io_job(struct kcopyd_job *job)
                 .client = job->kc->io_client,
         };
 
+        io_job_start(job->kc->throttle);
+
         if (job->rw == READ)
                 r = dm_io(&io_req, 1, &job->source, NULL);
         else
@@ -695,7 +813,7 @@ int kcopyd_cancel(struct kcopyd_job *job, int block)
 /*-----------------------------------------------------------------
  * Client setup
  *---------------------------------------------------------------*/
-struct dm_kcopyd_client *dm_kcopyd_client_create(void)
+struct dm_kcopyd_client *dm_kcopyd_client_create(struct dm_kcopyd_throttle *throttle)
 {
         int r = -ENOMEM;
         struct dm_kcopyd_client *kc;
@@ -708,6 +826,7 @@ struct dm_kcopyd_client *dm_kcopyd_client_create(void)
         INIT_LIST_HEAD(&kc->complete_jobs);
         INIT_LIST_HEAD(&kc->io_jobs);
         INIT_LIST_HEAD(&kc->pages_jobs);
+        kc->throttle = throttle;
 
         kc->job_pool = mempool_create_slab_pool(MIN_JOBS, _job_cache);
         if (!kc->job_pool)
diff --git a/drivers/md/dm-raid1.c b/drivers/md/dm-raid1.c
index e2ea97723e10..d053098c6a91 100644
--- a/drivers/md/dm-raid1.c
+++ b/drivers/md/dm-raid1.c
@@ -82,6 +82,9 @@ struct mirror_set {
         struct mirror mirror[0];
 };
 
+DECLARE_DM_KCOPYD_THROTTLE_WITH_MODULE_PARM(raid1_resync_throttle,
+                "A percentage of time allocated for raid resynchronization");
+
 static void wakeup_mirrord(void *context)
 {
         struct mirror_set *ms = context;
@@ -1111,7 +1114,7 @@ static int mirror_ctr(struct dm_target *ti, unsigned int argc, char **argv)
                 goto err_destroy_wq;
         }
 
-        ms->kcopyd_client = dm_kcopyd_client_create();
+        ms->kcopyd_client = dm_kcopyd_client_create(&dm_kcopyd_throttle);
         if (IS_ERR(ms->kcopyd_client)) {
                 r = PTR_ERR(ms->kcopyd_client);
                 goto err_destroy_wq;
diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index 58c2b5881377..c0e07026a8d1 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -124,6 +124,9 @@ struct dm_snapshot {
 #define RUNNING_MERGE          0
 #define SHUTDOWN_MERGE         1
 
+DECLARE_DM_KCOPYD_THROTTLE_WITH_MODULE_PARM(snapshot_copy_throttle,
+                "A percentage of time allocated for copy on write");
+
 struct dm_dev *dm_snap_origin(struct dm_snapshot *s)
 {
         return s->origin;
@@ -1108,7 +1111,7 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
                 goto bad_hash_tables;
         }
 
-        s->kcopyd_client = dm_kcopyd_client_create();
+        s->kcopyd_client = dm_kcopyd_client_create(&dm_kcopyd_throttle);
         if (IS_ERR(s->kcopyd_client)) {
                 r = PTR_ERR(s->kcopyd_client);
                 ti->error = "Could not create kcopyd client";
diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index 303e11da7c2a..35d9d0396cc2 100644
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -26,6 +26,9 @@
 #define PRISON_CELLS 1024
 #define COMMIT_PERIOD HZ
 
+DECLARE_DM_KCOPYD_THROTTLE_WITH_MODULE_PARM(snapshot_copy_throttle,
+                "A percentage of time allocated for copy on write");
+
 /*
  * The block size of the device holding pool data must be
  * between 64KB and 1GB.
@@ -1642,7 +1645,7 @@ static struct pool *pool_create(struct mapped_device *pool_md,
                 goto bad_prison;
         }
 
-        pool->copier = dm_kcopyd_client_create();
+        pool->copier = dm_kcopyd_client_create(&dm_kcopyd_throttle);
         if (IS_ERR(pool->copier)) {
                 r = PTR_ERR(pool->copier);
                 *error = "Error creating pool's kcopyd client";