[PATCH 01/12] btrfs: calculate discard delay based on number of extents

Dennis Zhou
2020-01-02T21:26:54Z

An earlier patch keeps track of discardable_extents. These are
undiscarded extents managed by the free space cache. Here, we will use
this to dynamically calculate the discard delay interval.

There are 3 rate to consider. The first is the target convergence rate,
the rate to discard all discardable_extents over the
BTRFS_DISCARD_TARGET_MSEC time frame. This is clamped by the lower
limit, the iops limit or BTRFS_DISCARD_MIN_DELAY (1ms), and the upper
limit, BTRFS_DISCARD_MAX_DELAY (1s). We reevaluate this delay every
transaction commit.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
---
fs/btrfs/ctree.h | 2 ++
fs/btrfs/discard.c | 55 +++++++++++++++++++++++++++++++++++++++---
fs/btrfs/discard.h | 1 +
fs/btrfs/extent-tree.c | 4 ++-
fs/btrfs/sysfs.c | 31 ++++++++++++++++++++++++
5 files changed, 88 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 7c1c236d13ae..c73bbc7e4491 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -468,6 +468,8 @@ struct btrfs_discard_ctl {
struct list_head discard_list[BTRFS_NR_DISCARD_LISTS];
atomic_t discardable_extents;
atomic64_t discardable_bytes;
+ unsigned long delay;
+ unsigned iops_limit;
};

/* delayed seq elem */
diff --git a/fs/btrfs/discard.c b/fs/btrfs/discard.c
index 173770bf8a2d..abcc3b2189d1 100644
--- a/fs/btrfs/discard.c
+++ b/fs/btrfs/discard.c
@@ -15,6 +15,12 @@
#define BTRFS_DISCARD_DELAY (120ULL * NSEC_PER_SEC)
#define BTRFS_DISCARD_UNUSED_DELAY (10ULL * NSEC_PER_SEC)

+/* Target completion latency of discarding all discardable extents */
+#define BTRFS_DISCARD_TARGET_MSEC (6 * 60 * 60UL * MSEC_PER_SEC)
+#define BTRFS_DISCARD_MIN_DELAY_MSEC (1UL)
+#define BTRFS_DISCARD_MAX_DELAY_MSEC (1000UL)
+#define BTRFS_DISCARD_MAX_IOPS (10U)
+
static struct list_head *get_discard_list(struct btrfs_discard_ctl *discard_ctl,
struct btrfs_block_group *block_group)
{
@@ -235,11 +241,18 @@ void btrfs_discard_schedule_work(struct btrfs_discard_ctl *discard_ctl,

block_group = find_next_block_group(discard_ctl, now);
if (block_group) {
- u64 delay = 0;
+ unsigned long delay = discard_ctl->delay;
+
+ /*
+ * This timeout is to hopefully prevent immediate discarding
+ * in a recently allocated block group.
+ */
+ if (now < block_group->discard_eligible_time) {
+ u64 bg_timeout = (block_group->discard_eligible_time -
+ now);

- if (now < block_group->discard_eligible_time)
- delay = nsecs_to_jiffies(
- block_group->discard_eligible_time - now);
+ delay = max(delay, nsecs_to_jiffies(bg_timeout));
+ }

mod_delayed_work(discard_ctl->discard_workers,
&discard_ctl->work, delay);
@@ -342,6 +355,38 @@ bool btrfs_run_discard_work(struct btrfs_discard_ctl *discard_ctl)
test_bit(BTRFS_FS_DISCARD_RUNNING, &fs_info->flags));
}

+/**
+ * btrfs_discard_calc_delay - recalculate the base delay
+ * @discard_ctl: discard control
+ *
+ * Recalculate the base delay which is based off the total number of
+ * discardable_extents. Clamp this between the lower_limit (iops_limit or 1ms)
+ * and the upper_limit (BTRFS_DISCARD_MAX_DELAY_MSEC).
+ */
+void btrfs_discard_calc_delay(struct btrfs_discard_ctl *discard_ctl)
+{
+ s32 discardable_extents =
+ atomic_read(&discard_ctl->discardable_extents);
+ unsigned iops_limit;
+ unsigned long delay, lower_limit = BTRFS_DISCARD_MIN_DELAY_MSEC;
+
+ if (!discardable_extents)
+ return;
+
+ spin_lock(&discard_ctl->lock);
+
+ iops_limit = READ_ONCE(discard_ctl->iops_limit);
+ if (iops_limit)
+ lower_limit = max_t(unsigned long, lower_limit,
+ MSEC_PER_SEC / iops_limit);
+
+ delay = BTRFS_DISCARD_TARGET_MSEC / discardable_extents;
+ delay = clamp(delay, lower_limit, BTRFS_DISCARD_MAX_DELAY_MSEC);
+ discard_ctl->delay = msecs_to_jiffies(delay);
+
+ spin_unlock(&discard_ctl->lock);
+}
+
/**
* btrfs_discard_update_discardable - propagate discard counters
* @block_group: block_group of interest
@@ -464,6 +509,8 @@ void btrfs_discard_init(struct btrfs_fs_info *fs_info)

atomic_set(&discard_ctl->discardable_extents, 0);
atomic64_set(&discard_ctl->discardable_bytes, 0);
+ discard_ctl->delay = BTRFS_DISCARD_MAX_DELAY_MSEC;
+ discard_ctl->iops_limit = BTRFS_DISCARD_MAX_IOPS;
}

void btrfs_discard_cleanup(struct btrfs_fs_info *fs_info)
diff --git a/fs/btrfs/discard.h b/fs/btrfs/discard.h
index 0f2f89b1b0b9..5250fe178e49 100644
--- a/fs/btrfs/discard.h
+++ b/fs/btrfs/discard.h
@@ -17,6 +17,7 @@ void btrfs_discard_schedule_work(struct btrfs_discard_ctl *discard_ctl,
bool btrfs_run_discard_work(struct btrfs_discard_ctl *discard_ctl);

/* Update operations */
+void btrfs_discard_calc_delay(struct btrfs_discard_ctl *discard_ctl);
void btrfs_discard_update_discardable(struct btrfs_block_group *block_group,
struct btrfs_free_space_ctl *ctl);

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 2c12366cfde5..0163fdd59f8f 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -2935,8 +2935,10 @@ int btrfs_finish_extent_commit(struct btrfs_trans_handle *trans)
cond_resched();
}

- if (btrfs_test_opt(fs_info, DISCARD_ASYNC))
+ if (btrfs_test_opt(fs_info, DISCARD_ASYNC)) {
+ btrfs_discard_calc_delay(&fs_info->discard_ctl);
btrfs_discard_schedule_work(&fs_info->discard_ctl, true);
+ }

/*
* Transaction is finished. We don't need the lock anymore. We
diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index e9dbdbbbebeb..e175aaf7a1e6 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -344,6 +344,36 @@ static const struct attribute_group btrfs_static_feature_attr_group = {
*/
#define discard_to_fs_info(_kobj) to_fs_info((_kobj)->parent->parent)

+static ssize_t btrfs_discard_iops_limit_show(struct kobject *kobj,
+ struct kobj_attribute *a,
+ char *buf)
+{
+ struct btrfs_fs_info *fs_info = discard_to_fs_info(kobj);
+
+ return snprintf(buf, PAGE_SIZE, "%u\n",
+ READ_ONCE(fs_info->discard_ctl.iops_limit));
+}
+
+static ssize_t btrfs_discard_iops_limit_store(struct kobject *kobj,
+ struct kobj_attribute *a,
+ const char *buf, size_t len)
+{
+ struct btrfs_fs_info *fs_info = discard_to_fs_info(kobj);
+ struct btrfs_discard_ctl *discard_ctl = &fs_info->discard_ctl;
+ unsigned iops_limit;
+ int ret;
+
+ ret = kstrtouint(buf, 10, &iops_limit);
+ if (ret)
+ return -EINVAL;
+
+ WRITE_ONCE(discard_ctl->iops_limit, iops_limit);
+
+ return len;
+}
+BTRFS_ATTR_RW(discard, iops_limit, btrfs_discard_iops_limit_show,
+ btrfs_discard_iops_limit_store);
+
static ssize_t btrfs_discardable_extents_show(struct kobject *kobj,
struct kobj_attribute *a,
char *buf)
@@ -367,6 +397,7 @@ static ssize_t btrfs_discardable_bytes_show(struct kobject *kobj,
BTRFS_ATTR(discard, discardable_bytes, btrfs_discardable_bytes_show);

static const struct attribute *discard_debug_attrs[] = {
+ BTRFS_ATTR_PTR(discard, iops_limit),
BTRFS_ATTR_PTR(discard, discardable_extents),
BTRFS_ATTR_PTR(discard, discardable_bytes),
NULL,
--
2.17.1