[PATCH v7 2/5] btrfs: create read policy framework

[Anand Jain <anand.jain@oracle.com>]

As of now we use %pid method to read stripped mirrored data, which means
process id determines the stripe id to be read. This type of routing
typically helps in a system with many small independent processes tying
to read random data. On the other hand the %pid based read IO policy is
inefficient because if there is a single process trying to read large
data the overall disk bandwidth remains under-utilized.

So this patch introduces read policy framework so that we could add more
read policies, such as IO routing based on device's wait-queue or manual
when we have a read-preferred device or a policy based on the target
storage caching.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
(rebased on 5.6)
v7: Fix missing /* fall through */ in the switch
    Removed Reviewed-by: Josef Bacik <josef@toxicpanda.com>
v6:-
v5: Title renamed from:- btrfs: add read_policy framework
    Change log updated.
    Unnecessary comment dropped, added more where necessary.
    Optimize code in the switch remove duplicate code.
    Define BTRFS_READ_POLICY_DEFAULT dropped.
    Rename enum btrfs_read_policy_type to enum btrfs_read_policy.
    Rename BTRFS_READ_BY_PID to BTRFS_READ_POLICY_PID.
    (As its mainly renames. Reviewed-by retained).
v4: -
v3: Declare fs_devices::readmirror as enum btrfs_readmirror_policy_type
v2: Declare fs_devices::readmirror as u8 instead of atomic_t 
    A small change in comment and change log wordings.

 fs/btrfs/volumes.c | 15 ++++++++++++++-
 fs/btrfs/volumes.h | 14 ++++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index c1909e5f4506..bafcf10f72ea 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1206,6 +1206,7 @@ static int open_fs_devices(struct btrfs_fs_devices *fs_devices,
 	fs_devices->latest_bdev = latest_dev->bdev;
 	fs_devices->total_rw_bytes = 0;
 	fs_devices->chunk_alloc_policy = BTRFS_CHUNK_ALLOC_REGULAR;
+	fs_devices->read_policy = BTRFS_READ_POLICY_PID;
 out:
 	return ret;
 }
@@ -5445,7 +5446,19 @@ static int find_live_mirror(struct btrfs_fs_info *fs_info,
 	else
 		num_stripes = map->num_stripes;
 
-	preferred_mirror = first + current->pid % num_stripes;
+	switch (fs_info->fs_devices->read_policy) {
+	default:
+		/*
+		 * Shouldn't happen, just warn and use pid instead of failing.
+		 */
+		btrfs_warn_rl(fs_info,
+			      "unknown read_policy type %u, fallback to pid",
+			      fs_info->fs_devices->read_policy);
+		/* fall through */
+	case BTRFS_READ_POLICY_PID:
+		preferred_mirror = first + current->pid % num_stripes;
+		break;
+	}
 
 	if (dev_replace_is_ongoing &&
 	    fs_info->dev_replace.cont_reading_from_srcdev_mode ==
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index f067b5934c46..f5ed864e4c5d 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -211,6 +211,15 @@ enum btrfs_chunk_allocation_policy {
 	BTRFS_CHUNK_ALLOC_REGULAR,
 };
 
+/*
+ * Read policies for the mirrored block groups, read picks the stripe based
+ * on these policies.
+ */
+enum btrfs_read_policy {
+	BTRFS_READ_POLICY_PID,
+	BTRFS_NR_READ_POLICY,
+};
+
 struct btrfs_fs_devices {
 	u8 fsid[BTRFS_FSID_SIZE]; /* FS specific uuid */
 	u8 metadata_uuid[BTRFS_FSID_SIZE];
@@ -264,6 +273,11 @@ struct btrfs_fs_devices {
 	struct completion kobj_unregister;
 
 	enum btrfs_chunk_allocation_policy chunk_alloc_policy;
+
+	/*
+	 * policy used to read the mirrored stripes
+	 */
+	enum btrfs_read_policy read_policy;
 };
 
 #define BTRFS_BIO_INLINE_CSUM_SIZE	64
-- 
2.23.0