btrfs: simplify the logic in need_preemptive_flushing

A lot of this was added all in one go with no explanation, and is a bit
unwieldy and confusing.  Simplify the logic to start preemptive flushing
if we've reserved more than half of our available free space.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
index 636a426..9befd22 100644 (file)
--- a/fs/btrfs/space-info.c
+++ b/fs/btrfs/space-info.c
@@ -786,11 +786,11 @@ btrfs_calc_reclaim_metadata_size(struct btrfs_fs_info *fs_info,
 }
 
 static bool need_preemptive_reclaim(struct btrfs_fs_info *fs_info,
-                                   struct btrfs_space_info *space_info,
-                                   u64 used)
+                                   struct btrfs_space_info *space_info)
 {
+       u64 ordered, delalloc;
        u64 thresh = div_factor_fine(space_info->total_bytes, 98);
-       u64 to_reclaim, expected;
+       u64 used;
 
        /* If we're just plain full then async reclaim just slows us down. */
        if ((space_info->bytes_used + space_info->bytes_reserved) >= thresh)
@@ -803,26 +803,52 @@ static bool need_preemptive_reclaim(struct btrfs_fs_info *fs_info,
        if (space_info->reclaim_size)
                return false;
 
-       to_reclaim = min_t(u64, num_online_cpus() * SZ_1M, SZ_16M);
-       if (btrfs_can_overcommit(fs_info, space_info, to_reclaim,
-                                BTRFS_RESERVE_FLUSH_ALL))
-               return false;
+       /*
+        * If we have over half of the free space occupied by reservations or
+        * pinned then we want to start flushing.
+        *
+        * We do not do the traditional thing here, which is to say
+        *
+        *   if (used >= ((total_bytes + avail) / 2))
+        *     return 1;
+        *
+        * because this doesn't quite work how we want.  If we had more than 50%
+        * of the space_info used by bytes_used and we had 0 available we'd just
+        * constantly run the background flusher.  Instead we want it to kick in
+        * if our reclaimable space exceeds 50% of our available free space.
+        */
+       thresh = calc_available_free_space(fs_info, space_info,
+                                          BTRFS_RESERVE_FLUSH_ALL);
+       thresh += (space_info->total_bytes - space_info->bytes_used -
+                  space_info->bytes_reserved - space_info->bytes_readonly);
+       thresh >>= 1;
 
-       used = btrfs_space_info_used(space_info, true);
-       if (btrfs_can_overcommit(fs_info, space_info, SZ_1M,
-                                BTRFS_RESERVE_FLUSH_ALL))
-               expected = div_factor_fine(space_info->total_bytes, 95);
-       else
-               expected = div_factor_fine(space_info->total_bytes, 90);
+       used = space_info->bytes_pinned;
 
-       if (used > expected)
-               to_reclaim = used - expected;
+       /*
+        * If we have more ordered bytes than delalloc bytes then we're either
+        * doing a lot of DIO, or we simply don't have a lot of delalloc waiting
+        * around.  Preemptive flushing is only useful in that it can free up
+        * space before tickets need to wait for things to finish.  In the case
+        * of ordered extents, preemptively waiting on ordered extents gets us
+        * nothing, if our reservations are tied up in ordered extents we'll
+        * simply have to slow down writers by forcing them to wait on ordered
+        * extents.
+        *
+        * In the case that ordered is larger than delalloc, only include the
+        * block reserves that we would actually be able to directly reclaim
+        * from.  In this case if we're heavy on metadata operations this will
+        * clearly be heavy enough to warrant preemptive flushing.  In the case
+        * of heavy DIO or ordered reservations, preemptive flushing will just
+        * waste time and cause us to slow down.
+        */
+       ordered = percpu_counter_sum_positive(&fs_info->ordered_bytes);
+       delalloc = percpu_counter_sum_positive(&fs_info->delalloc_bytes);
+       if (ordered >= delalloc)
+               used += fs_info->delayed_refs_rsv.reserved +
+                       fs_info->delayed_block_rsv.reserved;
        else
-               to_reclaim = 0;
-       to_reclaim = min(to_reclaim, space_info->bytes_may_use +
-                                    space_info->bytes_reserved);
-       if (!to_reclaim)
-               return false;
+               used += space_info->bytes_may_use;
 
        return (used >= thresh && !btrfs_fs_closing(fs_info) &&
                !test_bit(BTRFS_FS_STATE_REMOUNTING, &fs_info->fs_state));
@@ -1019,7 +1045,6 @@ static void btrfs_preempt_reclaim_metadata_space(struct work_struct *work)
        struct btrfs_block_rsv *delayed_refs_rsv;
        struct btrfs_block_rsv *global_rsv;
        struct btrfs_block_rsv *trans_rsv;
-       u64 used;
 
        fs_info = container_of(work, struct btrfs_fs_info,
                               preempt_reclaim_work);
@@ -1030,8 +1055,7 @@ static void btrfs_preempt_reclaim_metadata_space(struct work_struct *work)
        trans_rsv = &fs_info->trans_block_rsv;
 
        spin_lock(&space_info->lock);
-       used = btrfs_space_info_used(space_info, true);
-       while (need_preemptive_reclaim(fs_info, space_info, used)) {
+       while (need_preemptive_reclaim(fs_info, space_info)) {
                enum btrfs_flush_state flush;
                u64 delalloc_size = 0;
                u64 to_reclaim, block_rsv_size;
@@ -1092,7 +1116,6 @@ static void btrfs_preempt_reclaim_metadata_space(struct work_struct *work)
                flush_space(fs_info, space_info, to_reclaim, flush);
                cond_resched();
                spin_lock(&space_info->lock);
-               used = btrfs_space_info_used(space_info, true);
        }
        spin_unlock(&space_info->lock);
 }
@@ -1511,7 +1534,7 @@ static int __reserve_bytes(struct btrfs_fs_info *fs_info,
                 * the async reclaim as we will panic.
                 */
                if (!test_bit(BTRFS_FS_LOG_RECOVERING, &fs_info->flags) &&
-                   need_preemptive_reclaim(fs_info, space_info, used) &&
+                   need_preemptive_reclaim(fs_info, space_info) &&
                    !work_busy(&fs_info->preempt_reclaim_work)) {
                        trace_btrfs_trigger_flush(fs_info, space_info->flags,
                                                  orig_bytes, flush, "preempt");