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btrfs: avoid used space computation when reserving space

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In __reserve_bytes() we have 3 repeated calls to btrfs_space_info_used(),
one early on as soon as take the space_info's spinlock, another one when
we call btrfs_can_overcommit(), which calls btrfs_space_info_used() again,
and a final one when we are reserving for a flush emergency.

During all these calls we are holding the space_info's spinlock, which is
heavily used by the space reservation and flushing code, so it's desirable
to make the critical sections as short as possible.

So make this more efficient by:

1) Instead of calling btrfs_can_overcommit() call the new variant
   can_overcommit() which takes the space_info's used space as an argument
   and pass the value we already computed and have in the 'used' variable;

2) Instead of calling btrfs_space_info_used() with its second argument as
   false when we are doing a flush emergency, decrement the space_info's
   bytes_may_use counter from the 'used' variable, as the difference
   between passing true or false as the second argument to
   btrfs_space_info_used() is whether or not to include the space_info's
   bytes_may_use counter in the computation.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Filipe Manana
2025-10-17 16:10:04 +01:00
committed by David Sterba
parent a5f8f64aa3
commit 0ce6300fec
1 changed files with 2 additions and 2 deletions
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4
fs/btrfs/space-info.c
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@@ -1783,7 +1783,7 @@ static int __reserve_bytes(struct btrfs_space_info *space_info, u64 orig_bytes,
*/
if (!pending_tickets &&
((used + orig_bytes <= space_info->total_bytes) ||
btrfs_can_overcommit(space_info, orig_bytes, flush))) {
can_overcommit(space_info, used, orig_bytes, flush))) {
btrfs_space_info_update_bytes_may_use(space_info, orig_bytes);
ret = 0;
}
@@ -1794,7 +1794,7 @@ static int __reserve_bytes(struct btrfs_space_info *space_info, u64 orig_bytes,
* left to allocate for the block.
*/
if (ret && unlikely(flush == BTRFS_RESERVE_FLUSH_EMERGENCY)) {
used = btrfs_space_info_used(space_info, false);
used -= space_info->bytes_may_use;
if (used + orig_bytes <= space_info->total_bytes) {
btrfs_space_info_update_bytes_may_use(space_info, orig_bytes);
ret = 0;