Hi,
using the gcc option -fstack-usage I measured the stack usage and tried to get
rid of the worst offenders. The best improvement was in create_subvol where we
stored a 400B+ structure on stack, but otherwise it's not always clear why the
stack usage is that high. Most functions consume less than 300 bytes, and this
number can account for inlined functions or other invisible compiler
optimization magic.
A few samples of what's left:
scrub.c:3056:31:scrub_stripe 568 static
volumes.c:3984:12:btrfs_uuid_scan_kthread 536 static
scrub.c:2834:31:scrub_raid56_parity 384 static
ioctl.c:5225:12:btrfs_ioctl_set_fslabel 304 static
ioctl.c:1261:5:btrfs_defrag_file 304 static
tree-log.c:3593:21:copy_items 288 dynamic,bounded
ioctl.c:5202:12:btrfs_ioctl_get_fslabel 288 static
ioctl.c:3457:12:btrfs_clone 288 static
extent_io.c:2873:12:__do_readpage 288 static
file.c:691:5:__btrfs_drop_extents 272 static
file.c:2646:13:btrfs_fallocate 272 static
extent-tree.c:2469:21:__btrfs_run_delayed_refs 272 static
extent_io.c:3779:5:btree_write_cache_pages 272 static
extent_io.c:1730:6:extent_clear_unlock_delalloc 272 static
tree-log.c:4432:12:btrfs_log_inode 264 dynamic,bounded
tree-log.c:578:21:replay_one_extent 256 static
transaction.c:1322:21:create_pending_snapshot 256 static
ioctl.c:434:21:create_subvol 256 static
inode.c:1221:21:run_delalloc_nocow 256 static
extent_io.c:4145:5:extent_readpages 256 static
tree-log.c:4126:12:btrfs_log_changed_extents 248 dynamic,bounded
relocation.c:680:22:build_backref_tree 248 static
inode.c:4287:5:btrfs_truncate_inode_items 248 static
extent_io.c:3312:31:__extent_writepage_io 248 static
volumes.c:2948:12:insert_balance_item 240 static
relocation.c:1762:5:replace_path 240 static
...
More detailed info would be needed to decide whether it's worth to reshuffle
the stack variables, from what I've seen it would make the code readability
worse, so I've stopped.
----------------------------------------------------------------
The following changes since commit bb7ab3b92e46da06b580c6f83abe7894dc449cca:
btrfs: Fix misspellings in comments. (2016-03-14 15:05:02 +0100)
are available in the git repository at:
git://git.kernel.org/pub/scm/linux/kernel/git/kdave/linux.git dev/stack-reduce
for you to fetch changes up to 355ef15a6eafc0cafd49b049d7173040adb44f61:
btrfs: reuse existing variable in scrub_stripe, reduce stack usage (2016-03-24 18:06:34 +0100)
----------------------------------------------------------------
David Sterba (2):
btrfs: use dynamic allocation for root item in create_subvol
btrfs: reuse existing variable in scrub_stripe, reduce stack usage
fs/btrfs/ioctl.c | 49 ++++++++++++++++++++++++++-----------------------
fs/btrfs/scrub.c | 19 +++++++++----------
2 files changed, 35 insertions(+), 33 deletions(-)
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