[PATCHSET RFC block/for-next] writeback: cgroup writeback support
From: Tejun Heo
Date: Tue Jan 06 2015 - 16:26:33 EST
Hello,
blkio cgroup (blkcg) is severely crippled in that it can only control
read and direct write IOs. blkcg can't tell which cgroup should be
held responsible for a given writeback IO and charges all of them to
the root cgroup - all normal write traffic ends up in the root cgroup.
Although the problem has been identified years ago, mainly because it
interacts with so many subsystems, it hasn't been solved yet.
This patchset finally implements cgroup writeback support so that
writeback of a page is attributed to the corresponding blkcg of the
memcg that the page belongs to.
Overall design
--------------
* This requires cooperation between memcg and blkcg. The IOs are
charged to the blkcg that the page's memcg corresponds to. This
currently works only on the unified hierarchy.
* Each memcg maintains reference counted front and back pointers to
the correspending blkcg. Whenever a page gets dirtied or initiates
writeback, it uses the blkcg the front one points to. The reference
counting ensures that the association remains till the page is done
and having front and back pointers guarantees that the association
can change without being live-locked by pages being contiuously
dirtied.
* struct bdi_writeback (wb) was always embedded in struct
backing_dev_info (bdi) and the distinction between the two wasn't
clear. This patchset makes wb operate as an independent writeback
execution. bdi->wb is still embedded and serves the root cgroup but
other wb's can be associated with a single bdi each serving a
non-root wb.
* All writeback operations are made per-wb instead of per-bdi.
bdi-wide operations are split across all member wb's. If some
finite amount needs to be distributed, be it number of pages to
writeback or bdi->min/max_ratio, it's distributed according to the
bandwidth proportion a wb has in the bdi.
* Non-root wb's host and write back only dirty pages (I_DIRTY_PAGES).
I_DIRTY_[DATA]SYNC is always handled by the root wb.
* An inode may have pages dirtied by different memcgs, which naturally
means that it should be able to be dirtied against multiple wb's.
To support linking an inode against multiple wb's, iwbl
(inode_wb_link) is introduced. An inode has multiple iwbl's
associated with it if it's dirty against multiple wb's.
* Overall, cgroup writeback support adds 2.5 pointers to struct inode
where the 0.5 is masked by alignment if !CONFIG_IMA.
Missing pieces
--------------
* It requires some cooperation from the filesystem and currently only
works with ext2. The changes necessary on the filesystem side
aren't too big. I'll write up a documentation on it.
* When an inode has multiple iwbls, they're put on a sorted list.
Depending on the usage, this list can grow quite long. We really
want an RCU-safe balanced tree here, which doesn't exist in the
kernel yet. Bonsai tree should do.
* balance_dirty_pages currently doesn't consider the task's memcg when
calculating the number of dirtyable pages. This means that tasks in
memcg won't have the benefit of smooth background writeback and will
bump into direct reclaim all the time. This has always been like
this but with cgroup writeback support, this is also finally
fixable. I'll work on this as the earlier part gets settled.
* balance_dirty_pages sleeps while holding i_mutex, which means that
when an inode is being dirtied actively by multiple cgroups, the
slowest writeback will choke others by sleeping most of the time
while holding i_mutex. Once the above per-memcg dirty ratio issue
is solved, this can be worked around by deferring the actual pausing
to right before control returns to userland. This would lead to
partial write completions when the inode is contended for writes by
multiple cgroups.
* blk-throttle works but cfq-iosched isn't ready for writebacks coming
down with different cgroups. cfq-iosched should be updated to have
a writeback ioc per cgroup and route writeback IOs through it.
* More testing and polishing.
How to test
-----------
* Boot with kernel option "cgroup__DEVEL__legacy_files_on_dfl".
* umount /sys/fs/cgroup/memory
umount /sys/fs/cgroup/blkio
mkdir /sys/fs/cgroup/unified
mount -t cgroup -o __DEVEL__sane_behavior cgroup /sys/fs/cgroup/unified
echo +blkio > /sys/fs/cgroup/unified/cgroup.subtree_control
* Build the cgroup hierarchy (don't forget to enable blkio using
subtree_control) and put processes in cgroups and run tests on ext2
filesystems and blkio.throttle.* knobs.
This patchset contains the following 45 patches.
0001-writeback-add-struct-dirty_context.patch
0002-writeback-add-CONFIG-BDI_CAP-FS-_CGROUP_WRITEBACK.patch
0003-memcg-encode-page_cgflags-in-the-lower-bits-of-page-.patch
0004-memcg-writeback-implement-memcg_blkcg_ptr.patch
0005-writeback-make-backing_dev_info-host-cgroup-specific.patch
0006-writeback-blkcg-associate-each-blkcg_gq-with-the-cor.patch
0007-writeback-attribute-stats-to-the-matching-per-cgroup.patch
0008-writeback-let-balance_dirty_pages-work-on-the-matchi.patch
0009-writeback-make-congestion-functions-per-bdi_writebac.patch
0010-writeback-blkcg-restructure-blk_-set-clear-_queue_co.patch
0011-writeback-blkcg-propagate-non-root-blkcg-congestion-.patch
0012-writeback-implement-and-use-mapping_congested.patch
0013-writeback-implement-WB_has_dirty_io-wb_state-flag.patch
0014-writeback-implement-backing_dev_info-tot_write_bandw.patch
0015-writeback-make-bdi_has_dirty_io-take-multiple-bdi_wr.patch
0016-writeback-don-t-issue-wb_writeback_work-if-clean.patch
0017-writeback-make-bdi-min-max_ratio-handling-cgroup-wri.patch
0018-writeback-implement-bdi_for_each_wb.patch
0019-writeback-remove-bdi_start_writeback.patch
0020-writeback-make-laptop_mode_timer_fn-handle-multiple-.patch
0021-writeback-make-writeback_in_progress-take-bdi_writeb.patch
0022-writeback-make-bdi_start_background_writeback-take-b.patch
0023-writeback-make-wakeup_flusher_threads-handle-multipl.patch
0024-writeback-add-wb_writeback_work-auto_free.patch
0025-writeback-implement-bdi_wait_for_completion.patch
0026-writeback-implement-wb_wait_for_single_work.patch
0027-writeback-restructure-try_writeback_inodes_sb-_nr.patch
0028-writeback-make-writeback-initiation-functions-handle.patch
0029-writeback-move-i_wb_list-emptiness-test-into-inode_w.patch
0030-vfs-writeback-introduce-struct-inode_wb_link.patch
0031-vfs-writeback-add-inode_wb_link-data-point-to-the-as.patch
0032-vfs-writeback-move-inode-dirtied_when-into-inode-i_w.patch
0033-writeback-minor-reorganization-of-fs-fs-writeback.c.patch
0034-vfs-writeback-implement-support-for-multiple-inode_w.patch
0035-vfs-writeback-implement-inode-i_nr_syncs.patch
0036-writeback-dirty-inodes-against-their-matching-cgroup.patch
0037-writeback-make-writeback_control-carry-the-inode_wb_.patch
0038-writeback-make-cyclic-writeback-cursor-cgroup-writeb.patch
0039-writeback-make-DIRTY_PAGES-tracking-cgroup-writeback.patch
0040-writeback-make-write_cache_pages-cgroup-writeback-aw.patch
0041-writeback-make-__writeback_single_inode-cgroup-write.patch
0042-writeback-make-__filemap_fdatawrite_range-croup-writ.patch
0043-buffer-writeback-make-__block_write_full_page-honor-.patch
0044-mpage-make-__mpage_writepage-honor-cgroup-writeback.patch
0045-ext2-enable-cgroup-writeback-support.patch
0001-0002 are basic preps.
0003-0004 implement memcg-blkcg association that dirty and
under-writeback pages can use.
0005-0029 gradually convert writeback code so that wb (bdi_writeback)
operates as an independent writeback domain instead of bdi
(backing_dev_info), a single bdi can have multiple per-cgroup wb's
working for it, and per-bdi operations are translated and distributed
to all its member wb's.
0030-0042 introduce iwbl (inode_wb_link) so that an inode can be
associated against multiple wb's as it gets dirtied by different
cgroups and make inode-wide operations be distributed across them.
0043-0045 make lower layers to properly propagate the cgroup
association from the writeback layer and enable cgroup writeback on
ext2.
This patchset is on top of
-mm + percpu/for-3.20 + cgroup/for-3.20 as of today (2016-01-06)
+ [1] [PATCHSET v2] writeback: prepare for cgroup writeback support
and available in the following git branch.
git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git review-cgroup-writeback-20150106
diffstat follows. Thanks.
block/blk-cgroup.c | 26
block/blk-core.c | 68 +
fs/block_dev.c | 1
fs/buffer.c | 30
fs/ext2/super.c | 2
fs/fs-writeback.c | 1450 +++++++++++++++++++++++++++++++--------
fs/inode.c | 12
fs/mpage.c | 6
fs/xfs/xfs_aops.c | 7
include/linux/backing-dev-defs.h | 153 +++-
include/linux/backing-dev.h | 626 ++++++++++++++++
include/linux/blk-cgroup.h | 10
include/linux/blkdev.h | 19
include/linux/fs.h | 14
include/linux/memcontrol.h | 56 +
include/linux/mm.h | 3
include/linux/mm_types.h | 3
include/linux/writeback.h | 10
include/trace/events/writeback.h | 4
init/Kconfig | 5
mm/backing-dev.c | 304 +++++++-
mm/debug.c | 2
mm/fadvise.c | 2
mm/filemap.c | 5
mm/memcontrol.c | 522 +++++++++++++-
mm/page-writeback.c | 184 ++++
mm/readahead.c | 2
mm/truncate.c | 4
mm/vmscan.c | 12
29 files changed, 3074 insertions(+), 468 deletions(-)
--
tejun
[1] http://lkml.kernel.org/g/1420572557-11572-1-git-send-email-tj@xxxxxxxxxx
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