[PATCHSET for-5.3/block] block: add blkcg bio punt mechanism

[Tejun Heo <tj@kernel.org>]

Hello,

This patchset contains only the block part of the following

  [1] [PATCHSET v2 btrfs/for-next] blkcg, btrfs: fix cgroup writeback support

with the following changes

 * wbc_account_io() is renamed to wbc_account_cgroup_owner() and
   wbc->no_account_io to wbc->no_cgroup_owner for clarity.

When writeback is executed asynchronously (e.g. for compression), bios
are bounced to and issued by worker pool shared by all cgroups.  This
leads to significant priority inversions when cgroup IO control is in
use - IOs for a low priority cgroup can tie down the workers forcing
higher priority IOs to wait behind them.

This patchset adds an bio punt mechanism to blkcg and updates btrfs to
issue async IOs through it.  A bio tagged with REQ_CGROUP_PUNT flag is
bounced to the asynchronous issue context of the associated blkcg on
bio_submit().  As the bios are issued from per-blkcg work items,
there's no concern for priority inversions and it doesn't require
invasive changes to the filesystems.  The mechanism should be
generally useful for IO control support across different filesystems.

This patchset contains the following 5 patches.

 0001-cgroup-blkcg-Prepare-some-symbols-for-module-and-CON.patch
 0002-blkcg-writeback-Rename-wbc_account_io-to-wbc_account.patch
 0003-blkcg-writeback-Add-wbc-no_cgroup_owner.patch
 0004-blkcg-writeback-Implement-wbc_blkcg_css.patch
 0005-blkcg-implement-REQ_CGROUP_PUNT.patch

The patches are also available in the following branch.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git review-blkcg-punt

Thanks, diffstat follows.

 Documentation/admin-guide/cgroup-v2.rst |    2 -
 block/blk-cgroup.c                      |   54 ++++++++++++++++++++++++++++++++
 block/blk-core.c                        |    3 +
 fs/btrfs/extent_io.c                    |    4 +-
 fs/buffer.c                             |    2 -
 fs/ext4/page-io.c                       |    2 -
 fs/f2fs/data.c                          |    4 +-
 fs/fs-writeback.c                       |   13 ++++---
 fs/mpage.c                              |    2 -
 include/linux/backing-dev.h             |    1 
 include/linux/blk-cgroup.h              |   16 ++++++++-
 include/linux/blk_types.h               |   10 +++++
 include/linux/cgroup.h                  |    1 
 include/linux/writeback.h               |   41 ++++++++++++++++++++----
 14 files changed, 134 insertions(+), 21 deletions(-)

--
tejun

[1] http://lkml.kernel.org/r/20190615182453.843275-1-tj@kernel.org