Re: [PATCH cgroup/for-4.4 3/3] cgroup: replace unified-hierarchy.txt with a proper cgroup v2 documentation

[Tejun Heo]

Hello,

On Thu, Oct 22, 2015 at 11:36:05PM +0900, Tejun Heo wrote:
> It works with ext2 and 4 and btrfs.  Will document it.  Thanks.

Updated to include all writeback information from
blkio-controller.txt.

5-3-2. Writeback

Page cache is dirtied through buffered writes and shared mmaps and
written asynchronously to the backing filesystem by the writeback
mechanism.  Writeback sits between the memory and IO domains and
regulates the proportion of dirty memory by balancing dirtying and
write IOs.

The io controller, in conjunction with the memory controller,
implements control of page cache writeback IOs.  The memory controller
defines the memory domain that dirty memory ratio is calculated and
maintained for and the io controller defines the io domain which
writes out dirty pages for the memory domain.  Both system-wide and
per-cgroup dirty memory states are examined and the more restrictive
of the two is enforced.

cgroup writeback requires explicit support from the underlying
filesystem.  Currently, cgroup writeback is implemented on ext2, ext4
and btrfs.  On other filesystems, all writeback IOs are attributed to
the root cgroup.

There are inherent differences in memory and writeback management
which affects how cgroup ownership is tracked.  Memory is tracked per
page while writeback per inode.  For the purpose of writeback, an
inode is assigned to a cgroup and all IO requests to write dirty pages
from the inode are attributed to that cgroup.

As cgroup ownership for memory is tracked per page, there can be pages
which are associated with different cgroups than the one the inode is
associated with.  These are called foreign pages.  The writeback
constantly keeps track of foreign pages and, if a particular foreign
cgroup becomes the majority over a certain period of time, switches
the ownership of the inode to that cgroup.

While this model is enough for most use cases where a given inode is
mostly dirtied by a single cgroup even when the main writing cgroup
changes over time, use cases where multiple cgroups write to a single
inode simultaneously are not supported well.  In such circumstances, a
significant portion of IOs are likely to be attributed incorrectly.
As memory controller assigns page ownership on the first use and
doesn't update it until the page is released, even if writeback
strictly follows page ownership, multiple cgroups dirtying overlapping
areas wouldn't work as expected.  It's recommended to avoid such usage
patterns.

The sysctl knobs which affect writeback behavior are applied to cgroup
writeback as follows.

  vm.dirty_background_ratio
  vm.dirty_ratio

	These ratios apply the same to cgroup writeback with the
	amount of available memory capped by limits imposed by the
	memory controller and system-wide clean memory.

  vm.dirty_background_bytes
  vm.dirty_bytes

	For cgroup writeback, this is calculated into ratio against
	total available memory and applied the same way as
	vm.dirty[_background]_ratio.


P. Information on Kernel Programming

This section contains kernel programming information in the areas
where interacting with cgroup is necessary.  cgroup core and
controllers are not covered.


P-1. Filesystem Support for Writeback

A filesystem can support cgroup writeback by updating
address_space_operations->writepage[s]() to annotate bio's using the
following two functions.

  wbc_init_bio(@wbc, @bio)

	Should be called for each bio carrying writeback data and
	associates the bio with the inode's owner cgroup.  Can be
	called anytime between bio allocation and submission.

  wbc_account_io(@wbc, @page, @bytes)

	Should be called for each data segment being written out.
	While this function doesn't care exactly when it's called
	during the writeback session, it's the easiest and most
	natural to call it as data segments are added to a bio.

With writeback bio's annotated, cgroup support can be enabled per
super_block by setting SB_I_CGROUPWB in ->s_iflags.  This allows for
selective disabling of cgroup writeback support which is helpful when
certain filesystem features, e.g. journaled data mode, are
incompatible.

wbc_init_bio() binds the specified bio to its cgroup.  Depending on
the configuration, the bio may be executed at a lower priority and if
the writeback session is holding shared resources, e.g. a journal
entry, may lead to priority inversion.  There is no one easy solution
for the problem.  Filesystems can try to work around specific problem
cases by skipping wbc_init_bio() or using bio_associate_blkcg()
directly.

-- 
tejun
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