[RFC PATCH 0/3] cgroup: fsio throttle controller

[Andrea Righi]

This is a redesign of my old cgroup-io-throttle controller:
https://lwn.net/Articles/330531/

I'm resuming this old patch to point out a problem that I think is still
not solved completely.

= Problem =

The io.max controller works really well at limiting synchronous I/O
(READs), but a lot of I/O requests are initiated outside the context of
the process that is ultimately responsible for its creation (e.g.,
WRITEs).

Throttling at the block layer in some cases is too late and we may end
up slowing down processes that are not responsible for the I/O that
is being processed at that level.

= Proposed solution =

The main idea of this controller is to split I/O measurement and I/O
throttling: I/O is measured at the block layer for READS, at page cache
(dirty pages) for WRITEs, and processes are limited while they're
generating I/O at the VFS level, based on the measured I/O.

= Example =

Here's a trivial example: create 2 cgroups, set an io.max limit of
10MB/s, run a write-intensive workload on both and after a while, from a
root cgroup, run "sync".

 # cat /proc/self/cgroup
 0::/cg1
 # fio --rw=write --bs=1M --size=32M --numjobs=16 --name=seeker --time_based --runtime=30

 # cat /proc/self/cgroup
 0::/cg2
 # fio --rw=write --bs=1M --size=32M --numjobs=16 --name=seeker --time_based --runtime=30

 - io.max controller:

 # echo "259:0 rbps=10485760 wbps=10485760" > /sys/fs/cgroup/unified/cg1/io.max
 # echo "259:0 rbps=10485760 wbps=10485760" > /sys/fs/cgroup/unified/cg2/io.max

 # cat /proc/self/cgroup
 0::/
 # time sync

 real	0m51,241s
 user	0m0,000s
 sys	0m0,113s

Ideally "sync" should complete almost immediately, because the root
cgroup is unlimited and it's not doing any I/O at all, but instead it's
blocked for more than 50 sec with io.max, because the writeback is
throttled to satisfy the io.max limits.

- fsio controller:

 # echo "259:0 10 10" > /sys/fs/cgroup/unified/cg1/fsio.max_mbs
 # echo "259:0 10 10" > /sys/fs/cgroup/unified/cg2/fsio.max_mbs

 [you can find details about the syntax in the documentation patch]

 # cat /proc/self/cgroup
 0::/
 # time sync

 real	0m0,146s
 user	0m0,003s
 sys	0m0,001s

= Questions =

Q: Do we need another controller?
A: Probably no, I think it would be better to integrate this policy (or
   something similar) in the current blkio controller, this is just to
   highlight the problem and get some ideas on how to address it.

Q: What about proportional limits / latency?
A: It should be trivial to add latency-based limits if we integrate this in the
   current I/O controller. About proportional limits (weights), they're
   strictly related to I/O scheduling and since this controller doesn't touch
   I/O dispatching policies it's not trivial to implement proportional limits
   (bandwidth limiting is definitely more straightforward).

Q: Applying delays at the VFS layer doesn't prevent I/O spikes during
   writeback, right?
A: Correct, the tradeoff here is to tolerate I/O bursts during writeback to
   avoid priority inversion problems in the system.

Andrea Righi (3):
  fsio-throttle: documentation
  fsio-throttle: controller infrastructure
  fsio-throttle: instrumentation

 Documentation/cgroup-v1/fsio-throttle.txt | 142 +++++++++
 block/blk-core.c                          |  10 +
 include/linux/cgroup_subsys.h             |   4 +
 include/linux/fsio-throttle.h             |  43 +++
 include/linux/writeback.h                 |   7 +-
 init/Kconfig                              |  11 +
 kernel/cgroup/Makefile                    |   1 +
 kernel/cgroup/fsio-throttle.c             | 501 ++++++++++++++++++++++++++++++
 mm/filemap.c                              |  20 +-
 mm/page-writeback.c                       |  14 +-
 10 files changed, 749 insertions(+), 4 deletions(-)