[PATCH v2 0/3] blkcg: sync() isolation

[Andrea Righi]

= Problem =

When sync() is executed from a high-priority cgroup, the process is forced to
wait the completion of the entire outstanding writeback I/O, even the I/O that
was originally generated by low-priority cgroups potentially.

This may cause massive latencies to random processes (even those running in the
root cgroup) that shouldn't be I/O-throttled at all, similarly to a classic
priority inversion problem.

This topic has been previously discussed here:
https://patchwork.kernel.org/patch/10804489/

[ Thanks to Josef for the suggestions ]

= Solution =

Here's a slightly more detailed description of the solution, as suggested by
Josef and Tejun (let me know if I misunderstood or missed anything):

 - track the submitter of wb work (when issuing sync()) and the cgroup that
   originally dirtied any inode, then use this information to determine the
   proper "sync() domain" and decide if the I/O speed needs to be boosted or
   not in order to prevent priority-inversion problems

 - by default when sync() is issued, all the outstanding writeback I/O is
   boosted to maximum speed to prevent priority inversion problems

 - if sync() is issued by the same throttled cgroup that generated the dirty
   pages, the corresponding writeback I/O is still throttled normally

 - add a new flag to cgroups (io.sync_isolation) that would make sync()'ers in
   that cgroup only be allowed to write out dirty pages that belong to its
   cgroup

= Test =

Here's a trivial example to trigger the problem:

 - create 2 cgroups: cg1 and cg2

 # mkdir /sys/fs/cgroup/unified/cg1
 # mkdir /sys/fs/cgroup/unified/cg2

 - set an I/O limit of 1MB/s on cg1/io.ma:

 # echo "8:0 rbps=1048576 wbps=1048576" > /sys/fs/cgroup/unified/cg1/io.max

 - run a write-intensive workload in cg1

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

 - run sync in cg2 and measure time

== Vanilla kernel ==

 # cat /proc/self/cgroup
 0::/cg2

 # time sync
 real	9m32,618s
 user	0m0,000s
 sys	0m0,018s

Ideally "sync" should complete almost immediately, because cg2 is unlimited and
it's not doing any I/O at all. Instead, the entire system is totally sluggish,
waiting for the throttled writeback I/O to complete, and it also triggers many
hung task timeout warnings.

== With this patch set applied and io.sync_isolation=0 (default) ==

 # cat /proc/self/cgroup
 0::/cg2

 # time sync
 real	0m2,044s
 user	0m0,009s
 sys	0m0,000s

[ Time range goes from 2s to 4s ]

== With this patch set applied and io.sync_isolation=1 ==

 # cat /proc/self/cgroup
 0::/cg2

 # time sync

 real	0m0,768s
 user	0m0,001s
 sys	0m0,008s

[ Time range goes from 0.7s to 1.6s ]

Changes in v2:
 - fix: properly keep track of sync waiters when a blkcg is writing to
   many block devices at the same time

Andrea Righi (3):
  blkcg: prevent priority inversion problem during sync()
  blkcg: introduce io.sync_isolation
  blkcg: implement sync() isolation

 Documentation/admin-guide/cgroup-v2.rst |   9 +++
 block/blk-cgroup.c                      | 178 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 block/blk-throttle.c                    |  48 ++++++++++++++-
 fs/fs-writeback.c                       |  57 +++++++++++++++++-
 fs/inode.c                              |   1 +
 fs/sync.c                               |   8 ++-
 include/linux/backing-dev-defs.h        |   2 +
 include/linux/blk-cgroup.h              |  52 +++++++++++++++++
 include/linux/fs.h                      |   4 ++
 mm/backing-dev.c                        |   2 +
 mm/page-writeback.c                     |   1 +
 11 files changed, 355 insertions(+), 7 deletions(-)