= 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 ] 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 | 120 ++++++++++++++++++++++++++++++++ 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, 297 insertions(+), 7 deletions(-)
