On Fri, Jun 12, 2015 at 04:57:34PM -0500, Tejun Heo wrote: > Update Documentation/cgroups/blkio-controller.txt to reflect the > recently added cgroup writeback support. > > Signed-off-by: Tejun Heo <tj@xxxxxxxxxx> > Cc: Li Zefan <lizefan@xxxxxxxxxx> > Cc: Vivek Goyal <vgoyal@xxxxxxxxxx> > Cc: cgroups@xxxxxxxxxxxxxxx > Cc: linux-fsdevel@xxxxxxxxxxxxxxx > --- > Documentation/cgroups/blkio-controller.txt | 83 ++++++++++++++++++++++++++++-- Hi Tejun, This looks good to me. Thanks. IIRC, I had run into the issues with two fsync running into two cgroups. One cgroup was of really small limit and other was unlimited. At that point of time I think conclusion was that multiple transactions could not make progress at the same time. So slower cgroup had blocked unlimited cgroup process from opening a transaction (as IO from slower group was stuck inside throttling later). For some reason, in my limited testing I have not noticed it with your branch. May be things have changed since or I am just hazy on details. I will do some more testing. Thanks Vivek > 1 file changed, 78 insertions(+), 5 deletions(-) > > diff --git a/Documentation/cgroups/blkio-controller.txt b/Documentation/cgroups/blkio-controller.txt > index cd556b9..68b6a6a 100644 > --- a/Documentation/cgroups/blkio-controller.txt > +++ b/Documentation/cgroups/blkio-controller.txt > @@ -387,8 +387,81 @@ groups and put applications in that group which are not driving enough > IO to keep disk busy. In that case set group_idle=0, and CFQ will not idle > on individual groups and throughput should improve. > > -What works > -========== > -- Currently only sync IO queues are support. All the buffered writes are > - still system wide and not per group. Hence we will not see service > - differentiation between buffered writes between groups. > +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. > + > +On traditional cgroup hierarchies, relationships between different > +controllers cannot be established making it impossible for writeback > +to operate accounting for cgroup resource restrictions and all > +writeback IOs are attributed to the root cgroup. > + > +If both the blkio and memory controllers are used on the v2 hierarchy > +and the filesystem supports cgroup writeback, writeback operations > +correctly follow the resource restrictions imposed by both memory and > +blkio controllers. > + > +Writeback examines both system-wide and per-cgroup dirty memory status > +and enforces the more restrictive of the two. Also, writeback control > +parameters which are absolute values - vm.dirty_bytes and > +vm.dirty_background_bytes - are distributed across cgroups according > +to their current writeback bandwidth. > + > +There's a peculiarity stemming from the discrepancy in ownership > +granularity between memory controller and writeback. While memory > +controller tracks ownership per page, writeback operates on inode > +basis. cgroup writeback bridges the gap by tracking ownership by > +inode but migrating ownership if too many foreign pages, pages which > +don't match the current inode ownership, have been encountered while > +writing back the inode. > + > +This is a conscious design choice as writeback operations are > +inherently tied to inodes making strictly following page ownership > +complicated and inefficient. The only use case which suffers from > +this compromise is multiple cgroups concurrently dirtying disjoint > +regions of the same inode, which is an unlikely use case and decided > +to be unsupported. Note that as memory controller assigns page > +ownership on the first use and doesn't update it until the page is > +released, even if cgroup writeback strictly follows page ownership, > +multiple cgroups dirtying overlapping areas wouldn't work as expected. > +In general, write-sharing an inode across multiple cgroups is not well > +supported. > + > +Filesystem support for cgroup writeback > +--------------------------------------- > + > +A filesystem can make writeback IOs cgroup-aware 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 MS_CGROUPWB in ->s_flags. 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. > -- > 2.4.2 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
