Many userspace tools and services use the proportional-share policy of the blkio/io cgroups controller. The CFQ I/O scheduler implemented this policy for the legacy block layer. To modify the weight of a group in case CFQ was in charge, the 'weight' parameter of the group must be modified. On the other hand, the BFQ I/O scheduler implements the same policy in blk-mq, but, with BFQ, the parameter to modify has a different name: bfq.weight (forced choice until legacy block was present, because two different policies cannot share a common parameter in cgroups). Due to CFQ legacy, most if not all userspace configurations still use the parameter 'weight', and for the moment do not seem likely to be changed. But, when CFQ went away with legacy block, such a parameter ceased to exist. So, a simple workaround has been proposed by Johannes [1] to make all configurations work: add a symlink, named weight, to bfq.weight. This pair of patches adds: 1) the possibility to create a symlink to a cgroup file; 2) the above 'weight' symlink. Thanks, Paolo [1] https://lkml.org/lkml/2019/4/8/555 Angelo Ruocco (2): cgroup: let a symlink too be created with a cftype file block, bfq: add weight symlink to the bfq.weight cgroup parameter block/bfq-cgroup.c | 6 ++++-- include/linux/cgroup-defs.h | 3 +++ kernel/cgroup/cgroup.c | 33 +++++++++++++++++++++++++++++---- 3 files changed, 36 insertions(+), 6 deletions(-) -- 2.20.1
