[Devel] [PATCH RHEL7 COMMIT] ms/block, bfq: add weight symlink to the bfq.weight cgroup parameter

[Kirill Tkhai]

The commit is pushed to "branch-rh8-4.18.0-80.1.2.vz8.2.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh8-4.18.0-80.1.2.vz8.3.1
------>
commit a78e75c3a099c2555467448c981a86c5129c82c1
Author: Angelo Ruocco <angeloruocco90 at gmail.com>
Date:   Wed Jan 29 12:16:55 2020 +0000

    ms/block, bfq: add weight symlink to the bfq.weight cgroup parameter
    
    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 [1] to make all
    configurations work: add a symlink, named weight, to bfq.weight. This
    commit adds such a symlink.
    
    [1] https://lkml.org/lkml/2019/4/8/555
    
    Suggested-by: Johannes Thumshirn <jthumshirn at suse.de>
    Signed-off-by: Angelo Ruocco <angeloruocco90 at gmail.com>
    Signed-off-by: Paolo Valente <paolo.valente at linaro.org>
    Signed-off-by: Jens Axboe <axboe at kernel.dk>
    
    (cherry-picked from 19e9da9e86c43687ed6a56bd39a52c6649973a35)
    https://jira.sw.ru/browse/PSBM-101019
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 block/bfq-cgroup.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
index c6113af..27f158f 100644
--- a/block/bfq-cgroup.c
+++ b/block/bfq-cgroup.c
@@ -1054,7 +1054,8 @@ struct blkcg_policy blkcg_policy_bfq = {
 struct cftype bfq_blkcg_legacy_files[] = {
 	{
 		.name = "bfq.weight",
-		.flags = CFTYPE_NOT_ON_ROOT,
+		.link_name = "weight",
+		.flags = CFTYPE_NOT_ON_ROOT | CFTYPE_SYMLINKED,
 		.seq_show = bfq_io_show_weight,
 		.write_u64 = bfq_io_set_weight_legacy,
 	},
@@ -1174,7 +1175,8 @@ struct cftype bfq_blkcg_legacy_files[] = {
 struct cftype bfq_blkg_files[] = {
 	{
 		.name = "bfq.weight",
-		.flags = CFTYPE_NOT_ON_ROOT,
+		.link_name = "weight",
+		.flags = CFTYPE_NOT_ON_ROOT | CFTYPE_SYMLINKED,
 		.seq_show = bfq_io_show_weight,
 		.write = bfq_io_set_weight,
 	},