[Devel] [PATCH rh8 2/2] ms/block, bfq: add weight symlink to the bfq.weight cgroup parameter

[Andrey Ryabinin]

From: Angelo Ruocco <angeloruocco90 at gmail.com>

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 c6113af31960..27f158f2ef2c 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,
 	},
-- 
2.24.1