linux-kernel - [PATCH 2/2] block, bfq: add weight symlink to the bfq.weight cgroup parameter

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Date:   Tue, 21 May 2019 10:01:55 +0200
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        ulf.hansson@...aro.org, linus.walleij@...aro.org,
        broonie@...nel.org, bfq-iosched@...glegroups.com,
        oleksandr@...alenko.name, Angelo Ruocco <angeloruocco90@...il.com>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Paolo Valente <paolo.valente@...aro.org>
Subject: [PATCH 2/2] block, bfq: add weight symlink to the bfq.weight cgroup parameter

From: Angelo Ruocco <angeloruocco90@...il.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@...e.de>
Signed-off-by: Angelo Ruocco <angeloruocco90@...il.com>
Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
---
 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 b3796a40a61a..59f46904cb11 100644
--- a/block/bfq-cgroup.c
+++ b/block/bfq-cgroup.c
@@ -1046,7 +1046,8 @@ 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,
 	},
@@ -1166,7 +1167,8 @@ 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,
 	},
-- 
2.20.1