BFQ cgroup weights range

BFQ cgroup weights range

[Michal Koutný]

Hello.

The default weight for proportional IO control associated with a cgroup
is 100 [1]. The minimum allowed weight is 1 [2] and the maximum weight
1000 [3].  This is a bit inconsistent with general cgroup weight
semantic where def/min == max/def (i.e. symmetric adjustments to both
sides) [4].

1) Is there a reason why the maximum allowed weight is (only) 1000?
   (E.g. it won't be possible to ensure 10^4 ratio of proportional
   control but 10^3 is achievable.)
2) Is the default value 100 special or absolute in a sense? (I suspect
   it is, the unchangeable weight of root cgroup members. Therefore two
   siblings with equal ratios 10:100 and 100:1000 would behave same only
   when there's no interfering IO from the root.)

Thanks,
Michal

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/block/bfq-cgroup.c?h=v5.14-rc7#n513
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/block/bfq-iosched.h?h=v5.14-rc7#n18
[3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/block/bfq-iosched.h?h=v5.14-rc7#n19
[4] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/admin-guide/cgroup-v2.rst?h=v5.14-rc7#n602

Re: BFQ cgroup weights range

[Paolo Valente]

> Il giorno 24 ago 2021, alle ore 12:56, Michal Koutný <mkoutny@suse.com> ha scritto:
> 
> Hello.
> 
> The default weight for proportional IO control associated with a cgroup
> is 100 [1]. The minimum allowed weight is 1 [2] and the maximum weight
> 1000 [3].  This is a bit inconsistent with general cgroup weight
> semantic where def/min == max/def (i.e. symmetric adjustments to both
> sides) [4].
> 
> 1) Is there a reason why the maximum allowed weight is (only) 1000?
>   (E.g. it won't be possible to ensure 10^4 ratio of proportional
>   control but 10^3 is achievable.)
> 2) Is the default value 100 special or absolute in a sense? (I suspect
>   it is, the unchangeable weight of root cgroup members. Therefore two
>   siblings with equal ratios 10:100 and 100:1000 would behave same only
>   when there's no interfering IO from the root.)
> 

Hi,
BFQ inherited these constants when we forked it from CFQ.  I'm ok with
increasing max weight to 10000.  I only wonder whether this would
break some configuration, as the currently maximum weight would not be
the maximum weight any longer.
\x14
Thanks,
Paolo

> Thanks,
> Michal
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/block/bfq-cgroup.c?h=v5.14-rc7#n513
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/block/bfq-iosched.h?h=v5.14-rc7#n18
> [3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/block/bfq-iosched.h?h=v5.14-rc7#n19
> [4] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/admin-guide/cgroup-v2.rst?h=v5.14-rc7#n602

Re: BFQ cgroup weights range

[Michal Koutný]

On Tue, Aug 24, 2021 at 02:51:47PM +0200, Paolo Valente <paolo.valente@linaro.org> wrote:
> BFQ inherited these constants when we forked it from CFQ.  I'm ok with
> increasing max weight to 10000.  I only wonder whether this would
> break some configuration, as the currently maximum weight would not be
> the maximum weight any longer.

Thanks for the reply. Let me form the idea as a patch (and commit
message) and discuss based on that if needed (+ccrosspost into cgroups
ML).

-- >8 --
From: Michal Koutný <mkoutny@suse.com>
Subject: [PATCH] block, bfq: Accept symmetric weight adjustments

The allowed range for BFQ weights is currently 1..1000 with 100 being
the default. There is no apparent reason to not accept weight
adjustments of same ratio on both sides of the default. This change
makes the attribute domain consistent with other cgroup (v2) knobs with
the weight semantics.

This extension of the range does not restrict existing configurations
(quite the opposite). This may affect setups where weights >1000 were
attempted to be set but failed with the default 100. Such cgroups would
attain their intended weight now. This is a changed behavior but it
rectifies the situation (similar intention to the commit 69d7fde5909b
("blkcg: use CGROUP_WEIGHT_* scale for io.weight on the unified
hierarchy") for CFQ formerly (and v2 only)).

Additionally, the changed range does not imply all IO workloads can be
really controlled to achieve the widest possible ratio 1:10^4.

Signed-off-by: Michal Koutný <mkoutny@suse.com>
---
 Documentation/admin-guide/cgroup-v1/blkio-controller.rst | 2 +-
 Documentation/block/bfq-iosched.rst                      | 2 +-
 block/bfq-iosched.h                                      | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v1/blkio-controller.rst b/Documentation/admin-guide/cgroup-v1/blkio-controller.rst
index 16253eda192e..48559541c9d8 100644
--- a/Documentation/admin-guide/cgroup-v1/blkio-controller.rst
+++ b/Documentation/admin-guide/cgroup-v1/blkio-controller.rst
@@ -102,7 +102,7 @@ Proportional weight policy files
 	  on all the devices until and unless overridden by per device rule
 	  (see `blkio.bfq.weight_device` below).
 
-	  Currently allowed range of weights is from 1 to 1000. For more details,
+	  Currently allowed range of weights is from 1 to 10000. For more details,
           see Documentation/block/bfq-iosched.rst.
 
   blkio.bfq.weight_device
diff --git a/Documentation/block/bfq-iosched.rst b/Documentation/block/bfq-iosched.rst
index df3a8a47f58c..88b5251734ce 100644
--- a/Documentation/block/bfq-iosched.rst
+++ b/Documentation/block/bfq-iosched.rst
@@ -560,7 +560,7 @@ For each group, the following parameters can be set:
 
   weight
         This specifies the default weight for the cgroup inside its parent.
-        Available values: 1..1000 (default: 100).
+        Available values: 1..10000 (default: 100).
 
         For cgroup v1, it is set by writing the value to `blkio.bfq.weight`.
 
diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
index 99c2a3cb081e..786b7f926dd2 100644
--- a/block/bfq-iosched.h
+++ b/block/bfq-iosched.h
@@ -16,7 +16,7 @@
 #define BFQ_CL_IDLE_TIMEOUT	(HZ/5)
 
 #define BFQ_MIN_WEIGHT			1
-#define BFQ_MAX_WEIGHT			1000
+#define BFQ_MAX_WEIGHT			10000
 #define BFQ_WEIGHT_CONVERSION_COEFF	10
 
 #define BFQ_DEFAULT_QUEUE_IOPRIO	4
-- 
2.32.0

Re: BFQ cgroup weights range

[Tejun Heo]

On Thu, Aug 26, 2021 at 03:12:12PM +0200, Michal Koutný wrote:
> On Tue, Aug 24, 2021 at 02:51:47PM +0200, Paolo Valente <paolo.valente@linaro.org> wrote:
> > BFQ inherited these constants when we forked it from CFQ.  I'm ok with
> > increasing max weight to 10000.  I only wonder whether this would
> > break some configuration, as the currently maximum weight would not be
> > the maximum weight any longer.
> 
> Thanks for the reply. Let me form the idea as a patch (and commit
> message) and discuss based on that if needed (+ccrosspost into cgroups
> ML).
> 
> -- >8 --
> From: Michal Koutný <mkoutny@suse.com>
> Subject: [PATCH] block, bfq: Accept symmetric weight adjustments
> 
> The allowed range for BFQ weights is currently 1..1000 with 100 being
> the default. There is no apparent reason to not accept weight
> adjustments of same ratio on both sides of the default. This change
> makes the attribute domain consistent with other cgroup (v2) knobs with
> the weight semantics.
> 
> This extension of the range does not restrict existing configurations
> (quite the opposite). This may affect setups where weights >1000 were
> attempted to be set but failed with the default 100. Such cgroups would
> attain their intended weight now. This is a changed behavior but it
> rectifies the situation (similar intention to the commit 69d7fde5909b
> ("blkcg: use CGROUP_WEIGHT_* scale for io.weight on the unified
> hierarchy") for CFQ formerly (and v2 only)).
> 
> Additionally, the changed range does not imply all IO workloads can be
> really controlled to achieve the widest possible ratio 1:10^4.
> 
> Signed-off-by: Michal Koutný <mkoutny@suse.com>

Looks fine to me.

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun

Re: BFQ cgroup weights range

[Paolo Valente]

> Il giorno 26 ago 2021, alle ore 19:25, Tejun Heo <tj@kernel.org> ha scritto:
> 
> On Thu, Aug 26, 2021 at 03:12:12PM +0200, Michal Koutný wrote:
>> On Tue, Aug 24, 2021 at 02:51:47PM +0200, Paolo Valente <paolo.valente@linaro.org> wrote:
>>> BFQ inherited these constants when we forked it from CFQ.  I'm ok with
>>> increasing max weight to 10000.  I only wonder whether this would
>>> break some configuration, as the currently maximum weight would not be
>>> the maximum weight any longer.
>> 
>> Thanks for the reply. Let me form the idea as a patch (and commit
>> message) and discuss based on that if needed (+ccrosspost into cgroups
>> ML).
>> 
>> -- >8 --
>> From: Michal Koutný <mkoutny@suse.com>
>> Subject: [PATCH] block, bfq: Accept symmetric weight adjustments
>> 
>> The allowed range for BFQ weights is currently 1..1000 with 100 being
>> the default. There is no apparent reason to not accept weight
>> adjustments of same ratio on both sides of the default. This change
>> makes the attribute domain consistent with other cgroup (v2) knobs with
>> the weight semantics.
>> 
>> This extension of the range does not restrict existing configurations
>> (quite the opposite). This may affect setups where weights >1000 were
>> attempted to be set but failed with the default 100. Such cgroups would
>> attain their intended weight now. This is a changed behavior but it
>> rectifies the situation (similar intention to the commit 69d7fde5909b
>> ("blkcg: use CGROUP_WEIGHT_* scale for io.weight on the unified
>> hierarchy") for CFQ formerly (and v2 only)).
>> 
>> Additionally, the changed range does not imply all IO workloads can be
>> really controlled to achieve the widest possible ratio 1:10^4.
>> 
>> Signed-off-by: Michal Koutný <mkoutny@suse.com>
> 
> Looks fine to me.
> 
> Acked-by: Tejun Heo <tj@kernel.org>
> 

Acked-by: Paolo Valente <paolo.valente@linaro.org>

Thanks for this improvement,
Paolo

> Thanks.
> 
> -- 
> tejun

Re: BFQ cgroup weights range

[Paolo Valente]

> Il giorno 26 ago 2021, alle ore 19:25, Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org> ha scritto:
> 
> On Thu, Aug 26, 2021 at 03:12:12PM +0200, Michal Koutný wrote:
>> On Tue, Aug 24, 2021 at 02:51:47PM +0200, Paolo Valente <paolo.valente-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org> wrote:
>>> BFQ inherited these constants when we forked it from CFQ.  I'm ok with
>>> increasing max weight to 10000.  I only wonder whether this would
>>> break some configuration, as the currently maximum weight would not be
>>> the maximum weight any longer.
>> 
>> Thanks for the reply. Let me form the idea as a patch (and commit
>> message) and discuss based on that if needed (+ccrosspost into cgroups
>> ML).
>> 
>> -- >8 --
>> From: Michal Koutný <mkoutny-IBi9RG/b67k@public.gmane.org>
>> Subject: [PATCH] block, bfq: Accept symmetric weight adjustments
>> 
>> The allowed range for BFQ weights is currently 1..1000 with 100 being
>> the default. There is no apparent reason to not accept weight
>> adjustments of same ratio on both sides of the default. This change
>> makes the attribute domain consistent with other cgroup (v2) knobs with
>> the weight semantics.
>> 
>> This extension of the range does not restrict existing configurations
>> (quite the opposite). This may affect setups where weights >1000 were
>> attempted to be set but failed with the default 100. Such cgroups would
>> attain their intended weight now. This is a changed behavior but it
>> rectifies the situation (similar intention to the commit 69d7fde5909b
>> ("blkcg: use CGROUP_WEIGHT_* scale for io.weight on the unified
>> hierarchy") for CFQ formerly (and v2 only)).
>> 
>> Additionally, the changed range does not imply all IO workloads can be
>> really controlled to achieve the widest possible ratio 1:10^4.
>> 
>> Signed-off-by: Michal Koutný <mkoutny-IBi9RG/b67k@public.gmane.org>
> 
> Looks fine to me.
> 
> Acked-by: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> 

Acked-by: Paolo Valente <paolo.valente-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>

Thanks for this improvement,
Paolo

> Thanks.
> 
> -- 
> tejun