Re: BFQ cgroup weights range

[Tejun Heo <tj@kernel.org>]

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