Re: [RFC PATCH] blk-throttle: enable io throttle for root in cgroup v2

[Khazhy Kumykov]

On Fri, Sep 17, 2021 at 10:41 AM Michal Koutný <mkoutny@xxxxxxxx> wrote:
>
> Hello Yu.
>
> On Thu, Sep 09, 2021 at 10:08:15PM +0800, Yu Kuai <yukuai3@xxxxxxxxxx> wrote:
> > I'm not sure why this feature is disabled in the first place, is
> > there any problem or design constraint?
>
> The idea for v2 is that in the root cgroup remain only kernel threads that
> provide "global" services and any user workload that should be
> constrained is put into non-root cgroups. Additionally, if kernel
> threads carry out work associated with a cgroup they can charge it to
> the respective cgroup.
>
> [snip]
> > We want to limit the overall iops/bps of the device in cgroup v2,
>
> Cui bono? (I mean what is the reason for throttling on the global level
> when there's no other entity utiliting the residual?
> <joke>Your drives are too fast?</joke>)

We'd be interested in something like this as well. (at least for
io.max). Our use case is providing remote devices which are a shared
resource. A "global" throttle like this (which is set by a local
management daemon) allows for throttling before sending network
traffic. It's also useful since we can put this throttle on a dm, so
we can enforce an aggregate throttle without needing backchannels to
coordinate multiple targets.
(This does also bring up: if this is a useful thing, would it make
sense to tie to the device, vs. requiring cgroup. We happen to use
cgroups so that requirement doesn't affect us).

Khazhy
>
> Michal

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