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

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Message-ID: <20210917174103.GC13346@blackbody.suse.cz>
Date:   Fri, 17 Sep 2021 19:41:03 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     tj@...nel.org, axboe@...nel.dk, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com
Subject: Re: [RFC PATCH] blk-throttle: enable io throttle for root in cgroup
 v2

Hello Yu.

On Thu, Sep 09, 2021 at 10:08:15PM +0800, Yu Kuai <yukuai3@...wei.com> 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>)

Michal