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Subject: Re: [RFC PATCH] blk-throttle: enable io throttle for root in cgroup
 v2
To:     Khazhy Kumykov <khazhy@google.com>,
        =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny@suse.com>
CC:     <tj@kernel.org>, <axboe@kernel.dk>, <cgroups@vger.kernel.org>,
        <linux-block@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <yi.zhang@huawei.com>
References: <20210909140815.2600858-1-yukuai3@huawei.com>
 <20210917174103.GC13346@blackbody.suse.cz>
 <CACGdZYJiLuh6kED_tdWkYqbHDXc_18m-XJbevp-ri5ansvbtYg@mail.gmail.com>
From:   "yukuai (C)" <yukuai3@huawei.com>
Message-ID: <37f8c687-8549-104a-2501-532a0cfc9a48@huawei.com>
Date:   Sun, 19 Sep 2021 18:31:38 +0800
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On 2021/09/18 3:58, Khazhy Kumykov wrote:
> On Fri, Sep 17, 2021 at 10:41 AM Michal Koutný <mkoutny@suse.com> wrote:
>>
>> Hello Yu.
>>
>> On Thu, Sep 09, 2021 at 10:08:15PM +0800, Yu Kuai <yukuai3@huawei.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>)
> 
> 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

Our use case is similair to this, a host can provide several remote
devices to difierent client. If one client is under high io pressure,
other client might be affected. Thus we want to limit the overall
iops/bps from the client.

Thanks,
Kuai

> 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