linux-kernel - Re: [PATCH 5/6] cgroup/cpuset: Free DL BW in case can

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Message-ID: <5ff103f9-1366-0a9b-bd97-419ced1de07f@arm.com>
Date:   Thu, 30 Mar 2023 17:14:06 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Waiman Long <longman@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Qais Yousef <qyousef@...alina.io>, Tejun Heo <tj@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Hao Luo <haoluo@...gle.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        luca.abeni@...tannapisa.it, claudio@...dence.eu.com,
        tommaso.cucinotta@...tannapisa.it, bristot@...hat.com,
        mathieu.poirier@...aro.org, cgroups@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Wei Wang <wvw@...gle.com>, Rick Yiu <rickyiu@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Sudeep Holla <sudeep.holla@....com>
Subject: Re: [PATCH 5/6] cgroup/cpuset: Free DL BW in case can_attach() fails

On 29/03/2023 20:09, Waiman Long wrote:
> On 3/29/23 12:39, Dietmar Eggemann wrote:
>> On 29/03/2023 16:31, Waiman Long wrote:
>>> On 3/29/23 10:25, Waiman Long wrote:
>>>> On 3/29/23 08:55, Juri Lelli wrote:
>>>>> From: Dietmar Eggemann <dietmar.eggemann@....com>
>> [...]
>>
>>>>> @@ -2518,11 +2547,21 @@ static int cpuset_can_attach(struct
>>>>> cgroup_taskset *tset)
>>>>>    static void cpuset_cancel_attach(struct cgroup_taskset *tset)
>>>>>    {
>>>>>        struct cgroup_subsys_state *css;
>>>>> +    struct cpuset *cs;
>>>>>          cgroup_taskset_first(tset, &css);
>>>>> +    cs = css_cs(css);
>>>>>          mutex_lock(&cpuset_mutex);
>>>>> -    css_cs(css)->attach_in_progress--;
>>>>> +    cs->attach_in_progress--;
>>>>> +
>>>>> +    if (cs->nr_migrate_dl_tasks) {
>>>>> +        int cpu = cpumask_any(cs->effective_cpus);
>>>>> +
>>>>> +        dl_bw_free(cpu, cs->sum_migrate_dl_bw);
>>>>> +        reset_migrate_dl_data(cs);
>>>>> +    }
>>>>> +
>>> Another nit that I have is that you may have to record also the cpu
>>> where the DL bandwidth is allocated in cpuset_can_attach() and free the
>>> bandwidth back into that cpu or there can be an underflow if another cpu
>>> is chosen.
>> Many thanks for the review!
>>
>> But isn't the DL BW control `struct dl_bw` per `struct root_domain`
>> which is per exclusive cpuset. So as long cpu is from
>> `cs->effective_cpus` shouldn't this be fine?
> 
> Sorry for my ignorance on how the deadline bandwidth operation work. I
> check the bandwidth code and find that we are storing the bandwidth
> information in the root domain, not on the cpu. That shouldn't be a
> concern then.
> 
> However, I still have some question on how that works when dealing with
> cpuset. First of all, not all the CPUs in a given root domains are in
> the cpuset. So there may be enough bandwidth on the root domain, but it
> doesn't mean there will be enough bandwidth in the set of CPUs in a
> particular cpuset. Secondly, how do you deal with isolated CPUs that do
> not have a corresponding root domain? It is now possible to create a
> cpuset with isolated CPUs.

Sorry, I overlooked this email somehow.

IMHO, this is only done for exclusive cpusets:

  cpuset_can_attach()

    if (!cpumask_intersects(oldcs->effective_cpus, cs->effective_cpus))

So they should have their own root_domain congruent to their cpumask.