linux-kernel - Re: [PATCH v9 3/7] cpuset: Add cpuset.sched.load

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Date:   Thu, 31 May 2018 11:36:39 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kernel-team@...com, pjt@...gle.com, luto@...capital.net,
        Mike Galbraith <efault@....de>, torvalds@...ux-foundation.org,
        Roman Gushchin <guro@...com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v9 3/7] cpuset: Add cpuset.sched.load_balance flag to v2

On 05/31/2018 11:20 AM, Peter Zijlstra wrote:
> On Thu, May 31, 2018 at 09:54:27AM -0400, Waiman Long wrote:
>> On 05/31/2018 08:26 AM, Peter Zijlstra wrote:
>>> I still find all that a bit weird.
>>>
>>> So load_balance=0 basically changes a partition into a
>>> 'fully-partitioned partition' with the seemingly random side-effect that
>>> now sub-partitions are allowed to consume all CPUs.
>> Are you suggesting that we should allow sub-partition to consume all the
>> CPUs no matter the load balance state? I can live with that if you think
>> it is more logical.
> I'm on the fence myself; the only thing I'm fairly sure of is that tying
> this particular behaviour to the load-balance knob seems off.

The main reason for doing it this way is that I don't want to have
load-balanced partition with no cpu in it. How about we just don't allow
consume-all at all. Each partition must have at least 1 cpu.

>
>>> The rationale, only given in the Changelog above, seems to be to allow
>>> 'easy' emulation of isolcpus.
>>>
>>> I'm still not convinced this is a useful knob to have. You can do
>>> fully-partitioned by simply creating a lot of 1 cpu parititions.
>> That is certainly true. However, I think there are some additional
>> overhead in the scheduler side in maintaining those 1-cpu partitions. Right?
> cpuset-controller as such doesn't have much overhead scheduler wise,
> cpu-controller OTOH does, and there depth is the predominant factor, so
> many sibling groups should not matter there either.
>
>>> So this one knob does two separate things, both of which seem, to me,
>>> redundant.
>>>
>>> Can we please get better rationale for this?
>> I am fine getting rid of the load_balance flag if this is the consensus.
>> However, we do need to come up with a good migration story for those
>> users that need the isolcpus capability. I think Mike was the one asking
>> for supporting isolcpus. So Mike, what is your take on that.
> So I don't strictly mind having a knob that does the 'fully-partitioned
> partition' thing -- however odd that sounds -- but I feel we should have
> a solid use-case for it.
>
> I also think we should not mix the 'consume all' thing with the
> 'fully-partitioned' thing, as they are otherwise unrelated.

The "consume all" and "fully-partitioned" look the same to me. Are you
talking about allocating all the CPUs in a partition to sub-partitions
so that there is no CPU left in the parent partition?

Cheers,
Longman