linux-kernel - Re: [PATCH v2 3/6] cgroup/cpuset: Add a new isolated cpus.partition type

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Message-ID: <58c87587-417b-1498-185f-1db6bb612c82@redhat.com>
Date:   Thu, 24 Jun 2021 11:23:45 -0400
From:   Waiman Long <llong@...hat.com>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <shuah@...nel.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>, Phil Auld <pauld@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH v2 3/6] cgroup/cpuset: Add a new isolated cpus.partition
 type

On 6/24/21 8:51 AM, Michal Koutný wrote:
> Hello.
>
> On Mon, Jun 21, 2021 at 02:49:21PM -0400, Waiman Long <longman@...hat.com> wrote:
>>      cgroup/cpuset: Add a new isolated cpus.partition type
>>
>>      Cpuset v1 uses the sched_load_balance control file to determine if load
>>      balancing should be enabled.  Cpuset v2 gets rid of sched_load_balance
>>      as its use may require disabling load balancing at cgroup root.
>>
>>      For workloads that require very low latency like DPDK, the latency
>>      jitters caused by periodic load balancing may exceed the desired
>>      latency limit.
>>
>>      When cpuset v2 is in use, the only way to avoid this latency cost is to
>>      use the "isolcpus=" kernel boot option to isolate a set of CPUs. After
>>      the kernel boot, however, there is no way to add or remove CPUs from
>>      this isolated set. For workloads that are more dynamic in nature, that
>>      means users have to provision enough CPUs for the worst case situation
>>      resulting in excess idle CPUs.
>>
>>      To address this issue for cpuset v2, a new cpuset.cpus.partition type
>>      "isolated" is added which allows the creation of a cpuset partition
>>      without load balancing. This will allow system administrators to
>>      dynamically adjust the size of isolated partition to the current need
>>      of the workload without rebooting the system.
> I like this work.
> Would it be worth generalizing the API to be on par with what isolcpus=
> can configure? (I.e. not only load balancing but the other dimensions of
> isolation (like the flags nohz and managed_irq now).)
Good point, the isolated partition is equivalent to isolcpus=domain. I 
will need to evaluate the nohz and managed_irq options to see if they 
can be done dynamically without adding a lot of overhead. If so, we can 
extend the functionality to cover that in future patches. Right now, 
this is for the domain functionality only. If we can cover the nohz and 
managed_irq options, we can deprecate isolcpus and advocate the use of 
cgroup instead.
>
> I don't know if all such behaviors could be implemented dynamically
> (likely not easy) but the API could initially implement just what you do
> here with the "isolated" partition type.
>
> The variant I'm thinking of would keep just the "root" and "member"
> partitions type and the "root" type could be additionally configured via
> cpuset.cpus.partition.flags (for example).
>
> WDYT?

What I am thinking is that "isolated" means "isolated:domain" or one can 
do "isolated:nohz,domain,manged_irq" just like the current isolcpus boot 
option. I don't think we really need to add an extra flags control file.

Cheers,
Longman