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From:   Waiman Long <llong@redhat.com>
Subject: Re: [PATCH v3 6/9] cgroup/cpuset: Add a new isolated cpus.partition
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To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Shuah Khan <shuah@kernel.org>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kselftest@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>, Phil Auld <pauld@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny@suse.com>
References: <20210720141834.10624-1-longman@redhat.com>
 <20210720141834.10624-7-longman@redhat.com>
 <20210727114241.GA283787@lothringen>
Message-ID: <fe3d9fcb-c3af-9214-c69f-00ef36521c5c@redhat.com>
Date:   Tue, 27 Jul 2021 11:56:25 -0400
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On 7/27/21 7:42 AM, Frederic Weisbecker wrote:
> On Tue, Jul 20, 2021 at 10:18:31AM -0400, Waiman Long wrote:
>> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=TBD
>>
>> commit 994fb794cb252edd124a46ca0994e37a4726a100
>> Author: Waiman Long <longman@redhat.com>
>> Date:   Sat, 19 Jun 2021 13:28:19 -0400
>>
>>      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.
>>
>>      Signed-off-by: Waiman Long <longman@redhat.com>
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
> Nice! And while we are adding a new ABI, can we take advantage of that and
> add a specific semantic that if a new isolated partition matches a subset of
> "isolcpus=", it automatically maps to it. This means that any further
> modification to that isolated partition will also modify the associated
> isolcpus= subset.
>
> Or to summarize, when we create a new isolated partition, remove the associated
> CPUs from isolcpus= ?

We can certainly do that as a follow-on. Another idea that I have been 
thinking about is to automatically generating a isolated partition under 
root to match the given isolcpus parameter when the v2 filesystem is 
mounted. That needs more experimentation and testing to verify that it 
can work.

Cheers,
Longman