Re: [PATCH v8 0/6] cgroup/cpuset: Add new cpuset partition type & empty effecitve cpus

[Jan Kiszka <jan.kiszka@siemens.com>]

On 10.11.21 17:10, Marcelo Tosatti wrote:
> On Wed, Nov 10, 2021 at 03:21:54PM +0000, Moessbauer, Felix wrote:
>>
>>
>>> -----Original Message-----
>>> From: Michal Koutný <mkoutny@suse.com>
>>> Sent: Wednesday, November 10, 2021 2:57 PM
>>> To: Moessbauer, Felix (T RDA IOT SES-DE) <felix.moessbauer@siemens.com>
>>> Cc: longman@redhat.com; akpm@linux-foundation.org;
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>>> peterz@infradead.org; shuah@kernel.org; tj@kernel.org; Kiszka, Jan (T RDA
>>> IOT) <jan.kiszka@siemens.com>; Schild, Henning (T RDA IOT SES-DE)
>>> <henning.schild@siemens.com>
>>> Subject: Re: [PATCH v8 0/6] cgroup/cpuset: Add new cpuset partition type &
>>> empty effecitve cpus
>>>
>>> Hello.
>>>
>>> On Wed, Nov 10, 2021 at 12:13:57PM +0100, Felix Moessbauer
>>> <felix.moessbauer@siemens.com> wrote:
>>>> However, I was not able to see any latency improvements when using
>>>> cpuset.cpus.partition=isolated.
>>>
>>> Interesting. What was the baseline against which you compared it (isolcpus, no
>>> cpusets,...)?
>>
>> For this test, I just compared both settings cpuset.cpus.partition=isolated|root.
>> There, I did not see a significant difference (but I know, RT tuning depends on a ton of things).
>>
>>>
>>>> The test was performed with jitterdebugger on CPUs 1-3 and the following
>>> cmdline:
>>>> rcu_nocbs=1-4 nohz_full=1-4 irqaffinity=0,5-6,11 intel_pstate=disable
>>>> On the other cpus, stress-ng was executed to generate load.
>>>> [...]
>>>
>>>> This requires cgroup.type=threaded on both cgroups and changes to the
>>>> application (threads have to be born in non-rt group and moved to rt-group).
>>>
>>> But even with isolcpus the application would need to set affinity of threads to
>>> the selected CPUs (cf cgroup migrating). Do I miss anything?
>>
>> Yes, that's true. But there are two differences (given that you use isolcpus):
>> 1. the application only has to set the affinity for rt threads.
>>  Threads that do not explicitly set the affinity are automatically excluded from the isolated cores.
>>  Even common rt test applications like jitterdebugger do not pin their non-rt threads.
>> 2. Threads can be started on non-rt CPUs and then bound to a specific rt CPU.
>> This binding can be specified before thread creation via pthread_create.
>> By that, you can make sure that at no point in time a thread has a "forbidden" CPU in its affinities.
>>
>> With cgroup2, you cannot guarantee the second aspect, as thread creation and moving to a cgroup is not an atomic operation.
>> Also - please correct me if I'm wrong - you first have to create a thread before moving it into a group.
>> At creation time, you cannot set the final affinity mask (as you create it in the non-rt group and there the CPU is not in the cpuset.cpus).
>> Once you move the thread to the rt cgroup, it has a default mask and by that can be executed on other rt cores.
> 
> man clone3:
> 
>        CLONE_NEWCGROUP (since Linux 4.6)
>               Create  the  process  in  a  new cgroup namespace.  If this flag is not set, then (as with fork(2)) the
>               process is created in the same cgroup namespaces as the calling process.
> 
>               For further information on cgroup namespaces, see cgroup_namespaces(7).
> 
>               Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWCGROUP.
> 

Is there pthread_attr_setcgroup_np()?

Jan

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