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

[Waiman Long <longman@redhat.com>]

On 11/10/21 12:29, Marcelo Tosatti wrote:
> On Wed, Nov 10, 2021 at 05:15:41PM +0100, Jan Kiszka wrote:
>> 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;
>>>>> cgroups@vger.kernel.org; corbet@lwn.net; frederic@kernel.org; guro@fb.com;
>>>>> hannes@cmpxchg.org; juri.lelli@redhat.com; linux-doc@vger.kernel.org; linux-
>>>>> kernel@vger.kernel.org; linux-kselftest@vger.kernel.org;
>>>>> lizefan.x@bytedance.com; mtosatti@redhat.com; pauld@redhat.com;
>>>>> 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
> Don't know... Waiman?

I don't think there is such libpthread call yet.

-Longman