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

[Waiman Long]

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@xxxxxxxx>
> > > > > Sent: Wednesday, November 10, 2021 2:57 PM
> > > > > To: Moessbauer, Felix (T RDA IOT SES-DE) <felix.moessbauer@xxxxxxxxxxx>
> > > > > Cc: longman@xxxxxxxxxx; akpm@xxxxxxxxxxxxxxxxxxxx;
> > > > > cgroups@xxxxxxxxxxxxxxx; corbet@xxxxxxx; frederic@xxxxxxxxxx; guro@xxxxxx;
> > > > > hannes@xxxxxxxxxxx; juri.lelli@xxxxxxxxxx; linux-doc@xxxxxxxxxxxxxxx; linux-
> > > > > kernel@xxxxxxxxxxxxxxx; linux-kselftest@xxxxxxxxxxxxxxx;
> > > > > lizefan.x@xxxxxxxxxxxxx; mtosatti@xxxxxxxxxx; pauld@xxxxxxxxxx;
> > > > > peterz@xxxxxxxxxxxxx; shuah@xxxxxxxxxx; tj@xxxxxxxxxx; Kiszka, Jan (T RDA
> > > > > IOT) <jan.kiszka@xxxxxxxxxxx>; Schild, Henning (T RDA IOT SES-DE)
> > > > > <henning.schild@xxxxxxxxxxx>
> > > > > 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@xxxxxxxxxxx> 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