On Wed, Nov 10, 2021 at 01:10:20PM -0300, 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. > Err, CLONE_INTO_CGROUP.
