[systemd-devel] Antw: [EXT] Re: [systemd‑devel] cgroupsv2 and realtime processes
>>> Michal Koutný schrieb am 06.06.2022 um 18:29 in Nachricht <20220606162925.gh6...@blackbody.suse.cz>: > On Mon, Jun 06, 2022 at 05:59:32PM +0200, Michał Zegan > wrote: >> I assume if it would be on it would break any and all realtime >> usage...? > > Most likely (you'd not be able either: turn on RT policy, migrate the > process or enable CPU controller, i.e. a step that'd lead to an invalid > state). > > I'm curious, what would be your use case for turning RT group > schedulling on? Odd experiments? ;-) > > Thanks, > Michal
Re: [systemd-devel] cgroupsv2 and realtime processes
W dniu 6.06.2022 o 18:29, Michal Koutný pisze: On Mon, Jun 06, 2022 at 05:59:32PM +0200, Michał Zegan wrote: I assume if it would be on it would break any and all realtime usage...? Most likely (you'd not be able either: turn on RT policy, migrate the process or enable CPU controller, i.e. a step that'd lead to an invalid state). I'm curious, what would be your use case for turning RT group schedulling on? I don't necessarily have any usecase, I just forgot that option was even configurable. Even though i knew about it's existence previously. Arch disables it by default and this seems to be a good choice. but cgroupsv2 docs don't say that this behavior is conditioned on that option so I forgot it existed. Thanks, Michal OpenPGP_0xE6516A8A8E25955D.asc Description: OpenPGP public key OpenPGP_signature Description: OpenPGP digital signature
Re: [systemd-devel] cgroupsv2 and realtime processes
On Mon, Jun 06, 2022 at 05:59:32PM +0200, Michał Zegan wrote: > I assume if it would be on it would break any and all realtime > usage...? Most likely (you'd not be able either: turn on RT policy, migrate the process or enable CPU controller, i.e. a step that'd lead to an invalid state). I'm curious, what would be your use case for turning RT group schedulling on? Thanks, Michal
Re: [systemd-devel] cgroupsv2 and realtime processes
W dniu 6.06.2022 o 17:14, Michal Koutný pisze: On Mon, Jun 06, 2022 at 04:54:03PM +0200, Michał Zegan wrote: this note pointed to in the readme is quite cgroups v1 specific, I believe what it describes was true in v1, and v2 does not have any capability to control realtime processes in non root cgroups if I read correctly. Yes. And it extends to v2 too where there are even no userspace knobs to configure the RT attributes. Therefore it works [1] with CONFIG_RT_GROUP_SCHED unset since RT processes remain in the root cgroup (it's an implementation detail you won't see from /proc/$pid/cgroup, where is still the regular process membership). To prevent confusion -- this applies only to processes (threads) with RT policy and from all other perspectives these processes (threads) are still in the listed cgroup. Does that explain what you were after? yes. forgot about that option to be fair. I assume if it would be on it would break any and all realtime usage...? Michal [1] Unless your goal is to control per-cgroup RT attributes. I understood you just wanted to be able to place RT processes into non-root cgroups. OpenPGP_0xE6516A8A8E25955D.asc Description: OpenPGP public key OpenPGP_signature Description: OpenPGP digital signature
Re: [systemd-devel] cgroupsv2 and realtime processes
On Mon, Jun 06, 2022 at 04:54:03PM +0200, Michał Zegan wrote: > this note pointed to in the readme is quite cgroups v1 specific, I believe > what it describes was true in v1, and v2 does not have any capability to > control realtime processes in non root cgroups if I read correctly. Yes. And it extends to v2 too where there are even no userspace knobs to configure the RT attributes. Therefore it works [1] with CONFIG_RT_GROUP_SCHED unset since RT processes remain in the root cgroup (it's an implementation detail you won't see from /proc/$pid/cgroup, where is still the regular process membership). To prevent confusion -- this applies only to processes (threads) with RT policy and from all other perspectives these processes (threads) are still in the listed cgroup. Does that explain what you were after? Michal [1] Unless your goal is to control per-cgroup RT attributes. I understood you just wanted to be able to place RT processes into non-root cgroups.
Re: [systemd-devel] cgroupsv2 and realtime processes
W dniu 6.06.2022 o 16:02, Michal Koutný pisze: Hello Michał. On Sun, Jun 05, 2022 at 03:28:23PM +0200, Michał Zegan wrote: I have kernel 5.17 on archlinux. How is your kernel configured wrt CONFIG_RT_GROUP_SCHED? it is unset Is that still true? That depends :-) Yet, checking /proc/(pid)/cgroup states these processes are not in a root cgroup, yet the cpu controller is enabled on the root cgroup (/sys/fs/cgroup/cgroup.subtree_control lists "cpu" as one of the controllers and I see the interface files in children). Can anyone explain the situation? With v2 and CONFIG_RT_GROUP_SCHED there's no way how to assign realtime budgets to cgroups and therefore realtime tasks cannot run in them. With !CONFIG_RT_GROUP_SCHED, there's (internally) only the root cgroup for realtime tasks and things apparently work. See also [1]. this note pointed to in the readme is quite cgroups v1 specific, I believe what it describes was true in v1, and v2 does not have any capability to control realtime processes in non root cgroups if I read correctly. The cgroupsv2 documentation states that cgroup cpu controller currently does not support realtime processes, so to enable it all realtime processes must be moved to root cgroup. Will you send a docs patch with the CONFIG_RT_GROUP_SCHED reservation? :-p HTH, Michal [1] https://nam12.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fsystemd%2Fsystemd%2Fblob%2F369151c9c73b12fb7a88fc2b558499c2d4832982%2FREADME%23L140&data=05%7C01%7C%7C2497332c429b4459b16308da47c5367e%7C84df9e7fe9f640afb435%7C1%7C0%7C637901209614621463%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=dXYxa0cDT48qoJDUwJKA%2B2d7vBLYxdc1TuZztaQqQWI%3D&reserved=0 OpenPGP_0xE6516A8A8E25955D.asc Description: OpenPGP public key OpenPGP_signature Description: OpenPGP digital signature
Re: [systemd-devel] cgroupsv2 and realtime processes
Hello Michał. On Sun, Jun 05, 2022 at 03:28:23PM +0200, Michał Zegan wrote: > I have kernel 5.17 on archlinux. How is your kernel configured wrt CONFIG_RT_GROUP_SCHED? > Is that still true? That depends :-) > Yet, checking /proc/(pid)/cgroup states these processes are not in a root > cgroup, yet the cpu controller is enabled on the root cgroup > (/sys/fs/cgroup/cgroup.subtree_control lists "cpu" as one of the controllers > and I see the interface files in children). > > Can anyone explain the situation? With v2 and CONFIG_RT_GROUP_SCHED there's no way how to assign realtime budgets to cgroups and therefore realtime tasks cannot run in them. With !CONFIG_RT_GROUP_SCHED, there's (internally) only the root cgroup for realtime tasks and things apparently work. See also [1]. > > The cgroupsv2 documentation states that cgroup cpu controller > currently does not support realtime processes, so to enable it all > realtime processes must be moved to root cgroup. Will you send a docs patch with the CONFIG_RT_GROUP_SCHED reservation? :-p HTH, Michal [1] https://github.com/systemd/systemd/blob/369151c9c73b12fb7a88fc2b558499c2d4832982/README#L140
