Re: SCHED_DEADLINE tasks missing their deadline with SCHED_FLAG_RECLAIM jobs in the mix (using GRUB)

luca abeni
2025-05-30T09:21:14Z

Hi Marcel,

On Sun, 25 May 2025 21:29:05 +0200
Marcel Ziswiler <marcel.ziswiler@codethink.co.uk> wrote:
[...]
> > How do you configure systemd? I am having troubles in reproducing
> > your AllowedCPUs configuration... This is an example of what I am
> > trying: sudo systemctl set-property --runtime custom-workload.slice
> > AllowedCPUs=1 sudo systemctl set-property --runtime init.scope
> > AllowedCPUs=0,2,3 sudo systemctl set-property --runtime
> > system.slice AllowedCPUs=0,2,3 sudo systemctl set-property
> > --runtime user.slice AllowedCPUs=0,2,3 and then I try to run a
> > SCHED_DEADLINE application with sudo systemd-run --scope -p
> > Slice=custom-workload.slice <application>
>
> We just use a bunch of systemd configuration files as follows:
>
> [root@localhost ~]# cat /lib/systemd/system/monitor.slice
> # Copyright (C) 2024 Codethink Limited
> # SPDX-License-Identifier: GPL-2.0-only
[...]

So, I copied your *.slice files in /lib/systemd/system (and I added
them to the "Wants=" entry of /lib/systemd/system/slices.target,
otherwise the slices are not created), but I am still unable to run
SCHED_DEADLINE applications in these slices.

This is due to the fact that the kernel does not create a new root
domain for these cpusets (probably because the cpusets' CPUs are not
exclusive and the cpuset is not "isolated": for example,
/sys/fs/cgroup/safety1.slice/cpuset.cpus.partition is set to "member",
not to "isolated"). So, the "cpumask_subset(span, p->cpus_ptr)" in
sched_setsched() is still false and the syscall returns -EPERM.

Since I do not know how to obtain an isolated cpuset with cgroup v2 and
systemd, I tried using the old cgroup v1, as described in the
SCHED_DEADLINE documentation.

This worked fine, and enabling SCHED_FLAG_RECLAIM actually reduced the
number of missed deadlines (I tried with a set of periodic tasks having
the same parameters as the ones you described). So, it looks like
reclaiming is working correctly (at least, as far as I can see) when
using cgroup v1 to configure the CPU partitions... Maybe there is some
bug triggered by cgroup v2, or maybe I am misunderstanding your setup.

I think the experiment suggested by Juri can help in understanding
where the issue can be.

Thanks,
Luca

> [Unit]
> Description=Prioritized slice for the safety monitor.
> Before=slices.target
>
> [Slice]
> CPUWeight=1000
> AllowedCPUs=0
> MemoryAccounting=true
> MemoryMin=10%
> ManagedOOMPreference=omit
>
> [Install]
> WantedBy=slices.target
>
> [root@localhost ~]# cat /lib/systemd/system/safety1.slice
> # Copyright (C) 2024 Codethink Limited
> # SPDX-License-Identifier: GPL-2.0-only
> [Unit]
> Description=Slice for Safety case processes.
> Before=slices.target
>
> [Slice]
> CPUWeight=1000
> AllowedCPUs=1
> MemoryAccounting=true
> MemoryMin=10%
> ManagedOOMPreference=omit
>
> [Install]
> WantedBy=slices.target
>
> [root@localhost ~]# cat /lib/systemd/system/safety2.slice
> # Copyright (C) 2024 Codethink Limited
> # SPDX-License-Identifier: GPL-2.0-only
> [Unit]
> Description=Slice for Safety case processes.
> Before=slices.target
>
> [Slice]
> CPUWeight=1000
> AllowedCPUs=2
> MemoryAccounting=true
> MemoryMin=10%
> ManagedOOMPreference=omit
>
> [Install]
> WantedBy=slices.target
>
> [root@localhost ~]# cat /lib/systemd/system/safety3.slice
> # Copyright (C) 2024 Codethink Limited
> # SPDX-License-Identifier: GPL-2.0-only
> [Unit]
> Description=Slice for Safety case processes.
> Before=slices.target
>
> [Slice]
> CPUWeight=1000
> AllowedCPUs=3
> MemoryAccounting=true
> MemoryMin=10%
> ManagedOOMPreference=omit
>
> [Install]
> WantedBy=slices.target
>
> [root@localhost ~]# cat /lib/systemd/system/system.slice
> # Copyright (C) 2024 Codethink Limited
> # SPDX-License-Identifier: GPL-2.0-only
>
> #
> # This slice will control all processes started by systemd by
> # default.
> #
>
> [Unit]
> Description=System Slice
> Documentation=man:systemd.special(7)
> Before=slices.target
>
> [Slice]
> CPUQuota=150%
> AllowedCPUs=0
> MemoryAccounting=true
> MemoryMax=80%
> ManagedOOMSwap=kill
> ManagedOOMMemoryPressure=kill
>
> [root@localhost ~]# cat /lib/systemd/system/user.slice
> # Copyright (C) 2024 Codethink Limited
> # SPDX-License-Identifier: GPL-2.0-only
>
> #
> # This slice will control all processes started by systemd-logind
> #
>
> [Unit]
> Description=User and Session Slice
> Documentation=man:systemd.special(7)
> Before=slices.target
>
> [Slice]
> CPUQuota=25%
> AllowedCPUs=0
> MemoryAccounting=true
> MemoryMax=80%
> ManagedOOMSwap=kill
> ManagedOOMMemoryPressure=kill
>
> > However, this does not work because systemd is not creating an
> > isolated cpuset... So, the root domain still contains CPUs 0-3, and
> > the "custom-workload.slice" cpuset only has CPU 1. Hence, the check
> >                         /*
> >                          * Don't allow tasks with an affinity mask
> > smaller than
> >                          * the entire root_domain to become
> > SCHED_DEADLINE. We
> >                          * will also fail if there's no bandwidth
> > available. */
> >                         if (!cpumask_subset(span, p->cpus_ptr) ||
> >                             rq->rd->dl_bw.bw == 0) {
> >                                 retval = -EPERM;
> >                                 goto unlock;
> >                         }
> > in sched_setsched() fails.
> >
> >
> > How are you configuring the cpusets?
>
> See above.
>
> > Also, which kernel version are you using?
> > (sorry if you already posted this information in previous emails
> > and I am missing something obvious)
>
> Not even sure, whether I explicitly mentioned that other than that we
> are always running latest stable.
>
> Two months ago when we last run some extensive tests on this it was
> actually v6.13.6.
>
> > Thanks,
>
> Thank you!
>
> > Luca
>
> Cheers
>
> Marcel