Re: Question Regarding isolcpus

[Joseph Salisbury <joseph.salisbury@canonical.com>]

On 9/28/23 04:39, Sebastian Andrzej Siewior wrote:
> On 2023-09-26 12:45:14 [-0400], Joseph Salisbury wrote:
>> Hi All,
> Hi,
>
>> I have a question regarding the isolcpus parameter.  I've been seeing this
>> parameter commonly used. However, in the kernel.org documentation[0],
>> isolcpus is listed as depreciated.
>>
>> Is it the case that isolcpus should not be used at all?  I've seen it used
>> in conjunction with taskset.  However, should we now be telling rt users to
>> use only cpusets in cgroups?  I see that CPUAffinity can be set in
>> /etc/systemd/system.conf.  Is that the preferred method, so the process
>> scheduler will automatically migrate processes between the cpusets in the
>> cgroup cpuset or the list set by CPUAffinity?
> Frederic might know if there is an actual timeline to remove it. The
> suggestions since then is to use cpusets which should be more flexible.
> There was also some work (which went into v6.1 I think) to be able to
> reconfigure the partitions at run-time while isolcpus= is a boot time
> option.
>  From what I remember, you have a default/system cpuset which all tasks
> use by default and then you can add another cpuset for the "isolated"
> CPUs. Based on the partition it can be either the default one or
> isolated [0]. The latter would exclude the CPUs from load balancing
> which is what isolcpus= does.
>
> [0] f28e22441f353 ("cgroup/cpuset: Add a new isolated cpus.partition type")

This question may be for the cgroups folks.  The kernel.org 
documentation has a WARNING which states: "cgroup2 doesn't yet support 
control of realtime processes and the cpu controller can only be enabled 
when all RT processes are in the root cgroup "[0]. Does this mean 
real-time processes are only supported on cgroupsV1?

Also, this warning is stated for the "CPU" Controller, but there is no 
mention of this for a "cpuset" controller. Does this imply that 
real-time processes are supported with "cpuset" controllers?


Thanks,

Joe

[0] 
https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html#controllers

>
>> Thanks,
>>
>> Joe
> Sebastian
>