Re: Modifying isolcpus, nohz_full, and rcu_nocb kernel parameters at runtime

[Waiman Long <longman@redhat.com>]

On 12/12/23 09:04, Pandruvada, Srinivas wrote:
> Hi Fredric,
> On Tue, 2023-12-12 at 14:27 +0100, Frederic Weisbecker wrote:
>> On Fri, Dec 08, 2023 at 09:18:53AM -0500, Gianfranco Dutka wrote:
>>>> The isolcpus, nohz_full and rcu_nocbs are boot-time kernel
>>>> parameters. I am in the process of improving dynamic CPU
>>>> isolation at runtime. Right now, we are able to do
>>>> isolcpus=domain with the isolated cpuset partition functionality.
>>>> Other aspects of CPU isolation are being looked at with the goal
>>>> of reducing the gap of what one can do at boot time versus what
>>>> can be done at run time. It will certain take time to reach that
>>>> goal.
>>>>
>>>> Cheers,
>>>> Longman
>>>>
>>> Thank you Waiman for the response. It would seem that getting
>>> similar
>>> functionality through cgroups/cpusets is the only option at the
>>> moment. Is it
>>> completely out of the question to possibly patch the kernel to
>>> modify these
>>> parameters at runtime? Or would that entail a significant change
>>> that might
>>> not be so trivial to accomplish? For instance, the solution
>>> wouldn’t be as
>>> simple as patching the kernel to make these writeable and then
>>> calling the
>>> same functions which run at boot-time when these parameters are
>>> originally
>>> written?
>> As for nohz_full (which implies rcu_nocb), it's certainly possible to
>> make it
>> tunable at runtime via cpusets. If people really want it, I'm willing
>> to help.
>>
> We have a case for dynamically isolating CPUs at run time.
>
> https://lore.kernel.org/lkml/ZNM5qoUSCdBwNTuH@chenyu5-mobl2/
>
> It was suggested by Vincent to use house keeping cpumask for solving
> unnecessary wake ups on isolated CPUs. Can this house keeping cpu mask
> and type be updated at runtime?

The house keeping cpumasks are statically set up at boot time. The 
cpuset code will have an internal isolated_cpus cpumask that lists all 
the CPUs that are in isolated partitions. It will also expose it with a 
new cpuset_cpu_is_isolated() function. The code is currently in the 
cgroup tree and will be merged into the upcoming v6.8 kernel. As the new 
isolated_cpus mask can be changed at run time, some synchronization may 
be needed depending on how it is being used. So the cpumask itself will 
not be exported, new API can be provided to copy out the cpumask if needed.

Cheers,
Longman