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SubjectRe: [PATCH v2] isolcpus: affine kernel threads to specified cpumask
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I hadn't been keeping up with all the changes to the "isolcpus" boot 
arg. Given how it's been extended, I agree that it seems the logical
place to deal with this. Patch seems okay to me, but I've got a couple
of nits in the message portion.

If I want to specify both no_kthreads and managed_irq it then something
like "isolcpus=managed_irq,no_kthreads,2-16" would work?


On 3/24/2020 9:20 AM, Marcelo Tosatti wrote:
>
> This is a kernel enhancement to configure the cpu affinity of kernel
> threads via kernel boot option isolcpus=no_kthreads,<isolcpus_params>,<cpulist>

https://github.com/torvalds/linux/blob/master/Documentation/admin-guide/kernel-parameters.txt
says that "isolcpus" is deprecated. Are we un-deprecating it? Or is it
only really deprecated for the "domain" option?

> When this option is specified, the cpumask is immediately applied upon
> thread launch. This does not affect kernel threads that specify cpu
> and node.
>
> This allows CPU isolation (that is not allowing certain threads
> to execute on certain CPUs) without using the isolcpus=domain parameter,
> making it possible to enable load balancing on such CPUs
> during runtime (see

I think you're missing the rest of the sentence here.

> Note-1: this is based off on Wind River's patch at
> https://github.com/starlingx-staging/stx-integ/blob/master/kernel/kernel-std/centos/patches/affine-compute-kernel-threads.patch
>
> Difference being that this patch is limited to modifying
> kernel thread cpumask: Behaviour of other threads can
> be controlled via cgroups or sched_setaffinity.
>
> Note-2: MontaVista's patch was based off Christoph Lameter's patch at
> https://lwn.net/Articles/565932/ with the only difference being
> the kernel parameter changed from kthread to kthread_cpus.

Wind River, not MontaVista. I know all us embedded linux folks look the
same...

>
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
>
> ---
>
> v2: use isolcpus= subcommand (Thomas Gleixner)
>
> Documentation/admin-guide/kernel-parameters.txt | 8 ++++++++
> include/linux/cpumask.h | 5 +++++
> include/linux/sched/isolation.h | 1 +
> init/main.c | 1 +
> kernel/cpu.c | 13 +++++++++++++
> kernel/kthread.c | 4 ++--
> kernel/sched/isolation.c | 6 ++++++
> 7 files changed, 36 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index c07815d230bc..7318e3057383 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1959,6 +1959,14 @@
> the CPU affinity syscalls or cpuset.
> <cpu number> begins at 0 and the maximum value is
> "number of CPUs in system - 1".
> + When using cpusets, use the isolcpus option no_kthreads
> + to avoid creation of kernel threads on isolated CPUs.
> +
> + no_kthreads
> + Adjust the CPU affinity mask of unbound kernel threads to
> + not contain CPUs on the isolated list. This complements
> + the isolation provided by the cpusets mechanism described
> + above.

It also complements the "managed_irq" option below. In many cases I'd
expect the same set of CPUs to be isolated from both irqs and kernel
threads.


Chris

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