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SubjectRe: [PATCH v2] isolcpus: affine kernel threads to specified cpumask
On Tue, Mar 24, 2020 at 09:56:26AM -0600, Chris Friesen wrote:
> 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?

Yes.

> 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?

I don't think its deprecated (see the recent inclusion of managed_irq,
and the suggestion from Thomas to extend it).

Will send another patch to remove that sentence.

> > 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.

Right.

> > 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...

Doh^2.

>
> >
> > 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

Agree, will fix in -v3.

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