linux-kernel - Re: [PATCH v2] isolcpus: affine kernel threads to specified cpumask

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Message-ID: <f40c828e-5168-bd08-69b8-f7a54fd53f42@windriver.com>
Date:   Tue, 24 Mar 2020 09:56:26 -0600
From:   Chris Friesen <chris.friesen@...driver.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Christoph Lameter <cl@...ux.com>,
        Jim Somerville <Jim.Somerville@...driver.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2] isolcpus: affine kernel threads to specified cpumask

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@...hat.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