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Date:   Wed, 25 Mar 2020 01:30:00 +0100
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Marcelo Tosatti <mtosatti@redhat.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Chris Friesen <chris.friesen@windriver.com>,
        linux-kernel@vger.kernel.org, Christoph Lameter <cl@linux.com>,
        Jim Somerville <Jim.Somerville@windriver.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v2] isolcpus: affine kernel threads to specified cpumask
Message-ID: <20200325002956.GC20223@lenoir>
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On Tue, Mar 24, 2020 at 12:20:16PM -0300, 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>
> 
> 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
> 
> 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.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

I'm wondering, why do you need such a boot shift at all when you
can actually affine kthreads on runtime?

> 
> ---
> 
> 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.

Actually that should be "kthread" instead of no_kthreads. A flag of isolcpus
describes what we want a set of CPUs to be isolated from. Well, at least that's
how we started with "domain" and "managed_irq".

>  
>  			managed_irq
>  
> diff --git a/include/linux/sched/isolation.h b/include/linux/sched/isolation.h
> index 0fbcbacd1b29..d002332d00eb 100644
> --- a/include/linux/sched/isolation.h
> +++ b/include/linux/sched/isolation.h
> @@ -14,6 +14,7 @@ enum hk_flags {
>  	HK_FLAG_DOMAIN		= (1 << 5),
>  	HK_FLAG_WQ		= (1 << 6),
>  	HK_FLAG_MANAGED_IRQ	= (1 << 7),
> +	HK_FLAG_NO_KTHREADS	= (1 << 8),

Similarly that should be HK_FLAG_KTHREAD.

>  };
>  
>  #ifdef CONFIG_CPU_ISOLATION
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index b262f47046ca..be9c8d53a986 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -347,7 +347,7 @@ struct task_struct *__kthread_create_on_node(int (*threadfn)(void *data),
>  		 * The kernel thread should not inherit these properties.
>  		 */
>  		sched_setscheduler_nocheck(task, SCHED_NORMAL, &param);
> -		set_cpus_allowed_ptr(task, cpu_all_mask);
> +		set_cpus_allowed_ptr(task, cpu_kthread_mask);

I'm wondering, why are we using cpu_all_mask and not cpu_possible_mask here?
If we used the latter, you wouldn't need to create cpu_kthread_mask and
you could directly rely on housekeeping_cpumask(HK_FLAG_KTHREAD).

> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index 008d6ac2342b..e9d48729efd4 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -169,6 +169,12 @@ static int __init housekeeping_isolcpus_setup(char *str)
>  			continue;
>  		}
>  
> +		if (!strncmp(str, "no_kthreads,", 12)) {
> +			str += 12;
> +			flags |= HK_FLAG_NO_KTHREADS;

You will certainly want HK_FLAG_WQ as well since workqueue has its own
way to deal with unbound affinity.

> +			continue;
> +		}
> +
>  		pr_warn("isolcpus: Error, unknown flag\n");
>  		return 0;
>  	}
> 

Thanks.