linux-kernel - Re: [patch 2/3] isolcpus: affine kernel threads to specified cpumask

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Message-ID: <20200331005706.GA24647@lenoir>
Date:   Tue, 31 Mar 2020 02:57:08 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Frederic Weisbecker <fweisbec@...il.com>,
        Chris Friesen <chris.friesen@...driver.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jim Somerville <Jim.Somerville@...driver.com>,
        Christoph Lameter <cl@...ux.com>
Subject: Re: [patch 2/3] isolcpus: affine kernel threads to specified cpumask

On Sat, Mar 28, 2020 at 12:21:19PM -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 kernel-parameters.txt).
> 
> 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: Wind River'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@...hat.com>
> 
> ---
>  Documentation/admin-guide/kernel-parameters.txt |    8 ++++++++
>  include/linux/sched/isolation.h                 |    1 +
>  kernel/kthread.c                                |    6 ++++--
>  kernel/sched/isolation.c                        |    6 ++++++
>  4 files changed, 19 insertions(+), 2 deletions(-)
> 
> Index: linux-2.6/Documentation/admin-guide/kernel-parameters.txt
> ===================================================================
> --- linux-2.6.orig/Documentation/admin-guide/kernel-parameters.txt
> +++ linux-2.6/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 "kthread"
> +			  to avoid creation of kernel threads on isolated CPUs.
> +
> +			kthread
> +			  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 and by managed_irq option.

So, what about what I suggested with having "unbound" instead, which
includes all the CPU-unbound work?

 HK_FLAG_WQ | HK_FLAG_TIMER | HK_FLAG_RCU | HK_FLAG_MISC | HK_FLAG_KTHREAD | HK_FLAG_SCHED

(and yes your suggestion of including HK_FLAG_SCHED is good).

Because I don't see the point of exposing kthread isolation alone as an ABI
so far.

Later I suspect I'll turn all these flags into a single HK_FLAG_UNBOUND.

Thanks.