linux-kernel - Re: [PATCH 05/33] sched/isolation: Save boot defined domain flags

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Message-ID: <xhsmhecqtoc4b.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Thu, 23 Oct 2025 17:45:40 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>, LKML
 <linux-kernel@...r.kernel.org>
Cc: Frederic Weisbecker <frederic@...nel.org>, Michal Koutný
 <mkoutny@...e.com>, Andrew Morton <akpm@...ux-foundation.org>, Bjorn
 Helgaas <bhelgaas@...gle.com>, Catalin Marinas <catalin.marinas@....com>,
 Danilo Krummrich <dakr@...nel.org>, "David S . Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Gabriele Monaco
 <gmonaco@...hat.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Ingo Molnar <mingo@...hat.com>, Jakub Kicinski <kuba@...nel.org>, Jens
 Axboe <axboe@...nel.dk>, Johannes Weiner <hannes@...xchg.org>, Lai
 Jiangshan <jiangshanlai@...il.com>, Marco Crivellari
 <marco.crivellari@...e.com>, Michal Hocko <mhocko@...e.com>, Muchun Song
 <muchun.song@...ux.dev>, Paolo Abeni <pabeni@...hat.com>, Peter Zijlstra
 <peterz@...radead.org>, Phil Auld <pauld@...hat.com>, "Rafael J . Wysocki"
 <rafael@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>, Shakeel
 Butt <shakeel.butt@...ux.dev>, Simon Horman <horms@...nel.org>, Tejun Heo
 <tj@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Vlastimil Babka
 <vbabka@...e.cz>, Waiman Long <longman@...hat.com>, Will Deacon
 <will@...nel.org>, cgroups@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-block@...r.kernel.org,
 linux-mm@...ck.org, linux-pci@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 05/33] sched/isolation: Save boot defined domain flags

On 13/10/25 22:31, Frederic Weisbecker wrote:
> HK_TYPE_DOMAIN will soon integrate not only boot defined isolcpus= CPUs
> but also cpuset isolated partitions.
>
> Housekeeping still needs a way to record what was initially passed
> to isolcpus= in order to keep these CPUs isolated after a cpuset
> isolated partition is modified or destroyed while containing some of
> them.
>
> Create a new HK_TYPE_DOMAIN_BOOT to keep track of those.
>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Reviewed-by: Phil Auld <pauld@...hat.com>
> ---
>  include/linux/sched/isolation.h | 1 +
>  kernel/sched/isolation.c        | 5 +++--
>  2 files changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/sched/isolation.h b/include/linux/sched/isolation.h
> index d8501f4709b5..da22b038942a 100644
> --- a/include/linux/sched/isolation.h
> +++ b/include/linux/sched/isolation.h
> @@ -7,6 +7,7 @@
>  #include <linux/tick.h>
>
>  enum hk_type {
> +	HK_TYPE_DOMAIN_BOOT,
>       HK_TYPE_DOMAIN,
>       HK_TYPE_MANAGED_IRQ,
>       HK_TYPE_KERNEL_NOISE,
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index a4cf17b1fab0..8690fb705089 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -11,6 +11,7 @@
>  #include "sched.h"
>
>  enum hk_flags {
> +	HK_FLAG_DOMAIN_BOOT	= BIT(HK_TYPE_DOMAIN_BOOT),
>       HK_FLAG_DOMAIN		= BIT(HK_TYPE_DOMAIN),
>       HK_FLAG_MANAGED_IRQ	= BIT(HK_TYPE_MANAGED_IRQ),
>       HK_FLAG_KERNEL_NOISE	= BIT(HK_TYPE_KERNEL_NOISE),
> @@ -216,7 +217,7 @@ static int __init housekeeping_isolcpus_setup(char *str)
>
>               if (!strncmp(str, "domain,", 7)) {
>                       str += 7;
> -			flags |= HK_FLAG_DOMAIN;
> +			flags |= HK_FLAG_DOMAIN | HK_FLAG_DOMAIN_BOOT;
>                       continue;
>               }
>
> @@ -246,7 +247,7 @@ static int __init housekeeping_isolcpus_setup(char *str)
>
>       /* Default behaviour for isolcpus without flags */
>       if (!flags)
> -		flags |= HK_FLAG_DOMAIN;
> +		flags |= HK_FLAG_DOMAIN | HK_FLAG_DOMAIN_BOOT;

I got stupidly confused by the cpumask_andnot() used later on since these
are housekeeping cpumasks and not isolated ones; AFAICT HK_FLAG_DOMAIN_BOOT
is meant to be a superset of HK_FLAG_DOMAIN - or, put in a way my brain
comprehends, NOT(HK_FLAG_DOMAIN) (i.e. runtime isolated cpumask) is a
superset of NOT(HK_FLAG_DOMAIN_BOOT) (i.e. boottime isolated cpumask),
thus the final shape of cpu_is_isolated() makes sense:

  static inline bool cpu_is_isolated(int cpu)
  {
          return !housekeeping_test_cpu(cpu, HK_TYPE_DOMAIN);
  }

Could we document that to make it a bit more explicit? Maybe something like

  enum hk_type {
        /* Set at boot-time via the isolcpus= cmdline argument */
        HK_TYPE_DOMAIN_BOOT,
        /*
         * Updated at runtime via isolated cpusets; strict subset of
         * HK_TYPE_DOMAIN_BOOT as it accounts for boot-time isolated CPUs.
         */
        HK_TYPE_DOMAIN,
        ...
  }