linux-kernel - Re: [PATCH 2/2] sched/isolation: Add cpu_is

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Date:   Mon, 13 Feb 2023 14:34:26 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Leonardo <leobras@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, cgroups@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 2/2] sched/isolation: Add cpu_is_isolated() API

On Sat 04-02-23 00:24:09, Frederic Weisbecker wrote:
> Provide this new API to check if a CPU has been isolated either through
> isolcpus= or nohz_full= kernel parameter.
> 
> It aims at avoiding kernel load deemed to be safely spared on CPUs
> running sensitive workload that can't bear any disturbance, such as
> pcp cache draining.
> 
> Suggested-by: Michal Hocko <mhocko@...e.com>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>

Is there any locking required? I do not think so as these should be
boot time configured AFAIR. From the discussion around this I have
understood that this might change in the future once cpusets gain a
better isolation support. Maybe this should be documented at this stage?

Thanks!

> ---
>  include/linux/sched/isolation.h | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/include/linux/sched/isolation.h b/include/linux/sched/isolation.h
> index b645cc81fe01..088672f08469 100644
> --- a/include/linux/sched/isolation.h
> +++ b/include/linux/sched/isolation.h
> @@ -53,4 +53,10 @@ static inline bool housekeeping_cpu(int cpu, enum hk_type type)
>  	return true;
>  }
>  
> +static inline bool cpu_is_isolated(int cpu)
> +{
> +	return !housekeeping_test_cpu(cpu, HK_TYPE_DOMAIN) ||
> +		 !housekeeping_test_cpu(cpu, HK_TYPE_KERNEL_NOISE);
> +}
> +
>  #endif /* _LINUX_SCHED_ISOLATION_H */
> -- 
> 2.34.1

-- 
Michal Hocko
SUSE Labs