linux-kernel - Re: [PATCH 02/13] kernel/cpu: Add support for declaring CPU hotplug not supported

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Message-ID: <87il7dl32c.ffs@tglx>
Date:   Wed, 11 Oct 2023 15:08:11 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        kexec@...ts.infradead.org, linux-coco@...ts.linux.dev,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 02/13] kernel/cpu: Add support for declaring CPU hotplug
 not supported

On Thu, Oct 05 2023 at 16:13, Kirill A. Shutemov wrote:
> The function cpu_hotplug_not_supported() can be called to indicate that
> CPU hotplug should be disabled. It does not prevent the initial bring up
> of the CPU, but it stops subsequent offlining.

This tells me what the patch is doing, but not the why.

> This function is intended to replace CC_ATTR_HOTPLUG_DISABLED.

> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -132,6 +132,7 @@ extern void cpus_read_lock(void);
>  extern void cpus_read_unlock(void);
>  extern int  cpus_read_trylock(void);
>  extern void lockdep_assert_cpus_held(void);
> +extern void cpu_hotplug_not_supported(void);

This function name sucks.

The point is as you explained to prevent offlining, but not onlining. So
can we please make this very clear? Something like:

    cpu_hotplug_disable_offlining()

> +/* Cleared if platform declares CPU hotplug not supported */
> +static bool cpu_hotplug_supported = true;

Again. This is not about disabling hotplug all together. Something like:

static bool cpu_hotplug_offline_disabled;

Which expresses clearly what this is about and does not require this
awkward negation.

Thanks,

        tglx