linux-kernel - Re: [PATCH/RFC] cpuhp code and data usage on UP systems

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Message-ID: <alpine.DEB.2.20.1710022047200.2114@nanos>
Date:   Mon, 2 Oct 2017 20:52:23 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Nicolas Pitre <nicolas.pitre@...aro.org>
cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH/RFC] cpuhp code and data usage on UP systems

On Mon, 2 Oct 2017, Nicolas Pitre wrote:
> 
> On !SMP systems, we end up with the following array definition:
> 
> static struct cpuhp_step cpuhp_ap_states[] = {
>         [CPUHP_ONLINE] = {
>                 .name                   = "online",
>                 .startup.single         = NULL,
>                 .teardown.single        = NULL,
>         },
> };
> 
> where CPUHP_ONLINE = 187. That means up to 99.5% of this array is unused 
> but still allocated in the kernel's .data section i.e. 3760 bytes.
> 
> The same issue exists with cpuhp_bp_states being 1720 bytes.
> 
> The goal of this patch is mainly about illustrating the issue and 
> reducing .data usage. I have no clear idea when this stuff is really 
> needed besides standard CPU hotplug. Not knowing exactly what I'm doing 
> here, I made it conditional on CONFIG_SMP for now. There might be a case 
> for moving that code to a separate file (cpuhp.c maybe?) and omitting it 
> from the kernel when unneeded.
> 
> Comments?

Sure.

> +#ifdef CONFIG_CPUHP
> +#define ___P(proto, def_retcode) extern proto;
> +#else
> +#define ___P(proto, def_retcode) static inline proto { return def_retcode }
> +#endif
> +
> +___P(
>  int __cpuhp_setup_state(enum cpuhp_state state,	const char *name, bool invoke,
>  			int (*startup)(unsigned int cpu),
> -			int (*teardown)(unsigned int cpu), bool multi_instance);
> -
> +			int (*teardown)(unsigned int cpu), bool multi_instance), 0; )

What exactly is invoking the callbacks of states in the proper context and
manages instances on registration? Ditto for teardwon in case of modules.

Thanks,

	tglx