linux-kernel - Re: [PATCH v5 07/21] x86/resctrl: Abstract and use supports_mba

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Date:   Tue, 23 Aug 2022 09:42:37 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     James Morse <james.morse@....com>, <x86@...nel.org>,
        <linux-kernel@...r.kernel.org>
CC:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Babu Moger <Babu.Moger@....com>,
        <shameerali.kolothum.thodi@...wei.com>,
        D Scott Phillips OS <scott@...amperecomputing.com>,
        <lcherian@...vell.com>, <bobo.shaobowang@...wei.com>,
        <tan.shaopeng@...itsu.com>, Jamie Iles <quic_jiles@...cinc.com>,
        Cristian Marussi <cristian.marussi@....com>,
        "Xin Hao" <xhao@...ux.alibaba.com>, <xingxin.hx@...nanolis.org>,
        <baolin.wang@...ux.alibaba.com>
Subject: Re: [PATCH v5 07/21] x86/resctrl: Abstract and use
 supports_mba_mbps()

Hi James,

On 6/22/2022 9:46 AM, James Morse wrote:

...

> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index b32ceff8325a..49d9e2c3c743 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -1890,17 +1890,26 @@ void rdt_domain_reconfigure_cdp(struct rdt_resource *r)
>  }
>  
>  /*
> - * Enable or disable the MBA software controller
> - * which helps user specify bandwidth in MBps.
>   * MBA software controller is supported only if
>   * MBM is supported and MBA is in linear scale.
>   */
> +static bool supports_mba_mbps(void)
> +{
> +	struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_MBA].r_resctrl;
> +
> +	return (is_mbm_enabled() &&
> +		r->alloc_capable && is_mba_linear());
> +}
> +
> +/*
> + * Enable or disable the MBA software controller
> + * which helps user specify bandwidth in MBps.
> + */
>  static int set_mba_sc(bool mba_sc)
>  {
>  	struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_MBA].r_resctrl;
>  
> -	if (!is_mbm_enabled() || !is_mba_linear() ||
> -	    mba_sc == is_mba_sc(r))
> +	if (!supports_mba_mbps() || mba_sc == is_mba_sc(r))
>  		return -EINVAL;
>  
>  	r->membw.mba_sc = mba_sc;


It seems that this change is propagating an issue that exists
in the current implementation (not introduced by this work).

The supports_mba_mbps() test uses is_mbm_enabled() which is:
	(is_mbm_total_enabled() || is_mbm_local_enabled())

That is not accurate since the software controller is only called
when MBM local is enabled:
	mbm_update()
	{
		...
		if (is_mbm_local_enabled()) {
			...
			if (is_mba_sc(NULL))
				mbm_bw_count(rmid, &rr);
		}
	}
			

Could the "is_mbm_enabled()" in the new supports_mba_mbps() introduced
here be changed to use "is_mbm_local_enabled()" instead? It does not seem
an issue for existing hardware (that supports total and local) but
the kernel parameters do enable users to boot with "rdt=!mbmlocal" to
trigger problematic scenarios.

Reinette