linux-kernel - Re: [PATCH v4 1/2] x86/resctrl: Pass domain to target CPU

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Message-ID: <12766261-26b2-4aa4-a735-c2380c4963fd@intel.com>
Date: Mon, 4 Mar 2024 15:07:31 -0800
From: Reinette Chatre <reinette.chatre@...el.com>
To: Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>, "Peter
 Newman" <peternewman@...gle.com>, Jonathan Corbet <corbet@....net>, "Shuah
 Khan" <skhan@...uxfoundation.org>, <x86@...nel.org>
CC: Shaopeng Tan <tan.shaopeng@...itsu.com>, James Morse
	<james.morse@....com>, Jamie Iles <quic_jiles@...cinc.com>, Babu Moger
	<babu.moger@....com>, Randy Dunlap <rdunlap@...radead.org>, Drew Fustini
	<dfustini@...libre.com>, <linux-kernel@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, <patches@...ts.linux.dev>
Subject: Re: [PATCH v4 1/2] x86/resctrl: Pass domain to target CPU

Hi Tony,

On 2/28/2024 11:36 AM, Tony Luck wrote:
> reset_all_ctrls() and resctrl_arch_update_domains() use on_each_cpu_mask()
> to call rdt_ctrl_update() on potentially one CPU from each domain.
> 
> But this means rdt_ctrl_update() needs to figure out which domain to
> apply changes to. Doing so requires a search of all domains in a resource,
> which can only be done safely if cpus_lock is held. Both callers do hold
> this lock, but there isn't a way for a function called on another CPU
> via IPI to verify this.
> 
> Commit c0d848fcb09d ("x86/resctrl: Remove lockdep annotation that triggers
> false positive") removed the incorrect assertions.
> 
> Add the target domain to the msr_param structure and
> call rdt_ctrl_update() for each domain separately using
> smp_call_function_single(). This means that rdt_ctrl_update() doesn't
> need to search for the domain and get_domain_from_cpu() can safely assert
> that the cpus_lock is held since the remaining callers do not use IPI.
> 
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---

..

> @@ -315,27 +300,27 @@ int resctrl_arch_update_domains(struct rdt_resource *r, u32 closid)
>  	struct rdt_hw_domain *hw_dom;
>  	struct msr_param msr_param;
>  	enum resctrl_conf_type t;
> -	cpumask_var_t cpu_mask;
>  	struct rdt_domain *d;
> +	int cpu;
>  	u32 idx;
>  
>  	/* Walking r->domains, ensure it can't race with cpuhp */
>  	lockdep_assert_cpus_held();
>  
> -	if (!zalloc_cpumask_var(&cpu_mask, GFP_KERNEL))
> -		return -ENOMEM;
> -
> -	msr_param.res = NULL;
>  	list_for_each_entry(d, &r->domains, list) {
>  		hw_dom = resctrl_to_arch_dom(d);
> +		msr_param.res = NULL;
> +		msr_param.dom = d;
>  		for (t = 0; t < CDP_NUM_TYPES; t++) {
>  			cfg = &hw_dom->d_resctrl.staged_config[t];
>  			if (!cfg->have_new_ctrl)
>  				continue;
>  
>  			idx = get_config_index(closid, t);
> -			if (!apply_config(hw_dom, cfg, idx, cpu_mask))
> +			if (cfg->new_ctrl == hw_dom->ctrl_val[idx])
>  				continue;
> +			hw_dom->ctrl_val[idx] = cfg->new_ctrl;
> +			cpu = cpumask_any(&d->cpu_mask);
>  

cpu only needs to be assigned once. How about initializing cpu to
nr_cpu_ids at the same time msr_param.res and msr_param.dom is
initialized and only assign it when msr_param.res is assigned?
I think that will be more robust.

If you agree and do this then please feel free to add:
Reviewed-by: Reinette Chatre <reinette.chatre@...el.com>

Reinette