linux-kernel - Re: [PATCH v15 32/34] fs/resctrl: Disable BMEC event configuration when mbm

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Message-ID: <36d5cca8-f97c-4984-84b9-6730958ce5ec@amd.com>
Date: Wed, 23 Jul 2025 12:30:48 -0500
From: "Moger, Babu" <babu.moger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>, corbet@....net,
 tony.luck@...el.com, james.morse@....com, tglx@...utronix.de,
 mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com
Cc: Dave.Martin@....com, x86@...nel.org, hpa@...or.com,
 akpm@...ux-foundation.org, paulmck@...nel.org, rostedt@...dmis.org,
 Neeraj.Upadhyay@....com, david@...hat.com, arnd@...db.de, fvdl@...gle.com,
 seanjc@...gle.com, jpoimboe@...nel.org, pawan.kumar.gupta@...ux.intel.com,
 xin@...or.com, manali.shukla@....com, tao1.su@...ux.intel.com,
 sohil.mehta@...el.com, kai.huang@...el.com, xiaoyao.li@...el.com,
 peterz@...radead.org, xin3.li@...el.com, kan.liang@...ux.intel.com,
 mario.limonciello@....com, thomas.lendacky@....com, perry.yuan@....com,
 gautham.shenoy@....com, chang.seok.bae@...el.com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, peternewman@...gle.com, eranian@...gle.com
Subject: Re: [PATCH v15 32/34] fs/resctrl: Disable BMEC event configuration
 when mbm_event mode is enabled

Hi Reinette,

On 7/17/25 23:02, Reinette Chatre wrote:
> Hi Babu,
> 
> On 7/8/25 3:17 PM, Babu Moger wrote:
>>  
>> +/*
>> + * resctrl_bmec_files_show() — Controls the visibility of BMEC-related resctrl
>> + * files. When @show is true, the files are displayed; when false, the files
>> + * are hidden.
>> + */
>> +static void resctrl_bmec_files_show(struct rdt_resource *r, bool show)
> 
> The "void" return is unexpected since many of the calls can fail. It looks to me
> that this is indeed intentional since there is no BMEC feature checking, but instead
> the existence of the resctrl files are implicitly used as feature check. Doing so enables
> this code to be called on any system whether BMEC is supported or not and thus a failure
> should not be considered an actual failure. If this is the case, then it is subtle so please
> add this information to the function's comments. If this is not the case, could you
> please explain the void return?

Yes. Returned void because in some cases one of the config files may not
be available. Added a comment now.

"Don't treat kernfs_find_and_get failure as an error, since this function
may be called regardless of whether BMEC is supported or the event is
enabled."


> 
>> +{
>> +	struct kernfs_node *kn_config, *l3_mon_kn;
>> +	char name[32];
>> +
>> +	sprintf(name, "%s_MON", r->name);
>> +	l3_mon_kn = kernfs_find_and_get(kn_info, name);
>> +	if (!l3_mon_kn)
>> +		return;
>> +
>> +	kn_config = kernfs_find_and_get(l3_mon_kn, "mbm_total_bytes_config");
>> +	if (kn_config) {
>> +		kernfs_show(kn_config, show);
>> +		kernfs_put(kn_config);
>> +	}
>> +
>> +	kn_config = kernfs_find_and_get(l3_mon_kn, "mbm_local_bytes_config");
>> +	if (kn_config) {
>> +		kernfs_show(kn_config, show);
>> +		kernfs_put(kn_config);
>> +	}
>> +
>> +	kernfs_put(l3_mon_kn);
>> +}
>> +
>>  static int resctrl_mbm_assign_mode_show(struct kernfs_open_file *of,
>>  					struct seq_file *s, void *v)
>>  {
>> @@ -2659,6 +2690,12 @@ static int rdtgroup_mkdir_info_resdir(void *priv, char *name,
>>  			ret = resctrl_mkdir_event_configs(r, kn_subdir);
>>  			if (ret)
>>  				return ret;
>> +			/*
>> +			 * Hide BMEC related files if mbm_event mode
>> +			 * is enabled.
>> +			 */
>> +			if (resctrl_arch_mbm_cntr_assign_enabled(r))
>> +				resctrl_bmec_files_show(r, false);
> 
> Looks like the kernfs_find_and_get(kn_info, name) in resctrl_bmec_files_show()
> can be avoided by providing the kn as parameter. I think you may be doing it like
> this because of how resctrl_bmec_files_show() is used in resctrl_mbm_assign_mode_write()
> in the next patch. I think the kn can still be provided as parameter but 
> resctrl_mbm_assign_mode_write() will set it to NULL and only then does 
> resctrl_bmec_files_show() need to figure it out itself.

Sure. Passed kernfs_node now. If the node is NULL then use
kernfs_find_and_get() to get the pointer to the node.

> 
>>  		}
>>  	}
>>  
> 
> Reinette
> 

-- 
Thanks
Babu Moger