linux-kernel - Re: [PATCH v19 00/20] Add support for Sub-NUMA cluster (SNC) systems

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Date: Thu, 30 May 2024 10:55:48 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Tony Luck <tony.luck@...el.com>
CC: Fenghua Yu <fenghua.yu@...el.com>, Maciej Wieczor-Retman
	<maciej.wieczor-retman@...el.com>, Peter Newman <peternewman@...gle.com>,
	James Morse <james.morse@....com>, Babu Moger <babu.moger@....com>, "Drew
 Fustini" <dfustini@...libre.com>, Dave Martin <Dave.Martin@....com>,
	<x86@...nel.org>, <linux-kernel@...r.kernel.org>, <patches@...ts.linux.dev>
Subject: Re: [PATCH v19 00/20] Add support for Sub-NUMA cluster (SNC) systems

Hi Tony,

On 5/30/24 9:36 AM, Tony Luck wrote:
> On Wed, May 29, 2024 at 07:46:27PM -0700, Reinette Chatre wrote:
>> Hi Tony,
>>
>> On 5/29/24 1:20 PM, Tony Luck wrote:
>>> On Tue, May 28, 2024 at 03:55:29PM -0700, Reinette Chatre wrote:
>>>> Hi Tony,
>>>>> 13:	Wordsmith commit into imperative.
>>>>> 	I looked at using kobject_has_children() to check for empty
>>>>> 	directory, but it needs a "struct kobject *" and all I have
>>>>> 	is "struct kernfs_node *". I'm now checking how many CPUs
>>>>
>>>> Consider how kobject_has_children() uses that struct kobject *.
>>>> Specifically:
>>>> 	return kobj->sd && kobj->sd->dir.subdirs
>>>>
>>>> It operates on kobj->sd, which is exactly what you have: struct kernfs_node.
>>>
>>> So right. My turn to grumble about other peoples choice of names. If
>>> that field was named "kn" instead of "sd" I would have spotted this
>>> too.
>>>
>>>>> 	remain in ci->shared_cpu_map to detect whether this is the
>>>>> 	last SNC node.
>>>>
>>>> hmmm, ok, will take a look ... but please finalize discussion of a patch series
>>>> before submitting a new series that rejects feedback without discussion and
>>>> does something completely different in new version.
>>>
>>> Reinette,
>>>
>>> So here's what rmdir_mondata_subdir_allrdtgrp() looks like using the
>>> subdirs check. It might need an update/better header comment.
>>>
>>> -Tony
>>>
>>> ---
>>>
>>> /*
>>>    * Remove all subdirectories of mon_data of ctrl_mon groups
>>>    * and monitor groups with given domain id.
>>
>> (note comment still considers that domain id is parameter)
> 
> Will fix.
> 
>>>    */
>>> static void rmdir_mondata_subdir_allrdtgrp(struct rdt_resource *r,
>>> 					   struct rdt_mon_domain *d)
>>> {
>>> 	struct rdtgroup *prgrp, *crgrp;
>>> 	struct kernfs_node *kn;
>>> 	char subname[32];
>>
>> I wonder if static checkers will know that this cannot be used
>> uninitialized?
> 
> I wondered that too. There are no complaints from gcc. How do people
> deal with false positives from static checkers? Simplest would be to
> provide an initializer:
> 
> 	char subname[32] = "";
> 
> While that might shut up the static check, it would be more confusing
> for human readers.

or	char subname[32] = {};

Please elaborate how this will be confusing to human readers? A comment
may help to address that.

I took the time to run a static checker on this series and it did
not complain about this issue. I did not run it with this fixup though, with
just original submission that seem to have similar pattern. I do still think
it would be good to initialize the arrays.

btw ... the static checker I ran did have four other complaints, three about
uninitialized data and one about divide by zero. Most problems appear to be
in mbm_update() that does not initialize rr.sumdomains nor rr.ci before
calling __mon_event_count().

Please use available tools to check code before posting.

> 
>>> 	char name[32];
>>>
>>> 	sprintf(name, "mon_%s_%02d", r->name, d->ci->id);
>>> 	if (r->mon_scope != RESCTRL_L3_CACHE) {
>>> 		/*
>>> 		 * SNC mode: Unless the last domain is being removed must
>>> 		 * just remove the SNC subdomain.
>>> 		 */
>>> 		sprintf(subname, "mon_sub_%s_%02d", r->name, d->hdr.id);
>>> 	}
>>>
>>> 	list_for_each_entry(prgrp, &rdt_all_groups, rdtgroup_list) {
>>> 		kn = kernfs_find_and_get(prgrp->mon.mon_data_kn, name);
>>> 		if (!kn)
>>> 			continue;
>>>
>>> 		if (kn->dir.subdirs <= 1)
>>> 			kernfs_remove(kn);
>>> 		else
>>> 			kernfs_remove_by_name(kn, subname);
>>>
>>> 		list_for_each_entry(crgrp, &prgrp->mon.crdtgrp_list, mon.crdtgrp_list) {
>>> 			kn = kernfs_find_and_get(crgrp->mon.mon_data_kn, name);
>>> 			if (!kn)
>>> 				continue;
>>>
>>> 			if (kn->dir.subdirs <= 1)
>>> 				kernfs_remove(kn);
>>> 			else
>>> 				kernfs_remove_by_name(kn, subname);
>>> 		}
>>> 	}
>>> }
>>
>> This solution looks more intuitive to me. I do think that it may be
>> missing some kernfs_put()'s?
> 
> There aren't any kernfs_put()'s in the existing code.

Why should it? Existing code does not have the kernfs_put()'s because
the extra reference is only obtained in this new code.

Reinette