linux-kernel - Re: [RFC PATCH v3 17/17] x86/resctrl: Introduce interface to modify assignment states of the groups

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Date: Fri, 3 May 2024 14:15:45 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Dave Martin <Dave.Martin@....com>
CC: Babu Moger <babu.moger@....com>, <corbet@....net>, <fenghua.yu@...el.com>,
	<tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>, <x86@...nel.org>, <hpa@...or.com>,
	<paulmck@...nel.org>, <rdunlap@...radead.org>, <tj@...nel.org>,
	<peterz@...radead.org>, <yanjiewtw@...il.com>, <kim.phillips@....com>,
	<lukas.bulwahn@...il.com>, <seanjc@...gle.com>, <jmattson@...gle.com>,
	<leitao@...ian.org>, <jpoimboe@...nel.org>, <rick.p.edgecombe@...el.com>,
	<kirill.shutemov@...ux.intel.com>, <jithu.joseph@...el.com>,
	<kai.huang@...el.com>, <kan.liang@...ux.intel.com>,
	<daniel.sneddon@...ux.intel.com>, <pbonzini@...hat.com>,
	<sandipan.das@....com>, <ilpo.jarvinen@...ux.intel.com>,
	<peternewman@...gle.com>, <maciej.wieczor-retman@...el.com>,
	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<eranian@...gle.com>, <james.morse@....com>
Subject: Re: [RFC PATCH v3 17/17] x86/resctrl: Introduce interface to modify
 assignment states of the groups

Hi Dave,

On 5/3/2024 7:53 AM, Dave Martin wrote:
> On Thu, May 02, 2024 at 10:52:15AM -0700, Reinette Chatre wrote:
>> Hi Dave,
>>
>> On 5/2/2024 9:21 AM, Dave Martin wrote:
>>> On Thu, Mar 28, 2024 at 08:06:50PM -0500, Babu Moger wrote:
>>>> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
>>>> index 2d96565501ab..64ec70637c66 100644
>>>> --- a/Documentation/arch/x86/resctrl.rst
>>>> +++ b/Documentation/arch/x86/resctrl.rst
>>>> @@ -328,6 +328,77 @@ with the following files:
>>>>  	 None of events are assigned on this mon group. This is a child
>>>>  	 monitor group of the non default control mon group.
>>>>  
>>>> +	Assignment state can be updated by writing to this interface.
>>>> +
>>>> +	NOTE: Assignment on one domain applied on all the domains. User can
>>>> +	pass one valid domain and assignment will be updated on all the
>>>> +	available domains.
>>>> +
>>>> +	Format is similar to the list format with addition of op-code for the
>>>> +	assignment operation.
>>>> +
>>>> +        * Default CTRL_MON group:
>>>> +                "//<domain_id><op-code><assignment_flags>"
>>>> +
>>>> +        * Non-default CTRL_MON group:
>>>> +                "<CTRL_MON group>//<domain_id><op-code><assignment_flags>"
>>>> +
>>>> +        * Child MON group of default CTRL_MON group:
>>>> +                "/<MON group>/<domain_id><op-code><assignment_flags>"
>>>> +
>>>> +        * Child MON group of non-default CTRL_MON group:
>>>> +                "<CTRL_MON group>/<MON group>/<domain_id><op-code><assignment_flags>"
>>>
>>> The final bullet seems to cover everything, if we allow <CTRL_MON group>
>>> and <MON group> to be independently empty strings to indicate the
>>> default control and/or monitoring group respectively.
>>>
>>> Would that be simpler than treating this as four separate cases?
>>>
>>> Also, will this go wrong if someone creates a resctrl group with '\n'
>>> (i.e., a newline character) in the name?
>>
>> There is a check for this in rdtgroup_mkdir().
> 
> Ah, right.  Found it.  I guess that works.
> 
> On a (sort of) related point, are there any concerns about namespace
> clashes in resctrlfs?  This looks like a potential issue for the resctrl
> top-level directory at least.
> 
> It's not clear to me how userspace can pick a name for a resctrl group
> that is guaranteed not to clash with the name of one of resctrl's own
> files in a future kernel.
> 
> (Note, this is nothing to do with series; I haven't been sure where to
> fit this into the dicsussion...)

It is not obvious to me what scenario you have in mind. Could you please
give an example?

> 
>>
>>>
>>>> +
>>>> +	Op-code can be one of the following:
>>>> +	::
>>>> +
>>>> +	 = Update the assignment to match the flags
>>>> +	 + Assign a new state
>>>> +	 - Unassign a new state
>>>> +	 _ Unassign all the states
>>>
>>> I can't remember whether I already asked this, but is "_" really
>>> needed here?
>>
>> Asked twice:
>> https://lore.kernel.org/lkml/ZiaRXrmDDjc194JI@e133380.arm.com/
>> https://lore.kernel.org/lkml/ZiervIprcwoApAqw@e133380.arm.com/
>>
>> Answered:
>> https://lore.kernel.org/lkml/4cd220cc-2e8e-4193-b01a-d3cd798c7118@amd.com/
>>
>> You seemed ok with answer then:
>> https://lore.kernel.org/lkml/ZiffF93HM8bE3qo7@e133380.arm.com/
> 
> There, I was asking about "_" meaning "no flags" in "=_".

Apologies. I did not notice the difference. Yes, I agree, "_" is
not expected to be an operator.

Reinette