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Date: Fri, 3 May 2024 15:53:35 +0100
From: Dave Martin <Dave.Martin@arm.com>
To: Reinette Chatre <reinette.chatre@intel.com>
Cc: Babu Moger <babu.moger@amd.com>, corbet@lwn.net, fenghua.yu@intel.com,
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Subject: Re: [RFC PATCH v3 17/17] x86/resctrl: Introduce interface to modify
 assignment states of the groups
Message-ID: <ZjT6b/nbZWjm0vw+@e133380.arm.com>
References: <cover.1711674410.git.babu.moger@amd.com>
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 <4875492c-13cd-4a77-a42f-243fe0f868a2@intel.com>
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In-Reply-To: <4875492c-13cd-4a77-a42f-243fe0f868a2@intel.com>

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...)

> 
> > 
> >> +
> >> +	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 "=_".

> 
> > 
> > Wouldn't it be the case that
> > 
> > 	//*_
> > 
> > would mean just the same thing as
> > 
> > 	//*=_
> > 
> > ...?  (assuming the "*" = "all domains" convention already discussed)
> > 
> > Maybe I'm missing something here.
> 
> I believe have an explicit operator ("+", "=", or "-") simplifies
> parsing while providing an interface consistent with what users are already
> used to.
> 
> Reinette

That was the point I was trying to make here, apologies if I wasn't
clear.

Cheers
---Dave