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

Dave Martin
2024-05-03T14:53:42Z

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