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

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Date: Wed, 17 Apr 2024 13:56:28 -0700
From: Peter Newman <peternewman@...gle.com>
To: babu.moger@....com
Cc: corbet@....net, fenghua.yu@...el.com, reinette.chatre@...el.com, 
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
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	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 Babu,

On Wed, Apr 17, 2024 at 12:39 PM Moger, Babu <babu.moger@....com> wrote:
> On 4/17/24 12:45, Peter Newman wrote:
> > On Thu, Mar 28, 2024 at 6:10 PM Babu Moger <babu.moger@....com> 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.
> >
> > How would different assignments to different domains work? If the
> > allocations are global, then the allocated monitor ID is available to
> > all domains whether they use it or not.
>
> That is correct.
> [A] Hardware counters(max 2 per group) are allocated at the group level.
> So, those counters are available to all the domains on that group. I will
> maintain a bitmap at the domain level. The bitmap will be set on the
> domains where assignment is applied and IPIs are sent. IPIs will not be
> sent to other domains.

Unless the monitor allocation is scoped at the domain level, I don't
see much point in implementing the per-domain parsing today, as the
only benefit is avoiding IPIs to domains whose counters you don't plan
to read.

-Peter