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From: Peter Newman <peternewman@google.com>
Date: Mon, 20 May 2024 09:00:19 -0700
Message-ID: <CALPaoCihfQ9VtLYzyHB9-PsQzXLc06BW8bzhBXwj9-i+Q8RVFQ@mail.gmail.com>
Subject: Re: [RFC PATCH v3 00/17] x86/resctrl : Support AMD Assignable
 Bandwidth Monitoring Counters (ABMC)
To: babu.moger@amd.com
Cc: Reinette Chatre <reinette.chatre@intel.com>, corbet@lwn.net, fenghua.yu@intel.com, 
	tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, 
	dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, 
	paulmck@kernel.org, rdunlap@infradead.org, tj@kernel.org, 
	peterz@infradead.org, yanjiewtw@gmail.com, kim.phillips@amd.com, 
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	leitao@debian.org, jpoimboe@kernel.org, rick.p.edgecombe@intel.com, 
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	pbonzini@redhat.com, sandipan.das@amd.com, ilpo.jarvinen@linux.intel.com, 
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Hi Babu,

On Mon, May 20, 2024 at 7:25=E2=80=AFAM Moger, Babu <babu.moger@amd.com> wr=
ote:
>
> Hi Peter,
>
> On 5/17/24 16:51, Peter Newman wrote:
> > Hi Reinette, Babu,
> >
> > On Fri, May 3, 2024 at 2:15=E2=80=AFPM Reinette Chatre
> > <reinette.chatre@intel.com> wrote:
> >>
> >> Hi Peter,
> >>
> >> On 5/3/2024 2:00 PM, Peter Newman wrote:
> >>> Hi Babu,
> >>>
> >>> On Fri, May 3, 2024 at 1:44=E2=80=AFPM Moger, Babu <bmoger@amd.com> w=
rote:
> >>>>
> >>>> Hi Peter,
> >>>>
> >>>> On 5/2/2024 7:57 PM, Peter Newman wrote:
> >>>>> Hi Reinette,
> >>>>>
> >>>>> On Thu, May 2, 2024 at 4:21=E2=80=AFPM Reinette Chatre
> >>>>>> I do think ABMC should be enabled by default when available and it=
 looks
> >>>>>> to be what this series aims to do [1]. The way I reason about this=
 is
> >>>>>> that legacy user space gets more reliable monitoring behavior with=
out
> >>>>>> needing to change behavior.
> >>>>>
> >>>>> I don't like that for a monitor assignment-aware user, following th=
e
> >>>>> creation of new monitoring groups, there will be less monitors
> >>>>> available for assignment. If the user wants precise control over wh=
ere
> >>>>> monitors are allocated, they would need to manually unassign the
> >>>>> automatically-assigned monitor after creating new groups.
> >>>>>
> >>>>> It's an annoyance, but I'm not sure if it would break any realistic
> >>>>> usage model. Maybe if the monitoring agent operates independently o=
f
> >>>>
> >>>> Yes. Its annoyance.
> >>>>
> >>>> But if you think about it, normal users don't create too many groups=
.
> >>>> They wont have to worry about assign/unassign headache if we enable
> >>>> monitor assignment automatically. Also there is pqos tool which uses
> >>>> this interface. It does not have to know about assign/unassign stuff=
.
> >>>
> >>> Thinking about this again, I don't think it's much of a concern
> >>> because the automatic assignment on mongroup creation behavior can be
> >>> trivially disabled using a boolean flag.
> >>
> >> This could be a config option.
> >
> > I'd like to work out the details of this option.
> >
> > info/L3_MON/mbm_assign_on_mkdir?
> >
> > boolean (parsed with kstrtobool()), defaulting to true?
>
> I am thinking is not a big concern. We only have limited (32) counters.
> Automatic monitor assignment works only for first 16 groups(2 counters fo=
r
> each group). When the counters are exhausted auto assignment does not
> work. In your case(with more than 16 groups) the auto assignment does not
> work. I feel having a config option is really not necessary.

I'm not sure I follow the argument you're trying to make because it
doesn't sound like an argument against adding a config option. What
exactly do you mean by "work" vs "not work"?

Also it doesn't address my original concern about needing to manually
(and non-atomically) undo the auto assignment in order to account for
where the monitors are assigned or ensure that creating a new
monitoring group will succeed.

-Peter