linux-kernel - Re: [RFC PATCH v3 00/17] x86/resctrl : Support AMD Assignable Bandwidth Monitoring Counters (ABMC)

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Message-ID: <CALPaoCihfQ9VtLYzyHB9-PsQzXLc06BW8bzhBXwj9-i+Q8RVFQ@mail.gmail.com>
Date: Mon, 20 May 2024 09:00:19 -0700
From: Peter Newman <peternewman@...gle.com>
To: babu.moger@....com
Cc: Reinette Chatre <reinette.chatre@...el.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, 
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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 00/17] x86/resctrl : Support AMD Assignable
 Bandwidth Monitoring Counters (ABMC)

Hi Babu,

On Mon, May 20, 2024 at 7:25 AM Moger, Babu <babu.moger@....com> wrote:
>
> Hi Peter,
>
> On 5/17/24 16:51, Peter Newman wrote:
> > Hi Reinette, Babu,
> >
> > On Fri, May 3, 2024 at 2:15 PM Reinette Chatre
> > <reinette.chatre@...el.com> wrote:
> >>
> >> Hi Peter,
> >>
> >> On 5/3/2024 2:00 PM, Peter Newman wrote:
> >>> Hi Babu,
> >>>
> >>> On Fri, May 3, 2024 at 1:44 PM Moger, Babu <bmoger@....com> wrote:
> >>>>
> >>>> Hi Peter,
> >>>>
> >>>> On 5/2/2024 7:57 PM, Peter Newman wrote:
> >>>>> Hi Reinette,
> >>>>>
> >>>>> On Thu, May 2, 2024 at 4:21 PM 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 without
> >>>>>> needing to change behavior.
> >>>>>
> >>>>> I don't like that for a monitor assignment-aware user, following the
> >>>>> creation of new monitoring groups, there will be less monitors
> >>>>> available for assignment. If the user wants precise control over where
> >>>>> 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 of
> >>>>
> >>>> 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 for
> 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