linux-kernel - Re: [PATCH v6 19/22] x86/resctrl: Introduce the interface to switch between monitor modes

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Message-ID: <CALPaoCjmRyP00b9rTCjpxwLDJ2rYkQ8HuNMYJA+qVNo5a4Q9JQ@mail.gmail.com>
Date: Fri, 16 Aug 2024 10:16:06 -0700
From: Peter Newman <peternewman@...gle.com>
To: Reinette Chatre <reinette.chatre@...el.com>
Cc: James Morse <james.morse@....com>, Babu Moger <babu.moger@....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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	kirill.shutemov@...ux.intel.com, jithu.joseph@...el.com, kai.huang@...el.com, 
	kan.liang@...ux.intel.com, daniel.sneddon@...ux.intel.com, 
	pbonzini@...hat.com, sandipan.das@....com, ilpo.jarvinen@...ux.intel.com, 
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	fenghua.yu@...el.com, tglx@...utronix.de
Subject: Re: [PATCH v6 19/22] x86/resctrl: Introduce the interface to switch
 between monitor modes

Hi Reinette,

On Fri, Aug 16, 2024 at 10:01 AM Reinette Chatre
<reinette.chatre@...el.com> wrote:
>
> Hi James,
>
> On 8/16/24 9:31 AM, James Morse wrote:
> > Hi Babu,
> >
> > On 06/08/2024 23:00, Babu Moger wrote:
> >> Introduce interface to switch between ABMC and legacy modes.
> >>
> >> By default ABMC is enabled on boot if the feature is available.
> >> Provide the interface to go back to legacy mode if required.
> >
> > I may have missed it on an earlier version ... why would anyone want the non-ABMC
> > behaviour on hardware that requires it: counters randomly reset and randomly return
> > 'Unavailable'... is that actually useful?
> >
> > You default this to on, so there isn't a backward compatibility argument here.
> >
> > It seems like being able to disable this is a source of complexity - is it needed?
>
> The ability to go back to legacy was added while looking ahead to support the next
> "assignable counter" feature that is software based ("soft-RMID" .. "soft-ABMC"?).
>
> This series adds support for ABMC on recent AMD hardware to address the issue described
> in cover letter. This issue also exists on earlier AMD hardware that does not have the ABMC
> feature and Peter is working on a software solution to address the issue on non-ABMC hardware.
> This software solution is expected to have the same interface as the hardware solution but
> earlier discussions revealed that it may introduce extra latency that users may only want to
> accept during periods of active monitoring. Thus the option to disable the counter assignment
> mode.

Sorry again for the soft-RMID/soft-ABMC confusion[1], it was soft-RMID
that impacted context switch latency. Soft-ABMC does not require any
additional work at context switch.

The only disadvantage to soft-ABMC I can think of is that it also
limits reading llc_occupancy event counts to "assigned" groups,
whereas without it, llc_occupancy works reliably on all RMIDs on AMD
hardware.

-Peter

[1] https://lore.kernel.org/lkml/CALPaoChDv+irGEmccaQ6SpsuVS8PZ_cfzPgceq3hD3N2cqNjZA@mail.gmail.com/