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

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Message-ID: <983fded5-48f8-439d-8afe-45b60841985a@arm.com>
Date: Fri, 16 Aug 2024 17:31:33 +0100
From: James Morse <james.morse@....com>
To: Babu Moger <babu.moger@....com>
Cc: x86@...nel.org, hpa@...or.com, paulmck@...nel.org, rdunlap@...radead.org,
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Subject: Re: [PATCH v6 19/22] x86/resctrl: Introduce the interface to switch
 between monitor modes

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?


For MPAM I'm looking at enabling this on any platform that is short of monitors. If
user-space disables it I don't have a "at random" hardware behaviour to fall back on - its
extra work to invent a behaviour I'm not sure is useful...


> $ cat /sys/fs/resctrl/info/L3_MON/mbm_mode
> [mbm_cntr_assign]
> legacy
> 
> To enable the "mbm_cntr_assign" mode:
> $ echo "mbm_cntr_assign" > /sys/fs/resctrl/info/L3_MON/mbm_mode
> 
> To enable the legacy monitoring feature:
> $ echo "legacy" > /sys/fs/resctrl/info/L3_MON/mbm_mode
> 
> MBM event counters will reset when mbm_mode is changed.


Thanks,

James