linux-kernel - Re: [PATCH v11 17/23] x86/resctrl: Auto assign/unassign counters when mbm_cntr

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Message-ID: <84e31944-39a2-4e15-9772-56f82e70fbe6@intel.com>
Date: Fri, 21 Feb 2025 14:48:46 -0800
From: Reinette Chatre <reinette.chatre@...el.com>
To: "Moger, Babu" <bmoger@....com>, Dave Martin <Dave.Martin@....com>
CC: Peter Newman <peternewman@...gle.com>, Babu Moger <babu.moger@....com>,
	<corbet@....net>, <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>, <tony.luck@...el.com>, <x86@...nel.org>,
	<hpa@...or.com>, <paulmck@...nel.org>, <akpm@...ux-foundation.org>,
	<thuth@...hat.com>, <rostedt@...dmis.org>, <xiongwei.song@...driver.com>,
	<pawan.kumar.gupta@...ux.intel.com>, <daniel.sneddon@...ux.intel.com>,
	<jpoimboe@...nel.org>, <perry.yuan@....com>, <sandipan.das@....com>,
	<kai.huang@...el.com>, <xiaoyao.li@...el.com>, <seanjc@...gle.com>,
	<xin3.li@...el.com>, <andrew.cooper3@...rix.com>, <ebiggers@...gle.com>,
	<mario.limonciello@....com>, <james.morse@....com>,
	<tan.shaopeng@...itsu.com>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <maciej.wieczor-retman@...el.com>,
	<eranian@...gle.com>
Subject: Re: [PATCH v11 17/23] x86/resctrl: Auto assign/unassign counters when
 mbm_cntr_assign is enabled

Hi Babu,

On 2/21/25 10:23 AM, Moger, Babu wrote:
> Hi All,
> 
> On 2/21/2025 11:14 AM, Dave Martin wrote:
>> Hi,
>>
>> On Thu, Feb 20, 2025 at 09:08:17AM -0800, Reinette Chatre wrote:
>>> Hi Dave,
>>>
>>> On 2/20/25 5:40 AM, Dave Martin wrote:
>>>> On Thu, Feb 20, 2025 at 11:35:56AM +0100, Peter Newman wrote:
>>>>> Hi Reinette,
>>>>>
>>>>> On Wed, Feb 19, 2025 at 6:55 PM Reinette Chatre
>>>>> <reinette.chatre@...el.com> wrote:
>>
>> [...]
>>
>>>>>> Could you please remind me how a user will set this flag?
>>>>>
>>>>> Quoting my original suggestion[1]:
>>>>>
>>>>>   "info/L3_MON/mbm_assign_on_mkdir?
>>>>>
>>>>>    boolean (parsed with kstrtobool()), defaulting to true?"
>>>>>
>>>>> After mount, any groups that got counters on creation would have to be
>>>>> cleaned up, but at least that can be done with forward progress once
>>>>> the flag is cleared.
>>>>>
>>>>> I was able to live with that as long as there aren't users polling for
>>>>> resctrl to be mounted and immediately creating groups. For us, a
>>>>> single container manager service manages resctrl.
>>
>> [...]
>>
>>>> +1
>>>>
>>>> That's basically my position -- the auto-assignment feels like a
>>>> _potential_ nuisance for ABMC-aware users, but it depends on what they
>>>> are trying to do.  Migration of non-ABMC-aware users will be easier for
>>>> basic use cases if auto-assignment occurs by default (as in this
>>>> series).
>>>>
>>>> Having an explicit way to turn this off seems perfectly reasonable
>>>> (and could be added later on, if not provided in this series).
>>>>
>>>>
>>>> What about the question re whether turning mbm_cntr_assign mode on
>>>> should trigger auto-assignment?
>>>>
>>>> Currently turning this mode off and then on again has the effect of
>>>> removing all automatic assignments for extant groups.  This feels
>>>> surprising and/or unintentional (?)
>>>
>>> Connecting to what you start off by saying I also see auto-assignment
>>> as the way to provide a smooth transition for "non-ABMC-aware" users.
>>
>> I agree, and having this on by default also helps non-ABMC-aware users.
>>
>>> To me a user that turns this mode off and then on again can be
>>> considered as a user that is "ABMC-aware" and turning it "off and then
>>> on again" seems like an intuitive way to get to a "clean slate"
>>> wrt counter assignments. This may also be a convenient way for
>>> an "ABMC-aware" user space to unassign all counters and thus also
>>> helpful if resctrl supports the flag that Peter proposed. The flag
>>> seems to already keep something like this in its context with
>>> a name of "mbm_assign_on_mkdir" that could be interpreted as
>>> "only auto assign on mkdir"?
>>
>> Yes, that's reasonable.  It could be a good idea to document this
>> behaviour of switching the mbm_cntr_assign mode, if we think it is
>> useful and people are likely to rely on it.
>>
>> Since mkdir is an implementation detail of the resctrl interface, I'd
>> be tempted to go for a more generic name, say,
>> "mbm_assign_new_mon_groups".  But that's just bikeshedding.
>> The proposed behaviour seems fine.
>>
>> Either way, if this is not included in this series, it could be added
>> later without breaking anything.
> 
> How about more generic "mbm_cntr_assign_auto" ?

I would like to be careful to not make it _too_ generic. Dave already pointed
out that users may be surprised that counters are not auto-assigned when switching
between the different modes so using the the name to help highlight when this
auto-assignment can be expected to happen seems very useful.

Reinette