linux-kernel - Re: [PATCH v2 00/17] x86/resctrl : Support AMD Assignable Bandwidth Monitoring Counters (ABMC)

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Date: Fri, 23 Feb 2024 09:17:53 -0800
From: Reinette Chatre <reinette.chatre@...el.com>
To: <babu.moger@....com>, James Morse <james.morse@....com>, <corbet@....net>,
	<fenghua.yu@...el.com>, <tglx@...utronix.de>, <mingo@...hat.com>,
	<bp@...en8.de>, <dave.hansen@...ux.intel.com>
CC: <x86@...nel.org>, <hpa@...or.com>, <paulmck@...nel.org>,
	<rdunlap@...radead.org>, <tj@...nel.org>, <peterz@...radead.org>,
	<yanjiewtw@...il.com>, <kim.phillips@....com>, <lukas.bulwahn@...il.com>,
	<seanjc@...gle.com>, <jmattson@...gle.com>, <leitao@...ian.org>,
	<jpoimboe@...nel.org>, <rick.p.edgecombe@...el.com>,
	<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>,
	<peternewman@...gle.com>, <maciej.wieczor-retman@...el.com>,
	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<eranian@...gle.com>
Subject: Re: [PATCH v2 00/17] x86/resctrl : Support AMD Assignable Bandwidth
 Monitoring Counters (ABMC)



On 2/20/2024 12:48 PM, Moger, Babu wrote:
> On 2/20/24 09:21, James Morse wrote:
>> On 19/01/2024 18:22, Babu Moger wrote:

>>> e. Enable ABMC mode.
>>>
>>> 	#echo 1 > /sys/fs/resctrl/info/L3_MON/mbm_assign_enable
>>>         #cat /sys/fs/resctrl/info/L3_MON/mbm_assign_enable
>>>         1
>>
>> Why does this mode need enabling? Can't it be enabled automatically on hardware that
>> supports it, or enabled implicitly when the first assignment attempt arrives?
>>
>> I guess this is really needed for a reset - could we implement that instead? This way
>> there isn't an extra step user-space has to do to make the assignments work.
> 
> Mostly the new features are added as an opt-in method. So, kept it that
> way. If we enable this feature automatically, then we have provide an
> option to disable it.
> 

At the same time it sounds to me like ABMC can improve current users'
experience without requiring them to do anything. This sounds appealing.
For example, if I understand correctly, it may be possible to start resctrl
with ABMC enabled by default and the number of monitoring groups (currently
exposed to user space via "num_rmids") limited to the number of counters
supported by ABMC. Existing users would then by default obtain better behavior
of counters not resetting.

The "new feature" could then be viewed as adding support for more monitoring
groups than what hardware can support concurrently.

Reinette