Re: [PATCH 4/4] selftests/resctrl: Adjust SNC support messages

From: Reinette Chatre <reinette.chatre@intel.com>
To: "Luck, Tony" <tony.luck@intel.com>,
	"Wieczor-Retman, Maciej" <maciej.wieczor-retman@intel.com>
Cc: "Yu, Fenghua" <fenghua.yu@intel.com>,
	Shuah Khan <shuah@kernel.org>,
	"james.morse@arm.com" <james.morse@arm.com>,
	"ilpo.jarvinen@linux.intel.com" <ilpo.jarvinen@linux.intel.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-kselftest@vger.kernel.org"
	<linux-kselftest@vger.kernel.org>
Subject: Re: [PATCH 4/4] selftests/resctrl: Adjust SNC support messages
Date: Thu, 7 Mar 2024 11:52:41 -0800	[thread overview]
Message-ID: <0393c4ce-7e41-4dcc-940a-a6bea9437970@intel.com> (raw)
In-Reply-To: <SJ1PR11MB60832DAD58E864F99A16FCC4FC202@SJ1PR11MB6083.namprd11.prod.outlook.com>

Hi Tony,

On 3/7/2024 9:57 AM, Luck, Tony wrote:
>>> SNC2 enabled:
>>>
>>> $ cat /sys/fs/resctrl/info/L3_mon/ snc_nodes_per_l3_cache
>>> 2
>>>
>>
>> This would be useful. I believe "SNC" is architecture specific?
>> What if the file always exists and is named "nodes_per_l3_cache"?
>>
>> I assume that the internals of handling more nodes per L3 cache should
>> be hidden from user space and it should not be necessary for user space
>> to know if this is because of SNC or potentially some other mechanism on
>> another platform?
>>
>> I think that may reduce fragmentation of resctrl .... not having
>> resctrl look so different on different architectures but maintains
>> the promise of a generic interface.
>>
>> I am not sure if this is specific to monitoring though,
>> why not host file in /sys/fs/resctrl/info/L3 ?
> 
> Reinette,
> 
> On the name change - sure. It doesn't need the "snc_" prefix.
> 
> The Intel implementation of SNC has far more effect on monitoring
> than on control. The user can read separate cache occupancy and
> memory bandwidth values for each SNC node. But cache allocation
> bitmasks and memory throttling still have a single control point for
> each L3 cache instance, not for each node. There are still some
> impacts on control, e.g. each bit in a CAT bitmask represents
> less actual space in the L3 cache.

I understand the impact but I am trying to view this conceptually.
The info directory exists to "contain information about the enabled
resources" (as per documentation) and it seems appropriate to make
this a property of the L3 resource.

> 
> Maybe move it to the top level of the info/ directory:
> 
> $ cat /sys/fs/resctrl/info/nodes_per_l3_cache
> 3

Thinking about it even differently. The goal is to give information
to userspace so we need to think about what would help user space?
For example, what if there is a file in info that shows 
which CPUs are associated with each domain?

Reinette