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

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Message-ID: <88430722-67b3-4f7d-8db2-95ee52b6f0b0@arm.com>
Date: Fri, 15 Mar 2024 18:02:10 +0000
From: James Morse <james.morse@....com>
To: Tony Luck <tony.luck@...el.com>
Cc: Reinette Chatre <reinette.chatre@...el.com>,
 "Wieczor-Retman, Maciej" <maciej.wieczor-retman@...el.com>,
 "Yu, Fenghua" <fenghua.yu@...el.com>, Shuah Khan <shuah@...nel.org>,
 "ilpo.jarvinen@...ux.intel.com" <ilpo.jarvinen@...ux.intel.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH 4/4] selftests/resctrl: Adjust SNC support messages

Hi Tony,

On 08/03/2024 18:42, Tony Luck wrote:
> On Fri, Mar 08, 2024 at 06:06:45PM +0000, James Morse wrote:
>> Hi guys,
>>
>> On 07/03/2024 23:16, Tony Luck wrote:
>>> On Thu, Mar 07, 2024 at 02:39:08PM -0800, Reinette Chatre wrote:
>>>> Thank you for the example. I find that significantly easier to
>>>> understand than a single number in a generic "nodes_per_l3_cache".
>>>> Especially with potential confusion surrounding inconsistent "nodes"
>>>> between allocation and monitoring. 
>>>>
>>>> How about domain_cpu_list and domain_cpu_map ?
>>
>>> Like this (my test system doesn't have SNC, so all domains are the same):
>>>
>>> $ cd /sys/fs/resctrl/info/
>>> $ grep . */domain*
>>> L3/domain_cpu_list:0: 0-35,72-107
>>> L3/domain_cpu_list:1: 36-71,108-143
>>> L3/domain_cpu_map:0: 0000,00000fff,ffffff00,0000000f,ffffffff
>>> L3/domain_cpu_map:1: ffff,fffff000,000000ff,fffffff0,00000000
>>> L3_MON/domain_cpu_list:0: 0-35,72-107
>>> L3_MON/domain_cpu_list:1: 36-71,108-143
>>> L3_MON/domain_cpu_map:0: 0000,00000fff,ffffff00,0000000f,ffffffff
>>> L3_MON/domain_cpu_map:1: ffff,fffff000,000000ff,fffffff0,00000000
>>> MB/domain_cpu_list:0: 0-35,72-107
>>> MB/domain_cpu_list:1: 36-71,108-143
>>> MB/domain_cpu_map:0: 0000,00000fff,ffffff00,0000000f,ffffffff
>>> MB/domain_cpu_map:1: ffff,fffff000,000000ff,fffffff0,00000000
>>
>> This duplicates the information in /sys/devices/system/cpu/cpuX/cache/indexY ... is this
>> really because that information is, er, wrong on SNC systems. Is it possible to fix that?
> 
> On an SNC system the resctrl domain for L3_MON becomes the SNC node
> instead of the L3 cache instance. With 2, 3, or 4 SNC nodes per L3.
> 
> Even without the SNC issue this duplication may be a useful
> convienience. On Intel to get from a resctrl domain is a multi-step
> process to first find which of the indexY directories has level=3
> and then look for the "id" that matches the domain.
> 
>> >From Tony's earlier description of how SNC changes things, the MB controls remain
>> per-socket. To me it feels less invasive to fix the definition of L3 on these platforms to
>> describe how it behaves (assuming that is possible), and define a new 'MB' that is NUMA
>> scoped.
>> This direction of redefining L3 means /sys/fs/resctrl and /sys/devices have different
>> views of 'the' cache hierarchy.
> 
> I almost went partly in that direction when I started this epic voyage.
> The "almost" part was to change the names of the monitoring directories
> under mon_data from (legacy non-SNC system):
> 
> $ ls -l mon_data
> total 0
> dr-xr-xr-x. 2 root root 0 Mar  8 10:31 mon_L3_00
> dr-xr-xr-x. 2 root root 0 Mar  8 10:31 mon_L3_01
> 
> to (2 socket, SNC=2 system):
> 
> $ ls -l mon_data
> total 0
> dr-xr-xr-x. 2 root root 0 Mar  8 10:31 mon_NODE_00
> dr-xr-xr-x. 2 root root 0 Mar  8 10:31 mon_NODE_01
> dr-xr-xr-x. 2 root root 0 Mar  8 10:31 mon_NODE_02
> dr-xr-xr-x. 2 root root 0 Mar  8 10:31 mon_NODE_03

This would be useful for MPAM. I've seen a couple of MPAM systems that have per-NUMA MPAM
controls on the 'L3', but describe it as a single global L3. The MPAM driver currently
hides this by summing the NUMA node counters and reporting it as the global L3's value.


> While that is in some ways a more accurate view, it breaks a lot of
> legacy monitoring applications that expect the "L3" names.

True - but the behaviour is different from a non SNC system, if this software can read the
file - but goes wrong because the contents of the file represent something different, its
still broken.




Thanks,

James