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

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Message-ID: <ZetcM9GO2PH6SC0j@agluck-desk3>
Date: Fri, 8 Mar 2024 10:42:59 -0800
From: Tony Luck <tony.luck@...el.com>
To: James Morse <james.morse@....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

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

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

> (I also think that this be over the threshold on 'funny machines look funny' - but I bet
> someone builds an arm machine that looks like this too!)

-Tony