linux-kernel - Re: [PATCH v4 6/7] x86/resctrl: Update documentation with Sub-NUMA cluster changes

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Date:   Fri, 25 Aug 2023 10:50:27 -0700
From:   Tony Luck <tony.luck@...el.com>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Peter Newman <peternewman@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <skhan@...uxfoundation.org>, x86@...nel.org,
        Shaopeng Tan <tan.shaopeng@...itsu.com>,
        James Morse <james.morse@....com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        Babu Moger <babu.moger@....com>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        patches@...ts.linux.dev
Subject: Re: [PATCH v4 6/7] x86/resctrl: Update documentation with Sub-NUMA
 cluster changes

On Fri, Aug 11, 2023 at 10:33:18AM -0700, Reinette Chatre wrote:
> Hi Tony,
> 
> On 7/22/2023 12:07 PM, Tony Luck wrote:
> > With Sub-NUMA Cluster mode enabled the scope of monitoring resources is
> > per-NODE instead of per-L3 cache. Suffixes of directories with "L3" in
> > their name refer to Sub-NUMA nodes instead of L3 cache ids.
> > 
> > Signed-off-by: Tony Luck <tony.luck@...el.com>
> > Reviewed-by: Peter Newman <peternewman@...gle.com>
> > ---
> >  Documentation/arch/x86/resctrl.rst | 10 +++++++---
> >  1 file changed, 7 insertions(+), 3 deletions(-)
> > 
> > diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
> > index cb05d90111b4..4d9ddb91751d 100644
> > --- a/Documentation/arch/x86/resctrl.rst
> > +++ b/Documentation/arch/x86/resctrl.rst
> > @@ -345,9 +345,13 @@ When control is enabled all CTRL_MON groups will also contain:
> >  When monitoring is enabled all MON groups will also contain:
> >  
> >  "mon_data":
> > -	This contains a set of files organized by L3 domain and by
> > -	RDT event. E.g. on a system with two L3 domains there will
> > -	be subdirectories "mon_L3_00" and "mon_L3_01".	Each of these
> > +	This contains a set of files organized by L3 domain or by NUMA
> > +	node (depending on whether Sub-NUMA Cluster (SNC) mode is disabled
> > +	or enabled respectively) and by RDT event. E.g. on a system with
> > +	SNC mode disabled with two L3 domains there will be subdirectories
> > +	"mon_L3_00" and "mon_L3_01". The numerical suffix refers to the
> > +	L3 cache id.  With SNC enabled the directory names are the same,
> > +	but the numerical suffix refers to the node id.  Each of these
> >  	directories have one file per event (e.g. "llc_occupancy",
> >  	"mbm_total_bytes", and "mbm_local_bytes"). In a MON group these
> >  	files provide a read out of the current value of the event for
> 
> I think it would be helpful to add a modified version of the snippet
> (from previous patch changelog) regarding well-behaved NUMA apps.
> With the above it may be confusing that a single cache allocation has
> multiple cache occupancy counters. 
> 
> This also changes the meaning of the numbers in the directory names.
> The documentation already provides guidance on how to find the cache
> ID of a logical CPU (see section "Cache IDs"). I think it will be 
> helpful to add a snippet that makes it clear to users how to map
> a CPU to its node ID.

Added extra details as suggested.

> 
> Reinette

-Tony