linux-kernel - Re: [PATCH v5 7/8] x86/resctrl: Update documentation with Sub-NUMA cluster changes

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Date:   Thu, 28 Sep 2023 11:15:44 -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 v5 7/8] x86/resctrl: Update documentation with Sub-NUMA
 cluster changes

On Mon, Sep 25, 2023 at 04:30:22PM -0700, Reinette Chatre wrote:
> Hi Tony,
> 
> On 8/29/2023 4:44 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.
> > 
> > Users should be aware that SNC mode also affects the amount of L3 cache
> > available for allocation within each SNC node.
> > 
> > Signed-off-by: Tony Luck <tony.luck@...el.com>
> > ---
> >  Documentation/arch/x86/resctrl.rst | 25 ++++++++++++++++++++++---
> >  1 file changed, 22 insertions(+), 3 deletions(-)
> > 
> > diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
> > index cb05d90111b4..407764f43f25 100644
> > --- a/Documentation/arch/x86/resctrl.rst
> > +++ b/Documentation/arch/x86/resctrl.rst
> > @@ -345,9 +345,15 @@ 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.
> > +        Mappings from node ids to CPUs are available in the
> > +        /sys/devices/system/node/node*/cpulist files. Each of these
> 
> Please be consistent with tabs vs spaces.

I blame "vim" for this one. Typing <TAB> while working on a .rst file
throws in 8 spaces. Maybe there's a vim setting to stop this annoying
behaviour (set syntax=off didn't help). Had to use CTRL(V)<TAB> to make
it work.

> 
> >  	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
> > @@ -452,6 +458,19 @@ and 0xA are not.  On a system with a 20-bit mask each bit represents 5%
> >  of the capacity of the cache. You could partition the cache into four
> >  equal parts with masks: 0x1f, 0x3e0, 0x7c00, 0xf8000.
> >  
> > +Notes on Sub-NUMA Cluster mode
> > +==============================
> > +When SNC mode is enabled the "llc_occupancy", "mbm_total_bytes", and
> > +"mbm_local_bytes" will only give accurate results for well behaved NUMA
> > +applications. I.e. those that perform the majority of memory accesses
> > +to memory on the local NUMA node to the CPU where the task is executing.
> > +
> 
> Following our discussion of the cover letter I believe this will change to
> be specific about what user space can expect and what is actually "accurate".

I added more text about the importance of binding tasks to SNC nodes.
> 
> > +The cache allocation feature still provides the same number of
> > +bits in a mask to control allocation into the L3 cache. But each
> > +of those ways has its capacity reduced because the cache is divided
> > +between the SNC nodes. The values reported in the resctrl
> > +"size" files are adjusted accordingly.
> > +
> >  Memory bandwidth Allocation and monitoring
> >  ==========================================
> >  
> 
> Reinette