linux-kernel - Re: [PATCH v19 11/20] x86/resctrl: Allocate a new bit in union mon_data

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Message-ID: <546228e7-0a77-42d3-b2a3-ed6b507f018e@intel.com>
Date: Thu, 30 May 2024 13:21:59 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>,
	"Maciej Wieczor-Retman" <maciej.wieczor-retman@...el.com>, Peter Newman
	<peternewman@...gle.com>, James Morse <james.morse@....com>, Babu Moger
	<babu.moger@....com>, Drew Fustini <dfustini@...libre.com>, Dave Martin
	<Dave.Martin@....com>
CC: <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
	<patches@...ts.linux.dev>
Subject: Re: [PATCH v19 11/20] x86/resctrl: Allocate a new bit in union
 mon_data_bits

Hi Tony,

Regarding shortlog: isn't the purpose of this change to _avoid_
allocating a new bit?

On 5/28/24 3:19 PM, Tony Luck wrote:
> When Sub-NUMA Cluster (SNC) mode is enabled the legacy monitor reporting
> files must report the sum of the data from all of the SNC nodes that
> share the L3 cache that is referenced by the monitor file.
> 
> Resctrl squeezes all the attributes of these files into 32-bits so they
> can be stored in the "priv" field of struct kernfs_node.
> 
> Arbitrarily choose the "evtid" field to sacrifice one bit to make
> space for a new bit. This structure is purely internal to resctrl,

Missing explanation why this is ok for this field to sacrifice a bit.

> no ABI issues with modifying it. Subsequent changes may rearrange the
> allocation of bits between each of the fields as needed.
> 
> The stolen bit is given to a new "sum" field that indicates that reading

stolen? Is that necessary? It can just be "Give the bit ..." (also
note imperative tone)

> this file must sum across SNC nodes. This bit also indicates that the
> domid field is the l3_cache_id (instead of a domain id) to find which
> domains must be summed.

l3_cache_id looks like a variable that does not exist anywhere in this series
or in existing resctrl.

Reinette