linux-kernel - Re: [PATCH v22 09/18] x86/resctrl: Add a new field to struct rmid

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Date: Fri, 28 Jun 2024 13:54:08 -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 v22 09/18] x86/resctrl: Add a new field to struct
 rmid_read for summation of domains

Hi Tony,

On 6/27/24 1:38 PM, Tony Luck wrote:
> When a user reads a monitor file rdtgroup_mondata_show() calls
> mon_event_read() to package up all the required details into an rmid_read
> structure which is passed across the smp_call*() infrastructure to code
> that will read data from hardware and return the value (or error status)
> in the rmid_read structure.
> 
> Sub-NUMA Cluster (SNC) mode adds files with new semantics. These require
> the smp_call-ed code to sum event data from all domains that share an
> L3 cache.
> 
> Add a pointer to the L3 "cacheinfo" structure to struct rmid_read
> for the data collection routines to use to pick the domains to be
> summed.

This patch has evolved to a point asking for a split. Everything described below
would be a good fit for its own patch. I do not know if a single patch like this
one will be acceptable but at minimum I would propose that you motivate the
additional changes as a result of the semantic changes related to this struct.
For example, something like below inserted at this point in changelog:

	The new semantics rely on some struct rmid_read members having
	NULL values to distinguish between the SNC and non-SNC scenarios.
	resctrl can thus no longer rely on this struct not being initialized
	properly.

> 
> Initialize all on-stack declarations of struct rmid_read:
> 	rdtgroup_mondata_show()
> 	mbm_update()
> 	mkdir_mondata_subdir()
> to ensure that garbage values from the stack are not passed down
> to other functions.
> 
> Remove redundant rr->val = 0; from mon_event_read() and rr.first = false;
> from mbm_update() since the rmid_read structure is cleared by compiler.
> 
> Reinette suggested that the rmid_read structure has become complex enough
> to warrant documentation of each of its fields and provided the kerneldoc
> documentation for struct rmid_read.
> 
> Co-developed-by: Reinette Chatre <reinette.chatre@...el.com>
> Signed-off-by: Reinette Chatre <reinette.chatre@...el.com>
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---

With patch either split or its changes motivated to be logically connected:

| Reviewed-by: Reinette Chatre <reinette.chatre@...el.com>

Reinette