linux-kernel - Re: [PATCH v20 11/18] x86/resctrl: Allocate a new field in union mon_data

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Date: Thu, 20 Jun 2024 14:28:28 -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 v20 11/18] x86/resctrl: Allocate a new field in union
 mon_data_bits

Hi Tony,

(looks like we are stuck with "allocate")

On 6/10/24 11:35 AM, 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.
> 
> Currently only three monitor events are defined by enum resctrl_event_id
> so reducing it from 8-bits to 7-bits still provides more than enough
> space to represent all the known event types. But note that this choice
> was arbitrary. The "rid" field is also far wider than needed for the
> current number of resource id types.  This structure is purely internal
> to resctrl, no ABI issues with modifying it. Subsequent changes may
> rearrange the allocation of bits between each of the fields as needed.
> 
> Give the bit to a new "sum" field that indicates that reading this file
> must sum across SNC nodes. This bit also indicates that the domid field
> is the id of an L3 cache (instead of a domain id) to find which domains
> must be summed.
> 
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---
>   arch/x86/kernel/cpu/resctrl/internal.h | 10 ++++++++--
>   1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> index d29c7b58c151..77da29ced7eb 100644
> --- a/arch/x86/kernel/cpu/resctrl/internal.h
> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
> @@ -133,14 +133,20 @@ struct mon_evt {
>    *                     as kernfs private data
>    * @rid:               Resource id associated with the event file
>    * @evtid:             Event id associated with the event file
> - * @domid:             The domain to which the event file belongs
> + * @sum:               Set when event must be summed across multiple
> + *                     domains.
> + * @domid:             When @sum is zero this is the domain to which
> + *                     the event file belongs. When @sum is one this
> + *                     is the id of the L3 cache that all domains to be
> + *                     summed share.
>    * @u:                 Name of the bit fields struct
>    */

It is not obvious to me how to best maintain consistency with existing
kerneldoc. Perhaps just let @sum not end in period? No strong opinion here.

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

Reinette