linux-kernel - Re: [PATCH v8 14/32] x86,fs/resctrl: Support binary fixed point event counters

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Message-ID: <5056047f-95e9-4a33-abfb-95c9c9b266a2@intel.com>
Date: Wed, 13 Aug 2025 21:13:53 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghuay@...dia.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>, Chen Yu <yu.c.chen@...el.com>
CC: <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
	<patches@...ts.linux.dev>
Subject: Re: [PATCH v8 14/32] x86,fs/resctrl: Support binary fixed point event
 counters

Hi Tony,

On 8/11/25 11:16 AM, Tony Luck wrote:
> resctrl assumes that all monitor events can be displayed as unsigned
> decimal integers.
> 
> Hardware architecture counters may provide some telemetry events with
> greater precision where the event is not a simple count, but is a
> measurement of some sort (e.g. Joules for energy consumed).
> 
> Add a new argument to resctrl_enable_mon_event() for architecture code
> to inform the file system that the value for a counter is a fixed-point
> value with a specific number of binary places.
> Only allow architecture to use floating point format on events that it
> marked with mon_evt::is_floating_point.

I interpret two phrases above to mean that architecture controls whether
an event is floating point or not:
"architecture code to inform the file system that the value for a counter is a fixed-point
value"
and
"Only allow architecture to use floating point format on events that *it*
marked with mon_evt::is_floating_point."

I think this needs to be explicit that resctrl fs determines whether
an event is floating point or not and architecture cannot modify this, of
course it could set a floating point event's binary places to zero.

> 
> Display fixed point values with values rounded to an appropriate number
> of decimal places for the precision of the number of binary places
> provided. Add one extra decimal place for every three additional binary
> places, except for low precision binary values where exact representation
> is possible:
> 
>   1 binary place is 0.0 or 0.5.			=> 1 decimal place
>   2 binary places is 0.0, 0.25, 0.5, 0.75	=> 2 decimal places
>   3 binary places is 0.0, 0.125, etc.		=> 3 decimal places
> 
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---
>  include/linux/resctrl.h            |  5 +-
>  fs/resctrl/internal.h              |  5 ++
>  arch/x86/kernel/cpu/resctrl/core.c |  6 +--
>  fs/resctrl/ctrlmondata.c           | 84 ++++++++++++++++++++++++++++++
>  fs/resctrl/monitor.c               | 10 +++-
>  5 files changed, 104 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
> index 17a21f193a3d..38809c497b44 100644
> --- a/include/linux/resctrl.h
> +++ b/include/linux/resctrl.h
> @@ -379,7 +379,10 @@ u32 resctrl_arch_get_num_closid(struct rdt_resource *r);
>  u32 resctrl_arch_system_num_rmid_idx(void);
>  int resctrl_arch_update_domains(struct rdt_resource *r, u32 closid);
>  
> -void resctrl_enable_mon_event(enum resctrl_event_id eventid, bool any_cpu);
> +#define MAX_BINARY_BITS	27

This MAX_BINARY_BITS define can stay internal to resctrl until it is used by 
an arch.

> +
> +void resctrl_enable_mon_event(enum resctrl_event_id eventid, bool any_cpu,
> +			      unsigned int binary_bits);
>  
>  bool resctrl_is_mon_event_enabled(enum resctrl_event_id eventid);
>  

Reinette