linux-kernel - Re: [PATCH v15 11/32] x86,fs/resctrl: Handle events that can be read from any CPU

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Message-ID: <aTiIe6U6e5PlEyj5@agluck-desk3>
Date: Tue, 9 Dec 2025 12:37:15 -0800
From: "Luck, Tony" <tony.luck@...el.com>
To: Reinette Chatre <reinette.chatre@...el.com>
CC: 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>, <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
	<patches@...ts.linux.dev>
Subject: Re: [PATCH v15 11/32] x86,fs/resctrl: Handle events that can be read
 from any CPU

On Tue, Dec 09, 2025 at 11:39:56AM -0800, Reinette Chatre wrote:
> Hi Tony,
> 
> On 12/4/25 12:53 PM, Tony Luck wrote:
> > resctrl assumes that monitor events can only be read from a CPU in the
> > cpumask_t set of each domain.  This is true for x86 events accessed with an
> > MSR interface, but may not be true for other access methods such as MMIO.
> > 
> > Introduce and use flag mon_evt::any_cpu, settable by architecture, that
> > indicates there are no restrictions on which CPU can read that event.
> 
> I propose to append (something like): "This flag is not supported by the L3
> event reading that requires to be run on a CPU that belongs to the L3
> domain of the event being read."

Perahps simpler?

This flag is not supported for RDT_RESOURCE_L3 monitoring events.

> 
> > diff --git a/fs/resctrl/monitor.c b/fs/resctrl/monitor.c
> > index 340b847ab397..081ff659b52c 100644
> > --- a/fs/resctrl/monitor.c
> > +++ b/fs/resctrl/monitor.c
> > @@ -518,10 +518,12 @@ static int __mon_event_count(struct rdtgroup *rdtgrp, struct rmid_read *rr)
> >  {
> >  	switch (rr->r->rid) {
> >  	case RDT_RESOURCE_L3:
> > -		if (rr->hdr)
> > +		if (rr->hdr) {
> > +			WARN_ON_ONCE(rr->evt->any_cpu);
> 
> I appreciate that this addresses the feedback from v13 but it needs to be adjusted for the
> code changes made since then. This means that this warning applies to both
> __l3_mon_event_count() and __l3_mon_event_count_sum() , no?

I'l move the WARN_ON_ONCE() before the "if"

> 
> >  			return __l3_mon_event_count(rdtgrp, rr);
> > -		else
> > +		} else {
> >  			return __l3_mon_event_count_sum(rdtgrp, rr);
> > +		}
> >  	default:
> >  		rr->err = -EINVAL;
> >  		return -EINVAL;
> 
> Reinette

-Tony