linux-kernel - Re: [PATCH v14 07/32] x86,fs/resctrl: Use struct rdt_domain

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Message-ID: <aS9NEJAnx0MWoyaT@agluck-desk3>
Date: Tue, 2 Dec 2025 12:33:20 -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 v14 07/32] x86,fs/resctrl: Use struct rdt_domain_hdr when
 reading counters

On Tue, Dec 02, 2025 at 08:06:47AM -0800, Reinette Chatre wrote:
> Hi Tony,
> > +static int __l3_mon_event_count_sum(struct rdtgroup *rdtgrp, struct rmid_read *rr)
> > +{
> > +	int cpu = smp_processor_id();
> > +	u32 closid = rdtgrp->closid;
> > +	u32 rmid = rdtgrp->mon.rmid;
> > +	struct rdt_mon_domain *d;
> > +	int cntr_id = -ENOENT;
> > +	u64 tval = 0;
> > +	int err, ret;
> >  
> >  	/* Summing domains that share a cache, must be on a CPU for that cache. */
> >  	if (!cpumask_test_cpu(cpu, &rr->ci->shared_cpu_map))
> > @@ -480,7 +494,7 @@ static int __l3_mon_event_count(struct rdtgroup *rdtgrp, struct rmid_read *rr)
> >  			err = resctrl_arch_cntr_read(rr->r, d, closid, rmid, cntr_id,
> >  						     rr->evtid, &tval);
> 
> This is not safe. The current __mon_event_count() implementation being refactored by this series
> ensures that if rr->is_mbm_cntr is true then cntr_id is valid. This patch places the code doing so
> in __l3_mon_event_count() without an equivalent in the new __l3_mon_event_count_sum(). From what I
> can tell, since __l3_mon_event_count_sum() sets cntr_id to -ENOENT and never initializes it correctly, 
> resctrl_arch_cntr_read() will be called with an invalid cntr_id that it is not able to handle.
> 
> There is no overlap in support for SNC and assignable counters. Do you expect that this is something that
> should be supported? Even if it is, SNC is model specific so it may be reasonable to expect that when/if
> a system supporting both features arrives it would need enabling anyway. I thus propose for simplicity
> that the handling of assignable counters by __l3_mon_event_count_sum() be dropped, albeit with a loud
> complaint if it is ever called with rr->is_mbm_cntr set.
> 

Reinette,

Agreed. I see little liklihood that SNC and assignable counters will
meet on a system.

How does this look for the "loud complaint":


static int __l3_mon_event_count_sum(struct rdtgroup *rdtgrp, struct rmid_read *rr)
{
	int cpu = smp_processor_id();
	u32 closid = rdtgrp->closid;
	u32 rmid = rdtgrp->mon.rmid;
	struct rdt_mon_domain *d;
	u64 tval = 0;
	int err, ret;

	/*
	 * Summing across domains is only done for systems that implement
	 * Sub-NUMA Cluster. There is no overlap with systems that support
	 * assignable counters.
	 */
	if (rr->is_mbm_cntr) {
		pr_warn_once("Assignable counter on SNC system!\n");
		rr->err = -EINVAL;
		return -EINVAL;
	}

	/* Summing domains that share a cache, must be on a CPU for that cache. */
	if (!cpumask_test_cpu(cpu, &rr->ci->shared_cpu_map))
		return -EINVAL;

	/*
	 * Legacy files must report the sum of an event across all
	 * domains that share the same L3 cache instance.
	 * Report success if a read from any domain succeeds, -EINVAL
	 * (translated to "Unavailable" for user space) if reading from
	 * all domains fail for any reason.
	 */
	ret = -EINVAL;
	list_for_each_entry(d, &rr->r->mon_domains, hdr.list) {
		if (d->ci_id != rr->ci->id)
			continue;
		err = resctrl_arch_rmid_read(rr->r, &d->hdr, closid, rmid,
					     rr->evtid, &tval, rr->arch_mon_ctx);
		if (!err) {
			rr->val += tval;
			ret = 0;
		}
	}

	if (ret)
		rr->err = ret;

	return ret;
}

-Tony