linux-kernel - Re: [PATCH] x86/resctrl: Refactor to make adding extra MBM events easy

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Message-ID: <Z9hPuMV5n_FY02MM@agluck-desk3>
Date: Mon, 17 Mar 2025 09:37:12 -0700
From: "Luck, Tony" <tony.luck@...el.com>
To: "Moger, Babu" <babu.moger@....com>
Cc: Fenghua Yu <fenghuay@...dia.com>,
	Reinette Chatre <reinette.chatre@...el.com>,
	Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>,
	Peter Newman <peternewman@...gle.com>,
	James Morse <james.morse@....com>,
	Drew Fustini <dfustini@...libre.com>,
	Dave Martin <Dave.Martin@....com>, linux-kernel@...r.kernel.org,
	patches@...ts.linux.dev
Subject: Re: [PATCH] x86/resctrl: Refactor to make adding extra MBM events
 easy

Hi Babu,

Thanks for taking a look.

On Mon, Mar 17, 2025 at 09:54:59AM -0500, Moger, Babu wrote:
> >  static void mbm_update(struct rdt_resource *r, struct rdt_mon_domain *d,
> >  		       u32 closid, u32 rmid)
> >  {
> > +	int evt;
> > +
> >  	/*
> >  	 * This is protected from concurrent reads from user as both
> >  	 * the user and overflow handler hold the global mutex.
> >  	 */
> > -	if (resctrl_arch_is_mbm_total_enabled())
> > -		mbm_update_one_event(r, d, closid, rmid, QOS_L3_MBM_TOTAL_EVENT_ID);
> > -
> > -	if (resctrl_arch_is_mbm_local_enabled())
> > -		mbm_update_one_event(r, d, closid, rmid, QOS_L3_MBM_LOCAL_EVENT_ID);
> > +	for_each_set_bit(evt, &rdt_mon_features, sizeof(rdt_mon_features))
> > +		mbm_update_one_event(r, d, closid, rmid, evt);
> 
> You need a check resctrl_arch_is_mbm_event() here.

Right. This is definitely broken. Needs that check.

> Will that be reason for your hang issue?

Might have. But it isn't my only problem. After fixing this my
kernel dies on a deref NULL.

> > @@ -1075,20 +1062,37 @@ static void dom_data_exit(struct rdt_resource *r)
> >  	mutex_unlock(&rdtgroup_mutex);
> >  }
> >  
> > -static struct mon_evt llc_occupancy_event = {
> > -	.name		= "llc_occupancy",
> > -	.evtid		= QOS_L3_OCCUP_EVENT_ID,
> > +static struct mon_evt all_events[] = {
> 
> There is one empty event here. You may need to make that clear.

Yes. I went with "some wasted space" for code simplicity just
using the bit numbers in rdt_mon_features to index this array.

Also applies to the mbm_states[] field in rdt_mon_domain and
the arch_mbm_states[] array in rdt_hw_mon_domain. The wasted
space could be reclaimed with some macros to convert from
bit numbers to array indices should this patch ever be
considered for inclusion.

-Tony