linux-kernel - Re: [PATCH v15-RFC 6/8] x86/resctrl: Introduce snc_nodes_per_l3

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Date: Fri, 9 Feb 2024 11:35:12 -0800
From: Tony Luck <tony.luck@...el.com>
To: "Moger, Babu" <babu.moger@....com>
Cc: Fenghua Yu <fenghua.yu@...el.com>,
	Reinette Chatre <reinette.chatre@...el.com>,
	Peter Newman <peternewman@...gle.com>,
	Jonathan Corbet <corbet@....net>,
	Shuah Khan <skhan@...uxfoundation.org>, x86@...nel.org,
	Shaopeng Tan <tan.shaopeng@...itsu.com>,
	James Morse <james.morse@....com>,
	Jamie Iles <quic_jiles@...cinc.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Drew Fustini <dfustini@...libre.com>, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH v15-RFC 6/8] x86/resctrl: Introduce snc_nodes_per_l3_cache

On Fri, Feb 09, 2024 at 09:29:16AM -0600, Moger, Babu wrote:
> >  
> > +extern unsigned int snc_nodes_per_l3_cache;
> 
> I feel this can be part of rdt_resource instead of global.

Mixed emotions about that. It would be another field that appears
in every instance of rdt_resource, but only used by the RDT_RESOURCE_L3_MON
copy.

> 
> > +
> >  enum resctrl_res_level {
> >  	RDT_RESOURCE_L3_MON,
> >  	RDT_RESOURCE_L3,
> > diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
> > index b741cbf61843..dc886d2c9a33 100644
> > --- a/arch/x86/kernel/cpu/resctrl/core.c
> > +++ b/arch/x86/kernel/cpu/resctrl/core.c
> > @@ -48,6 +48,12 @@ int max_name_width, max_data_width;
> >   */
> >  bool rdt_alloc_capable;
> >  
> > +/*
> > + * Number of SNC nodes that share each L3 cache.  Default is 1 for
> > + * systems that do not support SNC, or have SNC disabled.
> > + */
> > +unsigned int snc_nodes_per_l3_cache = 1;
> > +
> >  static void
> >  mba_wrmsr_intel(struct rdt_domain *d, struct msr_param *m,
> >  		struct rdt_resource *r);
> > diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
> > index 080cad0d7288..357919bbadbe 100644
> > --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> > +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> > @@ -148,8 +148,18 @@ static inline struct rmid_entry *__rmid_entry(u32 rmid)
> >  
> >  static int __rmid_read(u32 rmid, enum resctrl_event_id eventid, u64 *val)
> >  {
> > +	struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
> 
> RDT_RESOURCE_L3_MON?

Second good catch.

> 
> > +	int cpu = smp_processor_id();
> > +	int rmid_offset = 0;
> >  	u64 msr_val;
> >  
> > +	/*
> > +	 * When SNC mode is on, need to compute the offset to read the
> > +	 * physical RMID counter for the node to which this CPU belongs.
> > +	 */
> > +	if (snc_nodes_per_l3_cache > 1)
> > +		rmid_offset = (cpu_to_node(cpu) % snc_nodes_per_l3_cache) * r->num_rmid;
> 
> Not sure if you have tested or not. r->num_rmid is initialized for the
> resource RDT_RESOURCE_L3_MON. For other resource it is always 0.

I hadn't got time on the SNC machine to try this out. Thanks
for catching this one, I'd have been scratching my head for a
while to track the symptoms of this problem back to this mistake.

Thanks

-Tony