linux-kernel - Re: [PATCH v12 7/8] x86/resctrl: Sub NUMA Cluster detection and enable

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Message-ID: <ZWkQBwwtSae4nGgH@agluck-desk3>
Date:   Thu, 30 Nov 2023 14:43:19 -0800
From:   Tony Luck <tony.luck@...el.com>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     Fam Zheng <fam@...hon.net>, Fenghua Yu <fenghua.yu@...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>,
        Babu Moger <babu.moger@....com>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        patches@...ts.linux.dev, Shaopeng Tan <tan.shaopeng@...fujitsu.com>
Subject: Re: [PATCH v12 7/8] x86/resctrl: Sub NUMA Cluster detection and
 enable

On Thu, Nov 30, 2023 at 01:47:10PM -0800, Reinette Chatre wrote:
> Hi Tony,
> > diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
> > index 3293ab4c58b0..85f8a1b3feaf 100644
> > --- a/arch/x86/kernel/cpu/resctrl/core.c
> > +++ b/arch/x86/kernel/cpu/resctrl/core.c
> > @@ -1056,12 +1056,13 @@ static __init int snc_get_config(void)
> >  	unsigned long *node_caches;
> >  	int mem_only_nodes = 0;
> >  	int cpu, node, ret;
> > +	int cache_id;
> >  	int num_l3_caches;
> 
> Please do maintain reverse fir order.

Fixed.

> 
> >  
> >  	if (!x86_match_cpu(snc_cpu_ids))
> >  		return 1;
> 
> I understand and welcome this change as motivated by robustness. Apart
> from that, with this being a model specific feature for this particular
> group of systems, it it not clear to me in which scenarios this could
> run on a system where a present CPU does not have access to L3 cache.

Agreed that on these systems there should always be an L3 cache. Should
I drop the check for "-1"?

> >  
> > -	node_caches = bitmap_zalloc(nr_node_ids, GFP_KERNEL);
> > +	node_caches = bitmap_zalloc(num_online_cpus(), GFP_KERNEL);
> 
> Please do take care to take new bitmap size into account in all
> places. From what I can tell there is a later bitmap_weight() call that
> still uses nr_node_ids as size.

Oops. I was also using num_online_cpus() before cpus_read_lock(), so
things could theoretically change before the bitmap_weight() call.
I switched to using num_present_cpus() in both places.

> >  	if (!node_caches)
> >  		return 1;
> >  
> > @@ -1072,10 +1073,13 @@ static __init int snc_get_config(void)
> >  
> >  	for_each_node(node) {
> >  		cpu = cpumask_first(cpumask_of_node(node));
> > -		if (cpu < nr_cpu_ids)
> > -			set_bit(get_cpu_cacheinfo_id(cpu, 3), node_caches);
> > -		else
> > +		if (cpu < nr_cpu_ids) {
> > +			cache_id = get_cpu_cacheinfo_id(cpu, 3);
> > +			if (cache_id != -1)
> > +				set_bit(cache_id, node_caches);
> > +		} else {
> >  			mem_only_nodes++;
> > +		}
> >  	}
> >  	cpus_read_unlock();
> >  
> 
> Could this code be made even more robust by checking the computed
> snc_nodes_per_l3_cache against the limited actually possible values?
> Forcing it to 1 if something went wrong?

Added a couple of extra sanity checks. See updated incremental patch
below.

-Tony


diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
index 3293ab4c58b0..3684c6bf8224 100644
--- a/arch/x86/kernel/cpu/resctrl/core.c
+++ b/arch/x86/kernel/cpu/resctrl/core.c
@@ -1057,11 +1057,12 @@ static __init int snc_get_config(void)
 	int mem_only_nodes = 0;
 	int cpu, node, ret;
 	int num_l3_caches;
+	int cache_id;
 
 	if (!x86_match_cpu(snc_cpu_ids))
 		return 1;
 
-	node_caches = bitmap_zalloc(nr_node_ids, GFP_KERNEL);
+	node_caches = bitmap_zalloc(num_present_cpus(), GFP_KERNEL);
 	if (!node_caches)
 		return 1;
 
@@ -1072,23 +1073,39 @@ static __init int snc_get_config(void)
 
 	for_each_node(node) {
 		cpu = cpumask_first(cpumask_of_node(node));
-		if (cpu < nr_cpu_ids)
-			set_bit(get_cpu_cacheinfo_id(cpu, 3), node_caches);
-		else
+		if (cpu < nr_cpu_ids) {
+			cache_id = get_cpu_cacheinfo_id(cpu, 3);
+			if (cache_id != -1)
+				set_bit(cache_id, node_caches);
+		} else {
 			mem_only_nodes++;
+		}
 	}
 	cpus_read_unlock();
 
-	num_l3_caches = bitmap_weight(node_caches, nr_node_ids);
+	num_l3_caches = bitmap_weight(node_caches, num_present_cpus());
 	kfree(node_caches);
 
 	if (!num_l3_caches)
 		return 1;
 
+	/* sanity check #1: Number of CPU nodes must be multiple of num_l3_caches */
+	if ((nr_node_ids - mem_only_nodes) % num_l3_caches)
+		return 1;
+
 	ret = (nr_node_ids - mem_only_nodes) / num_l3_caches;
 
-	if (ret > 1)
+	/* sanity check #2: Only valid results are 1, 2, 4 */
+	switch (ret) {
+	case 1:
+		break;
+	case 2:
+	case 4:
 		rdt_resources_all[RDT_RESOURCE_L3].r_resctrl.mon_scope = RESCTRL_NODE;
+		break;
+	default:
+		return 1;
+	}
 
 	return ret;
 }
-- 
2.41.0