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

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Date:   Thu, 30 Nov 2023 12:57:25 -0800
From:   Tony Luck <tony.luck@...el.com>
To:     Fam Zheng <fam@...hon.net>
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>,
        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 06:02:42PM +0000, Fam Zheng wrote:
> > +static __init int snc_get_config(void)
> > +{
> > +	unsigned long *node_caches;
> > +	int mem_only_nodes = 0;
> > +	int cpu, node, ret;
> > +	int num_l3_caches;
> > +
> > +	if (!x86_match_cpu(snc_cpu_ids))
> > +		return 1;
> > +
> > +	node_caches = bitmap_zalloc(nr_node_ids, GFP_KERNEL);
> > +	if (!node_caches)
> > +		return 1;
> > +
> > +	cpus_read_lock();
> > +
> > +	if (num_online_cpus() != num_present_cpus())
> > +		pr_warn("Some CPUs offline, SNC detection may be incorrect\n");
> > +
> > +	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);
> 
> Are we sure get_cpu_cacheinfo_id() is an valid index here? Looking at
> the function it could be -1 or larger than nr_node_ids.

Fam,

Return -1 is possible (in the case where first CPU on a node doesn't
have an L3 cache). Larger than nr_node_ids seems a bit more speculative.
It would mean a system with multiple L3 cache instances per node. I
suppose that's theoretically possible. In the limit case every CPU may
have its own personal L3 cache, but still have multiple CPUs grouped
together on a node.

Patch below (to be folded into part7 of next version). Increases the
size of the bitmap. Checks for get_cpu_cacheinfo_id() returning -1.
Patch just ignores the node in this case. I'm never quite sure how much
code to add for "Can't happen" scenarios.

-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;
 
 	if (!x86_match_cpu(snc_cpu_ids))
 		return 1;
 
-	node_caches = bitmap_zalloc(nr_node_ids, GFP_KERNEL);
+	node_caches = bitmap_zalloc(num_online_cpus(), GFP_KERNEL);
 	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();
 
-- 
2.41.0