Hi Tony,
On 11/30/2023 12:57 PM, Tony Luck wrote:
> 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.
>
Thank you.
> -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.
>
> 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.
>
> - 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.
> 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?
Reinette
