Re: [PATCH v6 2/2] perf pmu intel: Adjust cpumasks for sub-NUMA clusters on Sapphire Rapids and Emerald Rapids

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Missing IMC cpumask adjustment and silent failure for SNC4 configurations on Sapphire Rapids.
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commit 312ea8a1894ae6a4d9318072afccaf0abbbb29bd
Author: Chun-Tse Shao <ctshao@google.com>

perf pmu intel: Adjust cpumasks for sub-NUMA clusters on Sapphire Rapids and Emerald Rapids

This commit adjusts the cpumasks for sub-NUMA clusters on Sapphire Rapids
and Emerald Rapids processors. It updates the detection logic and the
mapping arrays to support SNC configurations for these architectures.

> diff --git a/tools/perf/arch/x86/util/pmu.c b/tools/perf/arch/x86/util/pmu.c
> index b8fab260c4238..09caec0a32a57 100644
> --- a/tools/perf/arch/x86/util/pmu.c
> +++ b/tools/perf/arch/x86/util/pmu.c
[ ... ]
> @@ -142,23 +146,43 @@ static int uncore_imc_snc(struct perf_pmu *pmu)
>  	// Compute the IMC SNC using lookup tables.
>  	unsigned int imc_num;
>  	int snc_nodes = snc_nodes_per_l3_cache();
[ ... ]
> +	/* snc_map is not inited yet. We only look up once to avoid expensive operations. */
> +	if (!snc_map) {
> +		switch (snc_nodes) {
> +		case 2:
[ ... ]
> +			break;
> +		case 3:
> +			snc_map = snc3_map;
> +			snc_map_len = ARRAY_SIZE(snc3_map);
> +			break;
> +		default:
> +			/* Error or no lookup support for SNC with >3 nodes. */
> +			return 0;

Sapphire Rapids natively supports 4-way sub-NUMA clustering, which would
make snc_nodes evaluate to 4. Does this regression silently skip IMC
cpumask adjustments for SNC4 configurations?

Because this default case returns 0 immediately, it entirely bypasses the
newly added warning block below.

> +		}
> +
> +		if (!snc_map) {
> +			pr_warning("Unexpected: can not find snc map config");
> +			return 0;
> +		}

Could this leave users unaware that the IMC PMUs are unmapped, even though
CHA PMUs are correctly adjusted for SNC4 elsewhere?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260515172710.428474-1-ctshao@google.com?part=2