Re: [PATCH 5/5] perf test: Add false_sharing workload exhibiting cross-CPU false sharing

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] perf/tests: sparse topology CPU discovery misses allowed CPUs
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commit 20aa23e4b03f823c93c6019527a360319667c8b2
Author: Arnaldo Carvalho de Melo <acme@redhat.com>

perf test: Add false_sharing workload exhibiting cross-CPU false sharing

This commit adds a 'perf test -w false_sharing' workload that simulates false
sharing across CPUs in a TCP connection structure to test data type profiling.

> diff --git a/tools/perf/tests/workloads/false_sharing.c b/tools/perf/tests/workloads/false_sharing.c
> new file mode 100644
> index 0000000000000..344875fb3a4d9
> --- /dev/null
> +++ b/tools/perf/tests/workloads/false_sharing.c

[ ... ]

> +	/* Pin against the allowed set, restricted cpusets still spread the threads. */
> +	ncpus = sysconf(_SC_NPROCESSORS_CONF);
> +	if (sched_getaffinity(0, sizeof(set), &set) == 0) {

[Severity: Medium]
Does this loop correctly handle sparse CPU topologies?
sysconf(_SC_NPROCESSORS_CONF) returns the count of configured CPUs, not the
maximum CPU ID. On a system where only higher CPU IDs are present (for
example, if only CPUs 16-19 are configured, ncpus is 4), bounding the loop by
i < ncpus will stop checking at ID 3. This could cause the test to miss
allowed CPUs and fall back to unpinned execution, compromising the false
sharing demonstration.

> +		for (i = 0; i < ncpus && i < CPU_SETSIZE; i++) {
> +			if (!CPU_ISSET(i, &set))
> +				continue;
> +			nr_allowed++;
> +		}
> +		allowed = malloc(nr_allowed * sizeof(int));
> +		if (allowed == NULL) {
> +			fprintf(stderr, "Error: malloc failed for CPU list\n");
> +			return 1;
> +		}

[Severity: Medium]
Is this second discovery loop also affected by the same boundary logic issue?

> +		for (i = 0; i < ncpus && i < CPU_SETSIZE; i++) {
> +			if (CPU_ISSET(i, &set))
> +				allowed[nallowed++] = i;
> +		}
> +	}

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Sashiko AI review · https://sashiko.dev/#/patchset/20260929201524.2507733-1-acme@kernel.org?part=5