Re: [PATCH 4/4] 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:
- [Low] perf test: fs_reader padding calculation ignores implicit padding
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commit 3c1c77347e93202ea6f68ec871d8e60e5a4defed
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 hammers a
shared struct from several CPUs, shaped as a TCP connection. The workload
creates threads sharing cache lines to test the data type profiler's
ability to resolve false sharing.

> diff --git a/tools/perf/tests/workloads/false_sharing.c b/tools/perf/tests/workloads/false_sharing.c
> new file mode 100644
> index 0000000000000..12e10d10b87e0
> --- /dev/null
> +++ b/tools/perf/tests/workloads/false_sharing.c
[ ... ]
> +struct fs_reader {
> +	pthread_t	thread;
> +	int		cpu;
> +	unsigned long	sum;
> +	char		__pad[64 - sizeof(pthread_t) - sizeof(int) - sizeof(unsigned long)];

[Severity: Low]
Does the manual padding computation in struct fs_reader ignore implicit
compiler padding?

On 64-bit architectures, there are 4 bytes of implicit compiler padding
required between the 4-byte cpu and the 8-byte sum. As a result, the struct
fields occupy 68 bytes before the __attribute__((aligned(64))) rounds the
final struct size up to 128 bytes, effectively doubling the intended footprint
and wasting 64 bytes per array element.

> +} __attribute__((aligned(64)));

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Sashiko AI review · https://sashiko.dev/#/patchset/20260928220634.2451784-1-acme@kernel.org?part=4