Re: [PATCH] perf test: Add test for events re-armed with PERF_EVENT_IOC_REFRESH

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] perf test: missing MADV_NOHUGEPAGE in test__event_refresh_sw() fails on THP
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commit f4e87f46b616d4e4e30869fe7bd06aad9b7c96f9
Author: Francesco Nigro <nigro.fra@gmail.com>

perf test: Add test for events re-armed with PERF_EVENT_IOC_REFRESH

This commit adds a perf test suite that opens a sampling event with
sample_period 1, re-arms it via PERF_EVENT_IOC_REFRESH from a SIGIO
handler, and verifies that 1000 software page-fault and tracepoint events
each deliver an overflow signal.

Link: https://github.com/async-profiler/async-profiler/issues/1813

> diff --git a/tools/perf/tests/event_refresh.c b/tools/perf/tests/event_refresh.c
> new file mode 100644
> index 0000000000000..480f738de9076
> --- /dev/null
> +++ b/tools/perf/tests/event_refresh.c
> @@ -0,0 +1,136 @@

[ ... ]

> +	pr_debug("%d events, %d overflows\n", NR_EVENTS, overflows);
> +	if (overflows == NR_EVENTS)
> +		err = TEST_OK;
> +
> +	sigaction(SIGIO, &old_sa, NULL);
> +out_close:
> +	close(fd);
> +	return err;
> +}
> +
> +static char *pages;
> +
> +static void touch_page(int i)
> +{
> +	pages[(long)i * page_size] = 1;
> +}
> +
> +static int test__event_refresh_sw(struct test_suite *test __maybe_unused,
> +				  int subtest __maybe_unused)
> +{
> +	struct perf_event_attr attr = {
> +		.type = PERF_TYPE_SOFTWARE,
> +		.config = PERF_COUNT_SW_PAGE_FAULTS,
> +	};
> +	int err;
> +
> +	pages = mmap(NULL, (long)NR_EVENTS * page_size, PROT_READ | PROT_WRITE,
> +		     MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> +	if (pages == MAP_FAILED)
> +		return TEST_FAIL;
> +
> +	err = count_overflows(&attr, touch_page);

[Severity: Medium]
In tools/perf/tests/event_refresh.c, test__event_refresh_sw() maps
NR_EVENTS * page_size bytes (~4 MiB with 4 KiB base pages) as a private
anonymous mapping and relies on each touch_page() write at page_size
stride to trigger a distinct PERF_COUNT_SW_PAGE_FAULTS event.

On systems with Transparent Huge Pages (THP) or multi-size THP (mTHP) set
to "always" (such as when CONFIG_TRANSPARENT_HUGEPAGE_ALWAYS=y or
/sys/kernel/mm/transparent_hugepage/enabled is set to "always"), the
first write in touch_page(0) can be satisfied by __handle_mm_fault() ->
create_huge_pmd() with a 2 MiB huge page (or a multi-page folio).

That maps up to 512 base pages in a single page fault, so subsequent
touch_page() calls within that huge page hit already-present PTEs/PMDs
without faulting. As a result, overflows only reaches 489 (or fewer)
instead of NR_EVENTS (1000), and count_overflows() returns TEST_FAIL.

Should test__event_refresh_sw() call madvise(pages, (long)NR_EVENTS *
page_size, MADV_NOHUGEPAGE) before count_overflows() so that each
touch_page() write faults in a single base page?

> +	munmap(pages, (long)NR_EVENTS * page_size);
> +	return err;
> +}

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261021131006.2101248-1-nigro.fra@gmail.com?part=1