Re: [RFC PATCH v13 3/4] selftests/bpf: add iocost cost model test

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Medium] selftests/bpf: pread() in serial_test_iocost_model_streams()
  hits page cache
--

commit 371817062abb0b837764b9a93458f77eb2057dd2
Author: Tao Cui <cuitao@kylinos.cn>

selftests/bpf: add iocost cost model test

This commit adds BPF selftests and two example struct_ops cost models for
blk-iocost using a configfs-backed null_blk device.

> diff --git a/tools/testing/selftests/bpf/prog_tests/iocost_model.c b/tools/testing/selftests/bpf/prog_tests/iocost_model.c
> new file mode 100644
> index 0000000000000..a714507c040e3
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/iocost_model.c
[ ... ]
> +void serial_test_iocost_model_streams(void)
> +{
[ ... ]
> +		devfd = open(path, O_RDONLY);
> +		if (ASSERT_OK(devfd < 0 ? -errno : 0, "open_dev_ms")) {
> +			char iobuf[4096];
> +			unsigned long long u1 = 0, u2 = 0;
> +
> +			ASSERT_OK(read_cost_usage("/iocost_ms",
> +						  dev, &u1), "cost_usage_pre");
> +			for (i = 0; i < 500; i++)
> +				(void)read(devfd, iobuf, sizeof(iobuf));
> +			/*
> +			 * Two interleaved sequential streams: exercise the
> +			 * stream table, not just a single cursor.
> +			 */
> +			for (i = 0; i < 200; i++) {
> +				(void)pread(devfd, iobuf, sizeof(iobuf),
> +									(i + 1) * sizeof(iobuf));
> +				(void)pread(devfd, iobuf, sizeof(iobuf),
> +									(1 << 20) +
> +									(i + 1) * sizeof(iobuf));
> +			}

[Severity: Medium]
Do these interleaved pread() calls get satisfied from the block device page
cache without reaching iocost_ms_calc_cost()?

Because devfd is opened without O_DIRECT, the preceding 500 sequential
4096-byte read() calls populate the page cache for bytes [0, 2048000).
Both pread() streams read within that cached range ([4096, 823296) and
[1052672, 1871872)), so no interleaved bios are submitted to the block
layer and the multi-stream slot table is not exercised while the
subsequent assertions pass from the initial read() loop alone.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261106122356.3224638-1-cui.tao@linux.dev?part=3