Re: [PATCH 3/5] selftests/bpf: add iocost cost model test

[Tao Cui <cui.tao@linux.dev>]

Hello Tejun,

在 2026/10/29 04:09, Tejun Heo 写道:
> Hello, Tao.
> 
> On Wed, Oct 28, 2026 at 10:35:32PM +0800, Tao Cui wrote:
>> +static bool iocost_bpf_supported(void)
>> +{
>> +	FILE *f;
>> +	char line[128];
>> +	bool any = false, on = false;
>> ...
>> +		if (strstr(line, "CONFIG_"))
>> +			any = true;
>> ...
>> +	return on;
> 
> any is set but never read, so every test skips without /proc/config.gz.
> 
I'll make the probe return whether CONFIG_BLK_CGROUP_IOCOST_BPF=y was
seen.

>> +	/* isolate the configfs and cgroup mounts from the host */
>> +	if (unshare(CLONE_NEWNS)) {
> 
> unshare(CLONE_NEWNS) alone doesn't make the mounts private, so the
> configfs mount propagates to the host, one more per test.
> 
I'll follow unshare() with mount(NULL, "/", NULL,
MS_REC | MS_PRIVATE, NULL).

>> +	ASSERT_OK(configfs_write("/sys/fs/cgroup/cgroup.subtree_control",
>> +				 "+io"), "subtree_io");
>> +	if (!ASSERT_OK(cgroup_mkdir("/sys/fs/cgroup/iocost_pairing"),
> 
> The "+io" write to the root cgroup's cgroup.subtree_control is never
> reverted, the cgroups are created directly under /sys/fs/cgroup and
> test_progs stays in the root cgroup. cgroup_helpers handles all of this.
> 
I'll restore the "+io" setting on exit and move the test process back
to the root cgroup before removing the test cgroups. I'll also switch
the test over to cgroup_helpers.

>> +	err = iocost_model__attach(skel);
>> +	if (err == -EOPNOTSUPP || err == -ENODEV) {
>> ...
>> +		test__skip();
> 
> -ENODEV and -EOPNOTSUPP from attaching to the test's own null_blk device
> are failures, not skips. The same goes for
> serial_test_iocost_model_streams().
> 
Both become ASSERT_OK failures.
>> +		ASSERT_GT(skel->bss->iocg_inits, 0, "pair_init");
> 
> This passes on the root blkg alone, so it doesn't show that the test
> cgroup got iocg_init(). The same goes for eject_had_inits and
> pair_init_again.
> 
Gone with the callbacks.
>> +		/*
>> +		 * detach implicitly by closing the link fd instead of
>> +		 * calling bpf_link__destroy() explicitly: the release
>> +		 * path must detach the model just like the explicit
>> +		 * call does
>> +		 */
>> +		lfd = bpf_link__fd(skel->links.iocost_2x);
>> +		bpf_link__disconnect(skel->links.iocost_2x);
>> +		close(lfd);
>> +		skel->links.iocost_2x = NULL;
> 
> This leaks libbpf's link struct. The reason in the comment doesn't hold
> either. bpf_link__destroy() on a struct_ops link does the same close().
> 
I'll use bpf_link__destroy() instead.

>> +void serial_test_iocost_model_streams(void)
>> ...
>> +	if (ASSERT_OK(err, "attach")) {
>> +		err = readback_model(dev, model, sizeof(model));
>> +		if (ASSERT_OK(err, "readback"))
>> +			ASSERT_STREQ(model, "bpf", "model_bpf");
>> +
>> +		bpf_link__destroy(skel->links.iocost_ms);
> 
> The test never enables the controller, so iocost_ms's calc_cost() never
> runs. The multi-stream results in the cover letter aren't exercised by
> the test.
> 
I'll enable the controller, move the test into a cgroup, drive IO
through the device, and assert that the model is exercised.

Thanks,
Tao

> Thanks.
> 
> --
> tejun