Re: [RFC PATCH v10 2/4] selftests/bpf: add iocost cost model test

[Tejun Heo <tj@kernel.org>]

Hello, Tao.

On Sat, 03 Oct 2026 20:40:43 +0800, Tao Cui wrote:
> +	dev = getenv("IOCOST_TEST_DEV");

Nothing in the tree sets IOCOST_TEST_DEV, so both tests always skip in CI.
The null_blk instance the v11 removal test creates could serve these two
as well.

> +		if (ASSERT_OK(err, "readback"))
> +			ASSERT_EQ(strcmp(model, "bpf"), 0, "model_bpf");

ASSERT_STREQ() prints both strings on failure.

> +		bpf_link__destroy(skel->links.iocost_2x);
> +		/* cleared, or the skeleton destroy below detaches again */
> +		skel->links.iocost_2x = NULL;

Without clearing, the skeleton destroy would call bpf_link__destroy() on
the freed link. It wouldn't detach again.

> + * The cursor handling follows the builtin: a zero cursor means "no
> + * previous IO", and the cursor is advanced for bios the builtin
> + * prices (READ/WRITE with a non-zero size; the builtin skips the
> + * whole update when the cost is zero), so flushes and discards
> + * leave it alone and long merged streams do not drift past the
> + * 16MB seek threshold.

The model advances the cursor for every merged READ or WRITE bio.
ioc_rqos_merge() only moves it on a back merge into the request ending at
the cursor, so a front merge leaves the builtin's cursor alone and moves
the model's.

> +	} else {
> +		base = randio;
> +		for (i = 1; i < NSLOTS; i++) {
> +			if (s->stamp[i] < s->stamp[victim])
> +				victim = i;
> +		}

A merged bio which matches no slot, e.g. a front merge or a merge into
another stream's request, takes the LRU slot and can evict a live stream.
Maybe skip the slot replacement for IOCOST_COST_F_MERGE?

Thanks.

-- 
tejun