Re: [PATCH 1/2] sched_ext: fix vtime priority queue inversion on wide vtime spread

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] The patch silently breaks the established cyclic `vtime` ordering contract by switching to an absolute `<` comparison, causing permanent task starvation upon wraparound and contradicting existing kfunc documentation and selftests.
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commit b0708e9275ee1ffad12d1e3811001b6d6ee4c741
Author: Tao Cui <cuitao@kylinos.cn>

sched_ext: fix vtime priority queue inversion on wide vtime spread

This commit replaces the time_before64() cyclic comparison in
scx_dsq_priq_less() with a plain less-than operation to prevent priority queue
inversion when vtimes have a wide spread.

> diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
> index 18183062f751c..db1ea9054c0ca 100644
> --- a/kernel/sched/ext/ext.c
> +++ b/kernel/sched/ext/ext.c
> @@ -1356,7 +1356,8 @@ static bool scx_dsq_priq_less(struct rb_node *node_a,
>  	const struct task_struct *b =
>  		container_of(node_b, struct task_struct, scx.dsq_priq);
>  
> -	return time_before64(a->scx.dsq_vtime, b->scx.dsq_vtime);
> +	/* dsq_vtime is arbitrary BPF input: keep a total order */
> +	return a->scx.dsq_vtime < b->scx.dsq_vtime;

[Severity: High]
Does this break the established cyclic vtime ordering contract and cause
permanent task starvation upon wraparound?

If a BPF scheduler assigns vtime to tasks, and the vtime advances and
eventually wraps around U64_MAX, tasks that are in the queue at the time of
wraparound (with vtime near U64_MAX) will be indefinitely starved. This happens
because new post-wrap tasks (with small absolute vtimes) will be strictly less
than the old ones and placed ahead of them.

The user-facing kfunc documentation for __scx_bpf_dsq_insert_vtime() in
kernel/sched/ext/ext.c still explicitly promises the cyclic wrapping behavior:

 * @args->vtime ordering is according to time_before64() which considers
 * wrapping. A numerically larger vtime may indicate an earlier position in the
 * ordering and vice-versa.

Additionally, BPF schedulers still actively rely on the cyclic vtime semantics,
as seen in tools/testing/selftests/sched_ext/select_cpu_vtime.bpf.c:

static inline bool vtime_before(u64 a, u64 b)
{
	return (s64)(a - b) < 0;
}

>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260901024038.730424-1-cui.tao@linux.dev?part=1