Re: [PATCH v2 2/2] sched_ext/scx_flatcg: make cgv_node_less() wraparound-safe

[Tejun Heo <tj@kernel.org>]

Hello,

On Tue, Sep 01, 2026 at 10:03:43PM +0800, Tao Cui wrote:
...
> long-running host. At the wrap instant the plain comparison puts the
> wrapped node behind everything else permanently.

Plain < puts the wrapped node at the front. The unwrapped ones get stuck
behind it. Also, each CPU picking a cgroup charges it a full slice, so the
wrap is closer than weeks.

...
> cyclic comparison is valid as an rbtree comparator only because
> cgrp_cap_budget() clamps every node to within max_budget behind
> cvtime_now, so any two nodes are far less than 2^63 apart and the
...

cgrp_cap_budget() only bounds the lag. The lead is bounded by the slice
charge plus pending cvtime_delta on re-insertion.

> Fixes: 7b742aa2c2c9 ("sched_ext: Add a cgroup scheduler which uses flattened hierarchy")

Not in mainline. The upstream commit is a4103eacc2ab.

> +	/* wrap-safe: cap_budget keeps nodes within 2^63 of each other */
> +	return (s64)(cgc_a->cvtime - cgc_b->cvtime) < 0;

Use time_before() and drop the comment.

Thanks.

--
tejun