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

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

Hello, Tejun.

在 2026/9/2 04:00, Tejun Heo 写道:
> 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 for both reviews, all points taken. Here's what changed:

1/2: "less than 2^63 apart", folded into the paragraph above.

2/2: wrapped node goes to the front (not behind), wrap timing
corrected (per-CPU slice charge), lag and lead bounds both stated,
Fixes points at a4103eacc2ab, comparator uses time_before(), and the
comment is dropped.

I'll send v3 shortly.

Thanks,
Tao

> Thanks.
> 
> --
> tejun