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

[bot+bpf-ci]

> cgv_node_less() compares cvtimes with a plain <, which breaks once
> cvtime wraps. A weight-1 cgroup in a hierarchy summing to 10000
> advances cvtime at up to 10000x wall time, so 2^64 ns of cvtime is
> weeks of continuous saturation away -- unlikely but reachable on a
> long-running host. At the wrap instant the plain comparison puts the
> wrapped node behind everything else permanently.
>
> Compare with (s64)(a - b) < 0 instead, as CFS does for vruntime. A
> 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
> cyclic order agrees with the true order.
>
> Compile-tested and smoke-tested in a VM: weight distribution and
> dispatch unaffected.
>
> Fixes: 7b742aa2c2c9 ("sched_ext: Add a cgroup scheduler which uses flattened hierarchy")
> Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
> Link: https://lore.kernel.org/r/3f1ce004-e259-4e72-a5f7-14a5050053bd@xxxxxxxxx
> Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>

Is the Fixes: tag correct? The tag references commit 7b742aa2c2c9, which
does not exist in this repository. The bug was actually introduced by
commit a4103eacc2ab4 ("sched_ext: Add a cgroup scheduler which uses
flattened hierarchy"), which added the buggy plain comparison
'cgc_a->cvtime < cgc_b->cvtime' that this commit fixes. Should the tag
be:

  Fixes: a4103eacc2ab4 ("sched_ext: Add a cgroup scheduler which uses flattened hierarchy")


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/33465241568