Re: [PATCH] sched/fair: clamp rescaled vlag in reweight_entity() to bound entity_key()

[Rik van Riel <riel@surriel.com>]

On Thu, 2026-06-11 at 10:19 +0200, Peter Zijlstra wrote:
> 
> IOW, the transform totally preserves the limit. If the vlag was bound
> before, it must be bound after.
> 
> What am I missing?

You're not missing anything.

After looking at the code, and tracing through things
for much of the afternoon, it turns out the culprit
is the cgroup throttle path.

Specifically, this bug happened on a 6.16 based kernel,
which has the new EEVDF zero_vruntime code, in order
to prevent the EEVDF starvation that could happen with
the old min_vruntime code, but also still has the old
cgroup throttling code.

This leaves a code path where lazy dequeue code combined
with the cgroup throttling code can result in entity_lag
and entity_key reaching ludicrous values, resulting in
the overflow.

This bug reproduces in about 5 minutes on that 6.16 based
kernel, but not with current upstream.

Nothing to do upstream, and it looks like there are a few
changesets upstream that may be able to fix the issue in
the 6.16 based kernel without even needing the clamp from
this patch.

-- 
All Rights Reversed.