Re: [PATCHSET sched_ext/for-7.3-fixes] sched_ext: Fix keep-last for sub-scheduler tasks and two scx_qmap placement loops

[Andrea Righi]

Hi Tejun,

On Sat, Sep 05, 2026 at 06:09:54AM -1000, Tejun Heo wrote:
> Hello,
> 
> Running scx_qmap as a sub-scheduler under scx_nitosis exposed a kernel bug
> and two scx_qmap bugs:
> 
> - dispatch_one() decides whether to keep running @prev by testing the root
>   scheduler's SCX_OPS_ENQ_LAST and bypass state. Those are properties of
>   @prev's own scheduler, and put_prev_task_scx() acts on that scheduler.
>   When the root sets the flag and @prev belongs to a sub-scheduler that
>   doesn't, the task is not kept and is enqueued with SCX_ENQ_LAST to a
>   scheduler that never opted in. This trips the WARN_ON_ONCE in
>   put_prev_task_scx(), and the task is queued without a follow-up scheduling
>   event, which can stall it. Patches 1-2 fix this.
> 
> - scx_qmap added SCX_ENQ_IMMED to its rescue inserts, which turns a rescue
>   request into a regular placement on a time-shared cid and loops with the
>   kernel's REENQ bounce until the reenqueue limit ejects the scheduler.
>   Patch 3 fixes this.
> 
> - scx_qmap placed tasks from the delegation split alone, ahead of the caps
>   in effect on the cpus, so highpri moves to a freshly granted cid were
>   denied and bounced. Patch 4 fixes this.

Apart from the comment about PATCH 4 (which I don't think it's a blocker)
everything else looks good to me.

For the series:

Reviewed-by: Andrea Righi <arighi@xxxxxxxxxx>

Thanks,
-Andrea

> 
> This patchset contains the following four patches:
> 
>  0001 sched_ext: Rename sch to root_sch in dispatch_one()
>  0002 sched_ext: Use @prev's scheduler for the keep decisions in dispatch_one()
>  0003 sched_ext: scx_qmap: Do not add IMMED to rescue inserts
>  0004 sched_ext: scx_qmap: Place only on cids whose caps are in effect
> 
> 0001-0002 fix the kernel side. 0003-0004 fix scx_qmap.
> 
> The patchset is based on sched_ext/for-7.3-fixes (0a85182723b6) and is also
> available in the following git branch:
> 
>  git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git sub-keep-last-rescue
> 
> diffstat follows. Thanks.
> 
>  kernel/sched/ext/ext.c         |  33 ++++++++------
>  tools/sched_ext/scx_qmap.bpf.c | 101 ++++++++++++++++++++++++++++++-----------
>  tools/sched_ext/scx_qmap.h     |   3 ++
>  3 files changed, 97 insertions(+), 40 deletions(-)
> 
> --
> tejun