Re: [PATCHSET v2 sched_ext/for-7.3-fixes] sched_ext: Idle claim recovery and online cid mask

[Andrea Righi <arighi@nvidia.com>]

Hi Tejun,

On Mon, Sep 14, 2026 at 10:27:23PM -1000, Tejun Heo wrote:
> Hello,
> 
> v2: - Dropped the idle-to-idle notification patch. Restoring an unused idle
>       claim from ops.dispatch() is the established pattern (Andrea Righi).
>       Replaced with a patch fixing scx_qmap to do that and documenting the
>       pattern in ops.update_idle().
>     - Reworded the online cmask getter kerneldoc (Andrea Righi).
> 
> v1: https://lore.kernel.org/r/20260914234259.3585373-1-tj@kernel.org
> 
> Two issues surfaced while converting a scheduler that maintains its own idle
> and online tracking.
> 
> An idle CPU can be claimed and kicked without receiving a task. When it
> picks idle again there is no idle transition, so ops.update_idle() stays
> silent and a scheduler tracking idle CPUs itself loses the CPU until an
> unrelated task runs there. scx_qmap has this hole. The first patch restores
> the claim from ops.dispatch(), where a kick guarantees a call, and documents
> the pattern in ops.update_idle().
> 
> A cid-form scheduler that installs its own mapping has no way to learn which
> cids are online: the online count identifies the set only under the default
> mapping and the CPU-form cpumask is unusable from cid programs. The second
> patch adds scx_bpf_online_cmask(), a kernel-maintained cmask in the
> scheduler's arena that follows the SCX hotplug notifications.
> 
> Verified by building the kernel and tools/sched_ext and running scx_qmap
> under CPU and fork load in a VM. The online cmask patch only changed
> kerneldoc since v1, where a local selftest offlined and onlined a CPU in a
> VM and checked the mask and the callbacks at each step.

Looks good, for the series:

Reviewed-by: Andrea Righi <arighi@nvidia.com>

Thanks,
-Andrea

> 
> Based on sched_ext/for-7.3-fixes (c7a1c6e8004a).
> 
> This patchset contains the following 2 patches.
> 
>  0001 sched_ext: scx_qmap: Restore unused idle claims from ops.dispatch()
>  0002 sched_ext: Maintain an online cid mask in the scheduler arena
> 
> The patchset is also available in the following git branch:
> 
>  git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git cid-online-cmask-v2
> 
> diffstat follows. Thanks.
> 
>  kernel/sched/ext/ext.c                   | 75 +++++++++++++++++++++++++++++---
>  kernel/sched/ext/internal.h              |  9 +++-
>  kernel/sched/ext/sub.c                   | 10 +++--
>  tools/sched_ext/include/scx/common.bpf.h |  1 +
>  tools/sched_ext/scx_qmap.bpf.c           | 14 ++++--
>  5 files changed, 94 insertions(+), 15 deletions(-)
> 
> --
> tejun