Re: [PATCH 09/13] sched_ext: Hook up hardlockup detector

[Tejun Heo <tj@kernel.org>]

On Tue, Nov 11, 2025 at 09:18:12AM -1000, Tejun Heo wrote:
> A poorly behaving BPF scheduler can trigger hard lockup. For example, on a
> large system with many tasks pinned to different subsets of CPUs, if the BPF
> scheduler puts all tasks in a single DSQ and lets all CPUs at it, the DSQ lock
> can be contended to the point where hardlockup triggers. Unfortunately,
> hardlockup can be the first signal out of such situations, thus requiring
> hardlockup handling.
> 
> Hook scx_hardlockup() into the hardlockup detector to try kicking out the
> current scheduler in an attempt to recover the system to a good state. The
> handling strategy can delay watchdog taking its own action by one polling
> period; however, given that the only remediation for hardlockup is crash, this
> is likely an acceptable trade-off.
> 
> v2: Add missing dummy scx_hardlockup() definition for
>     !CONFIG_SCHED_CLASS_EXT (kernel test bot).
> 
> Reported-by: Dan Schatzberg <schatzberg.dan@gmail.com>
> Cc: Emil Tsalapatis <etsal@meta.com>
> Cc: Douglas Anderson <dianders@chromium.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Reviewed-by: Andrea Righi <arighi@nvidia.com>
> Signed-off-by: Tejun Heo <tj@kernel.org>

I'll likely apply this patchset soon. Please holler if there are any
objections.

Thanks.

-- 
tejun