Re: [PATCH sched_ext/for-6.14-fixes] sched_ext: Fix pick_task_scx() picking non-queued tasks when it's called without balance()

[Tejun Heo <tj@kernel.org>]

On Tue, Feb 25, 2025 at 06:02:23AM -1000, Tejun Heo wrote:
> a6250aa251ea ("sched_ext: Handle cases where pick_task_scx() is called
> without preceding balance_scx()") added a workaround to handle the cases
> where pick_task_scx() is called without prececing balance_scx() which is due
> to a fair class bug where pick_taks_fair() may return NULL after a true
> return from balance_fair().
> 
> The workaround detects when pick_task_scx() is called without preceding
> balance_scx() and emulates SCX_RQ_BAL_KEEP and triggers kicking to avoid
> stalling. Unfortunately, the workaround code was testing whether @prev was
> on SCX to decide whether to keep the task running. This is incorrect as the
> task may be on SCX but no longer runnable.
> 
> This could lead to a non-runnable task to be returned from pick_task_scx()
> which cause interesting confusions and failures. e.g. A common failure mode
> is the task ending up with (!on_rq && on_cpu) state which can cause
> potential wakers to busy loop, which can easily lead to deadlocks.
> 
> Fix it by testing whether @prev has SCX_TASK_QUEUED set. This makes
> $prev_on_scx only used in one place. Open code the usage and improve the
> comment while at it.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Reported-by: Pat Cody <patcody@meta.com>
> Fixes: a6250aa251ea ("sched_ext: Handle cases where pick_task_scx() is called without preceding balance_scx()")
> Cc: stable@vger.kernel.org # v6.12+

Applied to sched_ext/for-6.14-fixes.

Thanks.

-- 
tejun