Messages in this thread |  | | Date | Sat, 7 Mar 2026 07:50:12 +0100 | | From | Andrea Righi <> | | Subject | Re: [PATCH sched_ext/for-7.0-fixes] sched_ext: Fix enqueue_task_scx() truncation of upper enqueue flags |
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On Fri, Mar 06, 2026 at 02:23:13PM -1000, Tejun Heo wrote: > enqueue_task_scx() takes int enq_flags from the sched_class interface. > SCX enqueue flags starting at bit 32 (SCX_ENQ_PREEMPT and above) are > silently truncated when passed through activate_task(). extra_enq_flags > was added as a workaround - storing high bits in rq->scx.extra_enq_flags > and OR-ing them back in enqueue_task_scx(). However, the OR target is > still the int parameter, so the high bits are lost anyway. > > The current impact is limited as the only affected flag is SCX_ENQ_PREEMPT > which is informational to the BPF scheduler - its loss means the scheduler > doesn't know about preemption but doesn't cause incorrect behavior. > > Fix by renaming the int parameter to core_enq_flags and introducing a > u64 enq_flags local that merges both sources. All downstream functions > already take u64 enq_flags. > > Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class") > Cc: stable@vger.kernel.org # v6.12+ > Signed-off-by: Tejun Heo <tj@kernel.org>
Good catch.
Acked-by: Andrea Righi <arighi@nvidia.com>
Thanks, -Andrea
> --- > kernel/sched/ext.c | 5 ++--- > 1 file changed, 2 insertions(+), 3 deletions(-) > > diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c > index XXXXXXX..XXXXXXX 100644 > --- a/kernel/sched/ext.c > +++ b/kernel/sched/ext.c > @@ -1728,16 +1728,15 @@ static void clr_task_runnable(struct task_struct *p) > p->scx.flags |= SCX_TASK_RESET_RUNNABLE_AT; > } > > -static void enqueue_task_scx(struct rq *rq, struct task_struct *p, int enq_flags) > +static void enqueue_task_scx(struct rq *rq, struct task_struct *p, int core_enq_flags) > { > struct scx_sched *sch = scx_task_sched(p); > int sticky_cpu = p->scx.sticky_cpu; > + u64 enq_flags = core_enq_flags | rq->scx.extra_enq_flags; > > if (enq_flags & ENQUEUE_WAKEUP) > rq->scx.flags |= SCX_RQ_IN_WAKEUP; > > - enq_flags |= rq->scx.extra_enq_flags; > - > /* > * Restoring a running task will be immediately followed by > * set_next_task_scx() which expects the task to not be on the BPF > -- > tejun
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