Hi Harshit,
Thanks for this!
On 07/03/25 20:42, Harshit Agarwal wrote:
This fix is the deadline version of the change made to the rt scheduler here: https://lore.kernel.org/lkml/20250225180553.167995-1-harshit@nutanix.com/ Please go through the original change for more details on the issue.
I don't think we want this kind of URLs in the changelog, as URL might disappear while the history remains (at least usually a little longer :). Maybe you could add a very condensed version of the description of the problem you have on the other fix?
In this fix we bail out or retry in the push_dl_task, if the task is no longer at the head of pushable tasks list because this list changed while trying to lock the runqueue of the other CPU.
Signed-off-by: Harshit Agarwal harshit@nutanix.com Cc: stable@vger.kernel.org
kernel/sched/deadline.c | 25 +++++++++++++++++++++---- 1 file changed, 21 insertions(+), 4 deletions(-)
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c index 38e4537790af..c5048969c640 100644 --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -2704,6 +2704,7 @@ static int push_dl_task(struct rq *rq) { struct task_struct *next_task; struct rq *later_rq;
- struct task_struct *task; int ret = 0;
next_task = pick_next_pushable_dl_task(rq); @@ -2734,15 +2735,30 @@ static int push_dl_task(struct rq *rq) /* Will lock the rq it'll find */ later_rq = find_lock_later_rq(next_task, rq);
- if (!later_rq) {
struct task_struct *task;
- task = pick_next_pushable_dl_task(rq);
- if (later_rq && (!task || task != next_task)) {
/** We must check all this again, since* find_lock_later_rq releases rq->lock and it is* then possible that next_task has migrated and* is no longer at the head of the pushable list.*/double_unlock_balance(rq, later_rq);if (!task) {/* No more tasks */goto out;}
put_task_struct(next_task);next_task = task;goto retry;
I fear we might hit a pathological condition that can lead us into a never ending (or very long) loop. find_lock_later_rq() tries to find a later_rq for at most DL_MAX_TRIES and it bails out if it can't.
Maybe to discern between find_lock_later_rq() callers we can use dl_throttled flag in dl_se and still implement the fix in find_lock_ later_rq()? I.e., fix similar to the rt.c patch in case the task is not throttled (so caller is push_dl_task()) and not rely on pick_next_ pushable_dl_task() if the task is throttled.
What do you think?
Thanks, Juri