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On 4/10/2025 3:30 PM, Peter Zijlstra wrote: > On Wed, Apr 09, 2025 at 11:15:35AM +0000, K Prateek Nayak wrote: > >> +static void fair_add_pushable_task(struct rq *rq, struct task_struct *p) >> +{ >> + if (fair_push_task(p)) { >> + plist_del(&p->pushable_tasks, &rq->cfs.pushable_tasks); >> + plist_node_init(&p->pushable_tasks, p->prio); > > I gotta aks, why do we care about ordering the push list on p->prio for > fair? This bit was carried over from Vincent's series. I assume it was inspired by the RT, DL's push mechanism. Perhaps fair can get away with MRU ordering similar to rq->cfs_tasks? > >> + plist_add(&p->pushable_tasks, &rq->cfs.pushable_tasks); >> + } >> +} >> + >> /* >> * select_task_rq_fair: Select target runqueue for the waking task in domains >> * that have the relevant SD flag set. In practice, this is SD_BALANCE_WAKE, > >> @@ -8914,6 +8978,12 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf >> put_prev_entity(cfs_rq, pse); >> set_next_entity(cfs_rq, se); >> >> + /* >> + * The previous task might be eligible for being pushed on >> + * another cpu if it is still active. >> + */ >> + fair_add_pushable_task(rq, prev); >> + >> __set_next_task_fair(rq, p, true); So __set_next_task_fair() removes the current task from the pushable list when it is picked ... >> } >> >> @@ -8986,6 +9056,13 @@ static void put_prev_task_fair(struct rq *rq, struct task_struct *prev, struct t >> cfs_rq = cfs_rq_of(se); >> put_prev_entity(cfs_rq, se); >> } >> + >> + /* >> + * The previous task might be eligible for being pushed on another cpu >> + * if it is still active. >> + */ >> + fair_add_pushable_task(rq, prev); >> + >> } > > These two are tricky; while they will work with a push balance callback, > they will cause some pain with pulling from the push lists; a-la > newidle. > > Notably, we must be sure to check ->on_cpu. ... so we should be safe here. Other considerations are covered by detach_task() but pushable task lists to only contain queued tasks that can run on more than one CPU and Vincent was cautious enough to place a bunch of WARN_ON_ONCE() in pick_next_pushable_fair_task() to catch those cases :) > > Perhaps later patches add this, let me continue reading... -- Thanks and Regards, Prateek