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    SubjectRe: [PATCH 14/31] sched_ext: Implement BPF extensible scheduler class
    Hello,

    On Fri, Dec 02, 2022 at 12:08:27PM -0500, Barret Rhoden wrote:
    > you might be able to avoid the double_lock_balance() by using
    > move_queued_task(), which internally hands off the old rq lock and returns
    > with the new rq lock.
    >
    > the pattern for consume_dispatch_q() would be something like:
    >
    > - kfunc from bpf, with this_rq lock held
    > - notice p isn't on this_rq, goto remote_rq:
    > - do sched_ext accounting, like the this_rq->dsq->nr--
    > - unlock this_rq
    > - p_rq = task_rq_lock(p)
    > - double_check p->rq didn't change to this_rq during that unlock
    > - new_rq = move_queued_task(p_rq, rf, p, new_cpu)
    > - do sched_ext accounting like new_rq->dsq->nr++
    > - unlock new_rq
    > - relock the original this_rq
    > - return to bpf
    >
    > you still end up grabbing both locks, but just not at the same time.

    Yeah, this probably would look better than the current double lock dancing,
    especially in the finish_dispatch() path.

    > plus, task_rq_lock() takes the guesswork out of whether you're getting p's
    > rq lock or not. it looks like you're using the holding_cpu to handle the
    > race where p moves cpus after you read task_rq(p) but before you lock that
    > task_rq. maybe you can drop the whole concept of the holding_cpu?

    ->holding_cpu is there to basically detect intervening dequeues, so if we
    lock them out with TASK_ON_RQ_MIGRATING, we might be able to drop it. I need
    to look into it more tho. Things get pretty subtle around there, so I could
    easily be missing something. I'll try this and let you know how it goes.

    Thanks.

    --
    tejun

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