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    SubjectRe: [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT_RT
    On Tue, Oct 27, 2020 at 06:05:15PM +0100, Thomas Gleixner wrote:
    > > Is there a way to raise a softirq and preferably place it on a given
    > > CPU without our IPI dance? That should be a win-win situation for
    > > everyone.
    >
    > Not really. Softirq pending bits are strictly per cpu and we don't have
    > locking or atomics to set them remotely. Even if we had that, then you'd
    > still need a mechanism to make sure that the remote CPU actually
    > processes them. So you'd still need an IPI of some sorts.

    Ok. I was hoping we could hide this in core code somehow, especially
    a peterz didn't like the use of smp_call_function_single_async in the
    blk-mq completion code very much.

    Sebastian, would this solve your preempt-rt and lockdep issues?


    diff --git a/block/blk-mq.c b/block/blk-mq.c
    index cdced4aca2e812..5c125fb11b5691 100644
    --- a/block/blk-mq.c
    +++ b/block/blk-mq.c
    @@ -626,19 +626,7 @@ static void __blk_mq_complete_request_remote(void *data)
    {
    struct request *rq = data;

    - /*
    - * For most of single queue controllers, there is only one irq vector
    - * for handling I/O completion, and the only irq's affinity is set
    - * to all possible CPUs. On most of ARCHs, this affinity means the irq
    - * is handled on one specific CPU.
    - *
    - * So complete I/O requests in softirq context in case of single queue
    - * devices to avoid degrading I/O performance due to irqsoff latency.
    - */
    - if (rq->q->nr_hw_queues == 1)
    - blk_mq_trigger_softirq(rq);
    - else
    - rq->q->mq_ops->complete(rq);
    + blk_mq_trigger_softirq(rq);
    }

    static inline bool blk_mq_complete_need_ipi(struct request *rq)
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