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SubjectRe: [PATCH 2/3] blk-mq: Always complete remote completions requests in softirq
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On 1/23/21 9:10 PM, Sebastian Andrzej Siewior wrote:
> Controllers with multiple queues have their IRQ-handelers pinned to a
> CPU. The core shouldn't need to complete the request on a remote CPU.
>
> Remove this case and always raise the softirq to complete the request.
>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
> block/blk-mq.c | 14 +-------------
> 1 file changed, 1 insertion(+), 13 deletions(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index f285a9123a8b0..90348ae518461 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -628,19 +628,7 @@ 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)
>
I don't get this.
This code is about _avoiding_ having to raise a softirq if the driver
exports more than one hardware queue.
So where exactly does the remote CPU case come in here?

Cheers,

Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
hare@suse.de +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer

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