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SubjectRe: [PATCH] blk-mq: fix selecting software ctx for request
On Sun, Feb 02, 2020 at 06:20:04PM +0800, Hillf Danton wrote:
>
> Select the current cpu if it's mapped to hardware to make helpers like
> blk_mq_rq_cpu() return correct value.
>
> Signed-off-by: Hillf Danton <hdanton@sina.com>
> ---
>
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -454,7 +454,10 @@ struct request *blk_mq_alloc_request_hct
> blk_queue_exit(q);
> return ERR_PTR(-EXDEV);
> }
> - cpu = cpumask_first_and(alloc_data.hctx->cpumask, cpu_online_mask);
> + cpu = raw_smp_processor_id();
> + if (!cpumask_test_cpu(cpu, alloc_data.hctx->cpumask))
> + cpu = cpumask_first_and(alloc_data.hctx->cpumask,
> + cpu_online_mask);

How can you know if there is any online CPU available for this hctx?

> alloc_data.ctx = __blk_mq_get_ctx(q, cpu);
>
> rq = blk_mq_get_request(q, NULL, &alloc_data);
>

It is really one NVMe specific issue, see the following discussion:

https://lore.kernel.org/linux-block/8f4402a0-967d-f12d-2f1a-949e1dda017c@grimberg.me/#r


Thanks,
Ming

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