Re: [PATCH v2] blk-mq: Remove 'running from the wrong CPU' warning

Ming Lei
2020-12-01T02:11:52Z

On Mon, Nov 30, 2020 at 11:19:21AM +0100, Daniel Wagner wrote:
> It's guaranteed that no request is in flight when a hctx is going
> offline. This warning is only triggered when the wq's CPU is hot
> plugged and the blk-mq is not synced up yet.
>
> As this state is temporary and the request is still processed
> correctly, better remove the warning as this is the fast path.
>
> Suggested-by: Ming Lei <ming.lei@redhat.com>
> Signed-off-by: Daniel Wagner <dwagner@suse.de>
> ---
>
> v2:
> - remove the warning as suggested by Ming
> v1:
> - initial version
> https://lore.kernel.org/linux-block/20201126095152.19151-1-dwagner@suse.de/
>
> block/blk-mq.c | 25 -------------------------
> 1 file changed, 25 deletions(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 55bcee5dc032..7e6761804f86 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1495,31 +1495,6 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
> {
> int srcu_idx;
>
> - /*
> - * We should be running this queue from one of the CPUs that
> - * are mapped to it.
> - *
> - * There are at least two related races now between setting
> - * hctx->next_cpu from blk_mq_hctx_next_cpu() and running
> - * __blk_mq_run_hw_queue():
> - *
> - * - hctx->next_cpu is found offline in blk_mq_hctx_next_cpu(),
> - * but later it becomes online, then this warning is harmless
> - * at all
> - *
> - * - hctx->next_cpu is found online in blk_mq_hctx_next_cpu(),
> - * but later it becomes offline, then the warning can't be
> - * triggered, and we depend on blk-mq timeout handler to
> - * handle dispatched requests to this hctx
> - */
> - if (!cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask) &&
> - cpu_online(hctx->next_cpu)) {
> - printk(KERN_WARNING "run queue from wrong CPU %d, hctx %s\n",
> - raw_smp_processor_id(),
> - cpumask_empty(hctx->cpumask) ? "inactive": "active");
> - dump_stack();
> - }
> -
> /*
> * We can't run the queue inline with ints disabled. Ensure that
> * we catch bad users of this early.
> --
> 2.16.4
>

Reviewed-by: Ming Lei <ming.lei@redhat.com>

Thanks,
Ming