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

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Date:   Tue, 1 Dec 2020 10:09:44 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Daniel Wagner <dwagner@...e.de>
Cc:     linux-block@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] blk-mq: Remove 'running from the wrong CPU' warning

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@...hat.com>
> Signed-off-by: Daniel Wagner <dwagner@...e.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@...hat.com>

Thanks,
Ming