linux-kernel - Re: [PATCH 3/3] blk-mq: Use llist_head for blk_cpu

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Date:   Thu, 29 Oct 2020 14:12:12 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-block@...r.kernel.org
Cc:     Christoph Hellwig <hch@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        David Runge <dave@...epmap.de>, linux-rt-users@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Wagner <dwagner@...e.de>, Mike Galbraith <efault@....de>
Subject: Re: [PATCH 3/3] blk-mq: Use llist_head for blk_cpu_done

On 2020-10-28 15:12:51 [+0100], To linux-block@...r.kernel.org wrote:
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -667,14 +632,21 @@ bool blk_mq_complete_request_remote(struct request *rq)
>  		return false;
>  
>  	if (blk_mq_complete_need_ipi(rq)) {
…
>  	} else {
>  		if (rq->q->nr_hw_queues > 1)
>  			return false;
> -		blk_mq_trigger_softirq(rq);
> +		cpu_list = this_cpu_ptr(&blk_cpu_done);
> +		if (llist_add(&rq->ipi_list, cpu_list))
> +			raise_softirq(BLOCK_SOFTIRQ);
>  	}
>  
>  	return true;

So Mike posted this:
| BUG: using smp_processor_id() in preemptible [00000000] code: usb-storage/841
| caller is blk_mq_complete_request_remote.part.0+0xa2/0x120
| CPU: 0 PID: 841 Comm: usb-storage Not tainted 5.10.0-rc1+ #61
| Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-1 04/01/2014
| Call Trace:
|  dump_stack+0x77/0x97
|  check_preemption_disabled+0xbe/0xc0
|  blk_mq_complete_request_remote.part.0+0xa2/0x120
|  blk_mq_complete_request+0x2e/0x40
|  usb_stor_control_thread+0x29a/0x300
|  kthread+0x14b/0x170
|  ret_from_fork+0x22/0x30

This comes from this_cpu_ptr() because usb_stor_control_thread() runs
with enabled preemption.

Adding preempt_disable() around it will make the warning go away but
will wake the ksoftirqd (this happens now, too).
Adding local_bh_disable() around it would perform the completion
immediately (instead of waking kssoftirqd) but local_bh_enable() feels
slightly more expensive.

Are there many drivers completing the SCSI requests in preemtible
context? In this case it would be more efficient to complete the request
directly (usb_stor_control_thread() goes to sleep after that anyway and
there is only one request at a time).

Sebastian