Re: [PATCH] blk-mq: Make running from the wrong CPU less scary

[Ming Lei <ming.lei@redhat.com>]

On Thu, Nov 26, 2020 at 10:51:52AM +0100, Daniel Wagner wrote:
> The current warning looks aweful like a proper crash. This is
> confusing. There is not much information to gained from the stack
> trace anyway, let's drop it.
> 
> While at it print the cpumask as there might be additial helpful
> information when debugging the sitation.
> 
> Signed-off-by: Daniel Wagner <dwagner@suse.de>
> ---
> Hi,
> 
> We got a report from a customer because he was concerned about the log
> entries. As it turns out, it fooled me too to be honest. What do you
> think about making it a bit less look-a-like a kernel oops?
> 
> 
>  smpboot: Booting Node 0 Processor 12 APIC 0x26                                                
> WARNING, didn't collect load info for all cpus, balancing is broken                            
>  run queue from wrong CPU 0, hctx active                                                       
>  CPU: 0 PID: 42300 Comm: kworker/13:2H Kdump: loaded Tainted: G           OE  X    5.3.18-109.$
>  Hardware name: IBM System x3650 M5 -[5462AC1]-/00KG915, BIOS -[TCE144J-3.11]- 12/03/2019      
>  Workqueue: kblockd blk_mq_run_work_fn                                                         
>  Call Trace:                                                                                   
>   dump_stack+0x66/0x8b                                                                         
>   __blk_mq_run_hw_queue+0xee/0x100                                                             
>   process_one_work+0x1f4/0x3e0                                                                 
>   worker_thread+0x2d/0x3e0                                                                     
>   ? process_one_work+0x3e0/0x3e0                                                               
>   kthread+0x10d/0x130                                                                          
>   ? kthread_park+0xa0/0xa0                                                                     
>   ret_from_fork+0x35/0x40                                                                      
>  run queue from wrong CPU 0, hctx active                                                       
>  CPU: 0 PID: 42300 Comm: kworker/13:2H Kdump: loaded Tainted: G           OE  X    5.3.18-109.$
>  Hardware name: IBM System x3650 M5 -[5462AC1]-/00KG915, BIOS -[TCE144J-3.11]- 12/03/2019      
>  Workqueue: kblockd blk_mq_run_work_fn    
> 
> 
> Thanks,
> Daniel
> 
>  block/blk-mq.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 55bcee5dc032..0427b719d9c4 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1514,10 +1514,8 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *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();
> +		printk(KERN_WARNING "run queue from wrong CPU %d, hctx %*pbl\n",
> +			raw_smp_processor_id(), cpumask_pr_args(hctx->cpumask));
>  	}

Now we have guaranteed that no any requests originated from one hctx exists
when this hctx is going to offline, which is strong enough for killing the check.

The reason why such warning is triggered is that wq's cpu hot unplug is
handled before blk-mq's handling.

I'd suggest to kill the whole branch in the fast path.

Thanks,
Ming