Re: [PATCH V4 0/5] blk-mq: improvement on handling IO during CPU hotplug

[John Garry <john.garry@huawei.com>]

On 29/10/2019 10:05, Ming Lei wrote:
>>> But this patch does wait for completion of in-flight request before
>>> shutdown the last CPU of this hctx.
>>>
>> Hi Ming,
>>
>> It may actually be a request from a hctx which is not shut down which errors
>> and causes the time out. I'm still checking.
> If that is the case, blk_mq_hctx_drain_inflight_rqs() will wait for
> completion of this request.
> 
> The only chance it is missed is that the last CPU of this hctx becomes
> offline just when this request stays in request list after it is
> retried from EH.
> 
>> BTW, Can you let me know exactly where you want the debug for "Or
>> blk_mq_hctx_next_cpu() may still run WORK_CPU_UNBOUND schedule after
>> all CPUs are offline, could you add debug message in that branch?"
> You can add the following debug message, then reproduce the issue and
> see if the debug log is dumped.
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 06081966549f..5a98a7b79c0d 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1452,6 +1452,10 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
>   		 */
>   		hctx->next_cpu = next_cpu;
>   		hctx->next_cpu_batch = 1;
> +
> +		printk(KERN_WARNING "CPU %d, schedule from (dead) hctx %s\n",
> +			raw_smp_processor_id(),
> +			cpumask_empty(hctx->cpumask) ? "inactive": "active");
>   		return WORK_CPU_UNBOUND;

We don't seem to be hitting this.

So the error generally happens after the CPUs have been hot unplugged 
for some time. I find a SCSI IO errors in the LLDD, having been run on 
an up hctx - blk-mq requeues it on same hctx, but it only dispatches 
eventually after some other SCSI IO timesout - I assume that IO timesout 
due to earlier scsi_set_blocked() call.

Continuing to look....

Thanks,
John

>   	}
>   
> 
> Thanks,
> Ming
> 
> .