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

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

>> There might be two reasons:
>>
>> 1) You are still testing a multiple reply-queue device?
> 
> As before, I am testing by exposing mutliple queues to the SCSI 
> midlayer. I had to make this change locally, as on mainline we still 
> only expose a single queue and use the internal reply queue when 
> enabling managed interrupts.
> 
> As I
>> mentioned last times, it is hard to map reply-queue into blk-mq
>> hctx correctly.
> 
> Here's my branch, if you want to check:
> 
> https://github.com/hisilicon/kernel-dev/commits/private-topic-sas-5.4-mq-v4
> 
> It's a bit messy (sorry), but you can see that the reply-queue in the 
> LLDD is removed in commit 087b95af374.
> 
> I am now thinking of actually making this change to the LLDD in mainline 
> to avoid any doubt in future.
> 
>>
>> 2) concurrent dispatch to device, which can be observed by the
>> following patch.
>>
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index 06081966549f..3590f6f947eb 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -679,6 +679,8 @@ void blk_mq_start_request(struct request *rq)
>>  {
>>         struct request_queue *q = rq->q;
>>
>> +       WARN_ON_ONCE(test_bit(BLK_MQ_S_INTERNAL_STOPPED, 
>> &rq->mq_hctx->state));
>> +
>>         trace_block_rq_issue(q, rq);
>>
>>         if (test_bit(QUEUE_FLAG_STATS, &q->queue_flags)) {
>>
>> However, I think it is hard to be 2#, since the current CPU is the last
>> CPU in hctx->cpu_mask.
>>
> 
> I'll try it.
> 

Hi Ming,

I am looking at this issue again.

I am using 
https://lore.kernel.org/linux-scsi/1571926881-75524-1-git-send-email-john.garry@huawei.com/T/#t 
with expose_mq_experimental set. I guess you're going to say that this 
series is wrong, but I think it's ok for this purpose.

Forgetting that for a moment, maybe I have found an issue.

For the SCSI commands which timeout, I notice that 
scsi_set_blocked(reason=SCSI_MLQUEUE_EH_RETRY) was called 30 seconds 
earlier.

  scsi_set_blocked+0x20/0xb8
  __scsi_queue_insert+0x40/0x90
  scsi_softirq_done+0x164/0x1c8
  __blk_mq_complete_request_remote+0x18/0x20
  flush_smp_call_function_queue+0xa8/0x150
  generic_smp_call_function_single_interrupt+0x10/0x18
  handle_IPI+0xec/0x1a8
  arch_cpu_idle+0x10/0x18
  do_idle+0x1d0/0x2b0
  cpu_startup_entry+0x24/0x40
  secondary_start_kernel+0x1b4/0x208

I also notice that the __scsi_queue_insert() call, above, seems to retry 
to requeue the request on a dead rq in calling 
__scsi_queue_insert()->blk_mq_requeue_requet()->__blk_mq_requeue_request(), 
***:

[ 1185.235243] psci: CPU1 killed.
[ 1185.238610] blk_mq_hctx_notify_dead cpu1 dead 
request_queue=0xffff0023ace24f60 (id=19)
[ 1185.246530] blk_mq_hctx_notify_dead cpu1 dead 
request_queue=0xffff0023ace23f80 (id=17)
[ 1185.254443] blk_mq_hctx_notify_dead cpu1 dead 
request_queue=0xffff0023ace22fa0 (id=15)
[ 1185.262356] blk_mq_hctx_notify_dead cpu1 dead 
request_queue=0xffff0023ace21fc0 (id=13)***
[ 1185.270271] blk_mq_hctx_notify_dead cpu1 dead 
request_queue=0xffff0023ace20fe0 (id=11)
[ 1185.939451] scsi_softirq_done NEEDS_RETRY rq=0xffff0023b7416000
[ 1185.945359] scsi_set_blocked reason=0x1057
[ 1185.949444] __blk_mq_requeue_request request_queue=0xffff0023ace21fc0 
id=13 rq=0xffff0023b7416000***

[...]

[ 1214.903455] scsi_timeout req=0xffff0023add29000 reserved=0
[ 1214.908946] scsi_timeout req=0xffff0023add29300 reserved=0
[ 1214.914424] scsi_timeout req=0xffff0023add29600 reserved=0
[ 1214.919909] scsi_timeout req=0xffff0023add29900 reserved=0

I guess that we're retrying as the SCSI failed in the LLDD for some reason.

So could this be the problem - we're attempting to requeue on a dead 
request queue?

Thanks,
John

> Thanks as always,
> John
> 
>>
>> Thanks,
>> Ming