[PATCH 2/2] blk-mq: simplify queue mapping & schedule with each possisble CPU

[jianchao.wang]

Hi ming

Sorry for delayed report this.

On 01/17/2018 05:57 PM, Ming Lei wrote:
> 2) hctx->next_cpu can become offline from online before __blk_mq_run_hw_queue
> is run, there isn't warning, but once the IO is submitted to hardware,
> after it is completed, how does the HBA/hw queue notify CPU since CPUs
> assigned to this hw queue(irq vector) are offline? blk-mq's timeout
> handler may cover that, but looks too tricky.

In theory, the irq affinity will be migrated to other cpu. This is done by
fixup_irqs() in the context of stop_machine.
However, in my test, I found this log:

[  267.161043] do_IRQ: 7.33 No irq handler for vector

The 33 is the vector used by nvme cq.
The irq seems to be missed and sometimes IO hang occurred.
It is not every time, I think maybe due to nvme_process_cq in nvme_queue_rq.

I add dump stack behind the error log and get following:
[  267.161043] do_IRQ: 7.33 No irq handler for vector migration/7
[  267.161045] CPU: 7 PID: 52 Comm: migration/7 Not tainted 4.15.0-rc7+ #27
[  267.161045] Hardware name: LENOVO 10MLS0E339/3106, BIOS M1AKT22A 06/27/2017
[  267.161046] Call Trace:
[  267.161047]  <IRQ>
[  267.161052]  dump_stack+0x7c/0xb5
[  267.161054]  do_IRQ+0xb9/0xf0
[  267.161056]  common_interrupt+0xa2/0xa2
[  267.161057]  </IRQ>
[  267.161059] RIP: 0010:multi_cpu_stop+0xb0/0x120
[  267.161060] RSP: 0018:ffffbb6c81af7e70 EFLAGS: 00000202 ORIG_RAX: ffffffffffffffde
[  267.161061] RAX: 0000000000000001 RBX: 0000000000000004 RCX: 0000000000000000
[  267.161062] RDX: 0000000000000006 RSI: ffffffff898c4591 RDI: 0000000000000202
[  267.161063] RBP: ffffbb6c826e7c88 R08: ffff991abc1256bc R09: 0000000000000005
[  267.161063] R10: ffffbb6c81af7db8 R11: ffffffff89c91d20 R12: 0000000000000001
[  267.161064] R13: ffffbb6c826e7cac R14: 0000000000000003 R15: 0000000000000000
[  267.161067]  ? cpu_stop_queue_work+0x90/0x90
[  267.161068]  cpu_stopper_thread+0x83/0x100
[  267.161070]  smpboot_thread_fn+0x161/0x220
[  267.161072]  kthread+0xf5/0x130
[  267.161073]  ? sort_range+0x20/0x20
[  267.161074]  ? kthread_associate_blkcg+0xe0/0xe0
[  267.161076]  ret_from_fork+0x24/0x30

The irq just occurred after the irq is enabled in multi_cpu_stop.

0xffffffff8112d655 is in multi_cpu_stop (/home/will/u04/source_code/linux-block/kernel/stop_machine.c:223).
218				 */
219				touch_nmi_watchdog();
220			}
221		} while (curstate != MULTI_STOP_EXIT);
222	
223		local_irq_restore(flags);
224		return err;
225	}


Thanks
Jianchao