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

jianchao.wang
2018-01-17T02:56:55Z

Hi ming

Thanks for your patch and kindly response.

On 01/16/2018 11:32 PM, Ming Lei wrote:
> OK, I got it, and it should have been the only corner case in which
> all CPUs mapped to this hctx become offline, and I believe the following
> patch should address this case, could you give a test?
>
> ---
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index c376d1b6309a..23f0f3ddffcf 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1416,21 +1416,44 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
> */
> static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
> {
> + bool tried = false;
> +
> if (hctx->queue->nr_hw_queues == 1)
> return WORK_CPU_UNBOUND;
>
> if (--hctx->next_cpu_batch <= 0) {
> int next_cpu;
> +select_cpu:
>
> next_cpu = cpumask_next_and(hctx->next_cpu, hctx->cpumask,
> cpu_online_mask);
> if (next_cpu >= nr_cpu_ids)
> next_cpu = cpumask_first_and(hctx->cpumask,cpu_online_mask);
>
> - hctx->next_cpu = next_cpu;
> + /*
> + * No online CPU can be found here when running from
> + * blk_mq_hctx_notify_dead(), so make sure hctx->next_cpu
> + * is set correctly.
> + */
> + if (next_cpu >= nr_cpu_ids)
> + hctx->next_cpu = cpumask_first_and(hctx->cpumask,
> + cpu_possible_mask);
> + else
> + hctx->next_cpu = next_cpu;
> hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
> }
>
> + /*
> + * Do unbound schedule if we can't find a online CPU for this hctx,
> + * and it should happen only if hctx->next_cpu is becoming DEAD.
> + */
> + if (!cpu_online(hctx->next_cpu)) {
> + if (!tried) {
> + tried = true;
> + goto select_cpu;
> + }
> + return WORK_CPU_UNBOUND;
> + }
> return hctx->next_cpu;
> }

I have tested this patch. The panic was gone, but I got the following:

[ 231.674464] WARNING: CPU: 0 PID: 263 at /home/will/u04/source_code/linux-block/block/blk-mq.c:1315 __blk_mq_run_hw_queue+0x92/0xa0
[ 231.674466] Modules linked in: .....
[ 231.674494] CPU: 0 PID: 263 Comm: kworker/2:1H Not tainted 4.15.0-rc7+ #12
[ 231.674495] Hardware name: LENOVO 10MLS0E339/3106, BIOS M1AKT22A 06/27/2017
[ 231.674496] Workqueue: kblockd blk_mq_run_work_fn
[ 231.674498] RIP: 0010:__blk_mq_run_hw_queue+0x92/0xa0
[ 231.674499] RSP: 0018:ffffa9c801fcfe60 EFLAGS: 00010202
[ 231.674500] RAX: 0000000000000001 RBX: ffff9c7c90231400 RCX: 0000000000000000
[ 231.674500] RDX: ffff9c7c9255b0f8 RSI: 0000000000000000 RDI: ffff9c7c90231400
[ 231.674500] RBP: ffff9c7ca2ca2140 R08: 0000000000000000 R09: 0000000000000000
[ 231.674501] R10: 00000000000003cb R11: 0000000000000000 R12: ffff9c7ca2ca8200
[ 231.674501] R13: 0000000000000000 R14: 0ffff9c7ca2ca820 R15: ffff9c7c3df25240
[ 231.674502] FS: 0000000000000000(0000) GS:ffff9c7ca2c00000(0000) knlGS:0000000000000000
[ 231.674502] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 231.674503] CR2: 0000000001727008 CR3: 0000000336409003 CR4: 00000000003606f0
[ 231.674504] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 231.674504] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 231.674504] Call Trace:
[ 231.674509] process_one_work+0x14b/0x420
[ 231.674510] worker_thread+0x47/0x420
[ 231.674512] kthread+0xf5/0x130
[ 231.674514] ? process_one_work+0x420/0x420
[ 231.674515] ? kthread_associate_blkcg+0xe0/0xe0
[ 231.674517] ? do_group_exit+0x3a/0xa0
[ 231.674518] ret_from_fork+0x24/0x30
[ 231.674520] Code: ff 48 89 df e8 e0 6b 00 00 8b 74 24 04 48 89 df e8 a4 fc ff ff 48 8b 44 24 08 65 48 33 04 25 28 00 00 00 75 0e 48 83 c4 10 5b c3 <0f> ff eb b7 0f ff eb c1 e8 e1 02 c6 ff 90 0f 1f 44 00 00 48 8b
[ 231.674537] ---[ end trace cc2de957e0e0fed4 ]---

It is here.
__blk_mq_run_hw_queue()
....
WARN_ON(!cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask) &&
cpu_online(hctx->next_cpu));
....

To eliminate this risk totally, we could blk_mq_hctx_next_cpu return the cpu even if the cpu is offlined and modify the cpu_online above to cpu_active.
The kworkers of the per-cpu pool must have be migrated back when the cpu is set active.
But there seems to be issues in DASD as your previous comment.
>>>>
That is the original version of this patch, and both Christian and Stefan
reported that system can't boot from DASD in this way[2], and I changed
to AND with cpu_online_mask, then their system can boot well
>>>>

On the other hand, there is also risk in

@@ -440,7 +440,7 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
blk_queue_exit(q);
return ERR_PTR(-EXDEV);
}
- cpu = cpumask_first(alloc_data.hctx->cpumask);
+ cpu = cpumask_first_and(alloc_data.hctx->cpumask, cpu_online_mask);
alloc_data.ctx = __blk_mq_get_ctx(q, cpu);

what if the cpus in alloc_data.hctx->cpumask are all offlined ?

Thanks
Jianchao