Re: [PATCH v13] blk-mq: Rework blk-mq timeout handling again

[Jens Axboe <axboe@kernel.dk>]

On 5/22/18 12:47 PM, Jens Axboe wrote:
> On 5/22/18 11:17 AM, Jens Axboe wrote:
>> On 5/22/18 10:44 AM, Jens Axboe wrote:
>>>
>>> On 5/22/18 10:25 AM, Bart Van Assche wrote:
>>>> Recently the blk-mq timeout handling code was reworked. See also Tejun
>>>> Heo, "[PATCHSET v4] blk-mq: reimplement timeout handling", 08 Jan 2018
>>>> (https://www.mail-archive.com/linux-block@vger.kernel.org/msg16985.html).
>>>> This patch reworks the blk-mq timeout handling code again. The timeout
>>>> handling code is simplified by introducing a state machine per request.
>>>> This change avoids that the blk-mq timeout handling code ignores
>>>> completions that occur after blk_mq_check_expired() has been called and
>>>> before blk_mq_rq_timed_out() has been called.
>>>
>>> I'll take a look at this again, getting rid of cmpxchg64 makes me
>>> much more comfortable.
>>
>> FWIW, a quick pass on runtime testing works fine. As expected, it's
>> more efficient than what's currently in the kernel, testing with both
>> null_blk (1 and nr_cpus worth of queues), and nvme as well. A huge win
>> is that we shrink the request size from 360 bytes to 312, and I did
>> a small followup patch that brings that to 304. That's a 15% reduction,
>> massive.
> 
> Ran into this, running block/014 from blktests:
> 
> [ 5744.949839] run blktests block/014 at 2018-05-22 12:41:25
> [ 5750.723000] null: rq 00000000ff68f103 timed out
> [ 5750.728181] WARNING: CPU: 45 PID: 2480 at block/blk-mq.c:585 __blk_mq_complete_request+0xa6/0x0
> [ 5750.738187] Modules linked in: null_blk(+) configfs nvme nvme_core sb_edac x86_pkg_temp_therma]
> [ 5750.765509] CPU: 45 PID: 2480 Comm: kworker/45:1H Not tainted 4.17.0-rc6+ #712
> [ 5750.774087] Hardware name: Dell Inc. PowerEdge T630/0NT78X, BIOS 2.3.4 11/09/2016
> [ 5750.783369] Workqueue: kblockd blk_mq_timeout_work
> [ 5750.789223] RIP: 0010:__blk_mq_complete_request+0xa6/0x110
> [ 5750.795850] RSP: 0018:ffff883ffb417d68 EFLAGS: 00010202
> [ 5750.802187] RAX: 0000000000000003 RBX: ffff881ff100d800 RCX: 0000000000000000
> [ 5750.810649] RDX: ffff88407fd9e040 RSI: ffff88407fd956b8 RDI: ffff881ff100d800
> [ 5750.819119] RBP: ffffe8ffffd91800 R08: 0000000000000000 R09: ffffffff82066eb8
> [ 5750.827588] R10: ffff883ffa386138 R11: ffff883ffa385900 R12: 0000000000000001
> [ 5750.836050] R13: ffff881fe7da6000 R14: 0000000000000020 R15: 0000000000000002
> [ 5750.844529] FS:  0000000000000000(0000) GS:ffff88407fd80000(0000) knlGS:0000000000000000
> [ 5750.854482] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 5750.861397] CR2: 00007ffc92f97f68 CR3: 000000000201d005 CR4: 00000000003606e0
> [ 5750.869861] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [ 5750.878333] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [ 5750.886805] Call Trace:
> [ 5750.890033]  bt_iter+0x42/0x50
> [ 5750.894000]  blk_mq_queue_tag_busy_iter+0x12b/0x220
> [ 5750.899941]  ? blk_mq_tag_to_rq+0x20/0x20
> [ 5750.904913]  ? __rcu_read_unlock+0x50/0x50
> [ 5750.909978]  ? blk_mq_tag_to_rq+0x20/0x20
> [ 5750.914948]  blk_mq_timeout_work+0x14b/0x240
> [ 5750.920220]  process_one_work+0x21b/0x510
> [ 5750.925197]  worker_thread+0x3a/0x390
> [ 5750.929781]  ? process_one_work+0x510/0x510
> [ 5750.934944]  kthread+0x11c/0x140
> [ 5750.939028]  ? kthread_create_worker_on_cpu+0x50/0x50
> [ 5750.945169]  ret_from_fork+0x1f/0x30
> [ 5750.949656] Code: 48 02 00 00 80 e6 80 74 29 8b 95 80 00 00 00 44 39 e2 75 3b 48 89 df ff 90 2 
> [ 5750.972139] ---[ end trace 40065cb1764bf500 ]---
> 
> which is this:
> 
>         WARN_ON_ONCE(blk_mq_rq_state(rq) != MQ_RQ_COMPLETE);

That check looks wrong, since TIMED_OUT -> COMPLETE is also a valid
state transition. So that check should be:

        WARN_ON_ONCE(blk_mq_rq_state(rq) != MQ_RQ_COMPLETE &&
                     blk_mq_rq_state(rq) != MQ_RQ_TIMED_OUT);

-- 
Jens Axboe