Date:   Tue, 7 Aug 2018 15:19:57 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Anchal Agarwal <anchalag@...zon.com>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        fllinden@...zon.com, sblbir@...zon.com, msw@...zon.com
Subject: Re: [PATCH] blk-wbt: Avoid lock contention and thundering herd issue
 in wbt_wait

On 8/7/18 3:12 PM, Anchal Agarwal wrote:
> On Tue, Aug 07, 2018 at 02:39:48PM -0600, Jens Axboe wrote:
>> On 8/7/18 2:12 PM, Anchal Agarwal wrote:
>>> On Tue, Aug 07, 2018 at 08:29:44AM -0600, Jens Axboe wrote:
>>>> On 8/1/18 4:09 PM, Jens Axboe wrote:
>>>>> On 8/1/18 11:06 AM, Anchal Agarwal wrote:
>>>>>> On Wed, Aug 01, 2018 at 09:14:50AM -0600, Jens Axboe wrote:
>>>>>>> On 7/31/18 3:34 PM, Anchal Agarwal wrote:
>>>>>>>> Hi folks,
>>>>>>>>
>>>>>>>> This patch modifies commit e34cbd307477a
>>>>>>>> (blk-wbt: add general throttling mechanism)
>>>>>>>>
>>>>>>>> I am currently running a large bare metal instance (i3.metal)
>>>>>>>> on EC2 with 72 cores, 512GB of RAM and NVME drives, with a
>>>>>>>> 4.18 kernel. I have a workload that simulates a database
>>>>>>>> workload and I am running into lockup issues when writeback
>>>>>>>> throttling is enabled,with the hung task detector also
>>>>>>>> kicking in.
>>>>>>>>
>>>>>>>> Crash dumps show that most CPUs (up to 50 of them) are
>>>>>>>> all trying to get the wbt wait queue lock while trying to add
>>>>>>>> themselves to it in __wbt_wait (see stack traces below).
>>>>>>>>
>>>>>>>> [    0.948118] CPU: 45 PID: 0 Comm: swapper/45 Not tainted 4.14.51-62.38.amzn1.x86_64 #1
>>>>>>>> [    0.948119] Hardware name: Amazon EC2 i3.metal/Not Specified, BIOS 1.0 10/16/2017
>>>>>>>> [    0.948120] task: ffff883f7878c000 task.stack: ffffc9000c69c000
>>>>>>>> [    0.948124] RIP: 0010:native_queued_spin_lock_slowpath+0xf8/0x1a0
>>>>>>>> [    0.948125] RSP: 0018:ffff883f7fcc3dc8 EFLAGS: 00000046
>>>>>>>> [    0.948126] RAX: 0000000000000000 RBX: ffff887f7709ca68 RCX: ffff883f7fce2a00
>>>>>>>> [    0.948128] RDX: 000000000000001c RSI: 0000000000740001 RDI: ffff887f7709ca68
>>>>>>>> [    0.948129] RBP: 0000000000000002 R08: 0000000000b80000 R09: 0000000000000000
>>>>>>>> [    0.948130] R10: ffff883f7fcc3d78 R11: 000000000de27121 R12: 0000000000000002
>>>>>>>> [    0.948131] R13: 0000000000000003 R14: 0000000000000000 R15: 0000000000000000
>>>>>>>> [    0.948132] FS:  0000000000000000(0000) GS:ffff883f7fcc0000(0000) knlGS:0000000000000000
>>>>>>>> [    0.948134] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>>>>>> [    0.948135] CR2: 000000c424c77000 CR3: 0000000002010005 CR4: 00000000003606e0
>>>>>>>> [    0.948136] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>>>>>>> [    0.948137] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>>>>>>> [    0.948138] Call Trace:
>>>>>>>> [    0.948139]  <IRQ>
>>>>>>>> [    0.948142]  do_raw_spin_lock+0xad/0xc0
>>>>>>>> [    0.948145]  _raw_spin_lock_irqsave+0x44/0x4b
>>>>>>>> [    0.948149]  ? __wake_up_common_lock+0x53/0x90
>>>>>>>> [    0.948150]  __wake_up_common_lock+0x53/0x90
>>>>>>>> [    0.948155]  wbt_done+0x7b/0xa0
>>>>>>>> [    0.948158]  blk_mq_free_request+0xb7/0x110
>>>>>>>> [    0.948161]  __blk_mq_complete_request+0xcb/0x140
>>>>>>>> [    0.948166]  nvme_process_cq+0xce/0x1a0 [nvme]
>>>>>>>> [    0.948169]  nvme_irq+0x23/0x50 [nvme]
>>>>>>>> [    0.948173]  __handle_irq_event_percpu+0x46/0x300
>>>>>>>> [    0.948176]  handle_irq_event_percpu+0x20/0x50
>>>>>>>> [    0.948179]  handle_irq_event+0x34/0x60
>>>>>>>> [    0.948181]  handle_edge_irq+0x77/0x190
>>>>>>>> [    0.948185]  handle_irq+0xaf/0x120
>>>>>>>> [    0.948188]  do_IRQ+0x53/0x110
>>>>>>>> [    0.948191]  common_interrupt+0x87/0x87
>>>>>>>> [    0.948192]  </IRQ>
>>>>>>>> ....
>>>>>>>> [    0.311136] CPU: 4 PID: 9737 Comm: run_linux_amd64 Not tainted 4.14.51-62.38.amzn1.x86_64 #1
>>>>>>>> [    0.311137] Hardware name: Amazon EC2 i3.metal/Not Specified, BIOS 1.0 10/16/2017
>>>>>>>> [    0.311138] task: ffff883f6e6a8000 task.stack: ffffc9000f1ec000
>>>>>>>> [    0.311141] RIP: 0010:native_queued_spin_lock_slowpath+0xf5/0x1a0
>>>>>>>> [    0.311142] RSP: 0018:ffffc9000f1efa28 EFLAGS: 00000046
>>>>>>>> [    0.311144] RAX: 0000000000000000 RBX: ffff887f7709ca68 RCX: ffff883f7f722a00
>>>>>>>> [    0.311145] RDX: 0000000000000035 RSI: 0000000000d80001 RDI: ffff887f7709ca68
>>>>>>>> [    0.311146] RBP: 0000000000000202 R08: 0000000000140000 R09: 0000000000000000
>>>>>>>> [    0.311147] R10: ffffc9000f1ef9d8 R11: 000000001a249fa0 R12: ffff887f7709ca68
>>>>>>>> [    0.311148] R13: ffffc9000f1efad0 R14: 0000000000000000 R15: ffff887f7709ca00
>>>>>>>> [    0.311149] FS:  000000c423f30090(0000) GS:ffff883f7f700000(0000) knlGS:0000000000000000
>>>>>>>> [    0.311150] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>>>>>> [    0.311151] CR2: 00007feefcea4000 CR3: 0000007f7016e001 CR4: 00000000003606e0
>>>>>>>> [    0.311152] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>>>>>>> [    0.311153] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>>>>>>> [    0.311154] Call Trace:
>>>>>>>> [    0.311157]  do_raw_spin_lock+0xad/0xc0
>>>>>>>> [    0.311160]  _raw_spin_lock_irqsave+0x44/0x4b
>>>>>>>> [    0.311162]  ? prepare_to_wait_exclusive+0x28/0xb0
>>>>>>>> [    0.311164]  prepare_to_wait_exclusive+0x28/0xb0
>>>>>>>> [    0.311167]  wbt_wait+0x127/0x330
>>>>>>>> [    0.311169]  ? finish_wait+0x80/0x80
>>>>>>>> [    0.311172]  ? generic_make_request+0xda/0x3b0
>>>>>>>> [    0.311174]  blk_mq_make_request+0xd6/0x7b0
>>>>>>>> [    0.311176]  ? blk_queue_enter+0x24/0x260
>>>>>>>> [    0.311178]  ? generic_make_request+0xda/0x3b0
>>>>>>>> [    0.311181]  generic_make_request+0x10c/0x3b0
>>>>>>>> [    0.311183]  ? submit_bio+0x5c/0x110
>>>>>>>> [    0.311185]  submit_bio+0x5c/0x110
>>>>>>>> [    0.311197]  ? __ext4_journal_stop+0x36/0xa0 [ext4]
>>>>>>>> [    0.311210]  ext4_io_submit+0x48/0x60 [ext4]
>>>>>>>> [    0.311222]  ext4_writepages+0x810/0x11f0 [ext4]
>>>>>>>> [    0.311229]  ? do_writepages+0x3c/0xd0
>>>>>>>> [    0.311239]  ? ext4_mark_inode_dirty+0x260/0x260 [ext4]
>>>>>>>> [    0.311240]  do_writepages+0x3c/0xd0
>>>>>>>> [    0.311243]  ? _raw_spin_unlock+0x24/0x30
>>>>>>>> [    0.311245]  ? wbc_attach_and_unlock_inode+0x165/0x280
>>>>>>>> [    0.311248]  ? __filemap_fdatawrite_range+0xa3/0xe0
>>>>>>>> [    0.311250]  __filemap_fdatawrite_range+0xa3/0xe0
>>>>>>>> [    0.311253]  file_write_and_wait_range+0x34/0x90
>>>>>>>> [    0.311264]  ext4_sync_file+0x151/0x500 [ext4]
>>>>>>>> [    0.311267]  do_fsync+0x38/0x60
>>>>>>>> [    0.311270]  SyS_fsync+0xc/0x10
>>>>>>>> [    0.311272]  do_syscall_64+0x6f/0x170
>>>>>>>> [    0.311274]  entry_SYSCALL_64_after_hwframe+0x42/0xb7
>>>>>>>>
>>>>>>>> In the original patch, wbt_done is waking up all the exclusive
>>>>>>>> processes in the wait queue, which can cause a thundering herd
>>>>>>>> if there is a large number of writer threads in the queue. The
>>>>>>>> original intention of the code seems to be to wake up one thread
>>>>>>>> only however, it uses wake_up_all() in __wbt_done(), and then
>>>>>>>> uses the following check in __wbt_wait to have only one thread
>>>>>>>> actually get out of the wait loop:
>>>>>>>>
>>>>>>>> if (waitqueue_active(&rqw->wait) &&
>>>>>>>>             rqw->wait.head.next != &wait->entry)
>>>>>>>>                 return false;
>>>>>>>>
>>>>>>>> The problem with this is that the wait entry in wbt_wait is
>>>>>>>> define with DEFINE_WAIT, which uses the autoremove wakeup function.
>>>>>>>> That means that the above check is invalid - the wait entry will
>>>>>>>> have been removed from the queue already by the time we hit the
>>>>>>>> check in the loop.
>>>>>>>>
>>>>>>>> Secondly, auto-removing the wait entries also means that the wait
>>>>>>>> queue essentially gets reordered "randomly" (e.g. threads re-add
>>>>>>>> themselves in the order they got to run after being woken up).
>>>>>>>> Additionally, new requests entering wbt_wait might overtake requests
>>>>>>>> that were queued earlier, because the wait queue will be
>>>>>>>> (temporarily) empty after the wake_up_all, so the waitqueue_active
>>>>>>>> check will not stop them. This can cause certain threads to starve
>>>>>>>> under high load.
>>>>>>>>
>>>>>>>> The fix is to leave the woken up requests in the queue and remove
>>>>>>>> them in finish_wait() once the current thread breaks out of the
>>>>>>>> wait loop in __wbt_wait. This will ensure new requests always
>>>>>>>> end up at the back of the queue, and they won't overtake requests
>>>>>>>> that are already in the wait queue. With that change, the loop
>>>>>>>> in wbt_wait is also in line with many other wait loops in the kernel.
>>>>>>>> Waking up just one thread drastically reduces lock contention, as
>>>>>>>> does moving the wait queue add/remove out of the loop.
>>>>>>>>
>>>>>>>> A significant drop in lockdep's lock contention numbers is seen when
>>>>>>>> running the test application on the patched kernel.
>>>>>>>
>>>>>>> I like the patch, and a few weeks ago we independently discovered that
>>>>>>> the waitqueue list checking was bogus as well. My only worry is that
>>>>>>> changes like this can be delicate, meaning that it's easy to introduce
>>>>>>> stall conditions. What kind of testing did you push this through?
>>>>>>>
>>>>>>> -- 
>>>>>>> Jens Axboe
>>>>>>>
>>>>>> I ran the following tests on both real HW with NVME devices attached
>>>>>> and emulated NVME too:
>>>>>>
>>>>>> 1. The test case I used to reproduce the issue, spawns a bunch of threads 
>>>>>>    to concurrently read and write files with random size and content. 
>>>>>>    Files are randomly fsync'd. The implementation is a FIFO queue of files. 
>>>>>>    When the queue fills the test starts to verify and remove the files. This 
>>>>>>    test will fail if there's a read, write, or hash check failure. It tests
>>>>>>    for file corruption when lots of small files are being read and written 
>>>>>>    with high concurrency.
>>>>>>
>>>>>> 2. Fio for random writes with a root NVME device of 200GB
>>>>>>   
>>>>>>   fio --name=randwrite --ioengine=libaio --iodepth=1 --rw=randwrite --bs=4k 
>>>>>>   --direct=0 --size=10G --numjobs=2 --runtime=60 --group_reporting
>>>>>>   
>>>>>>   fio --name=randwrite --ioengine=libaio --iodepth=1 --rw=randwrite --bs=4k
>>>>>>   --direct=0 --size=5G --numjobs=2 --runtime=30 --fsync=64 --group_reporting
>>>>>>   
>>>>>>   I did see an improvement in the bandwidth numbers reported on the patched
>>>>>>   kernel. 
>>>>>>
>>>>>> Do you have any test case/suite in mind that you would suggest me to 
>>>>>> run to be sure that patch does not introduce any stall conditions?
>>>>>
>>>>> One thing that is always useful is to run xfstest, do a full run on
>>>>> the device. If that works, then do another full run, this time limiting
>>>>> the queue depth of the SCSI device to 1. If both of those pass, then
>>>>> I'd feel pretty good getting this applied for 4.19.
>>>>
>>>> Did you get a chance to run this full test?
>>>>
>>>> -- 
>>>> Jens Axboe
>>>>
>>>>
>>>
>>> Hi Jens,
>>> Yes I did run the tests and was in the process of compiling concrete results
>>> I tested following environments against xfs/auto group
>>> 1. Vanilla 4.18.rc kernel
>>> 2. 4.18 kernel with the blk-wbt patch
>>> 3. 4.18 kernel with the blk-wbt patch + io_queue_depth=2. I 
>>> understand you asked for queue depth for SCSI device=1 however, I have NVME 
>>> devices in my environment and 2 is the minimum value for io_queue_depth allowed 
>>> according to the NVME driver code. The results pretty much look same with no 
>>> stalls or exceptional failures. 
>>> xfs/auto ran 296 odd tests with 3 failures and 130 something "no runs". 
>>> Remaining tests passed. "Skipped tests"  were mostly due to missing features
>>> (eg: reflink support on scratch filesystem)
>>> The failures were consistent across runs on 3 different environments. 
>>> I am also running full test suite but it is taking long time as I am 
>>> hitting kernel BUG in xfs code in some generic tests. This BUG is not 
>>> related to the patch and  I see them in vanilla kernel too. I am in 
>>> the process of excluding these kind of tests as they come and 
>>> re-run the suite however, this proces is time taking. 
>>> Do you have any specific tests in mind that you would like me 
>>> to run apart from what I have already tested above?
>>
>> Thanks, I think that looks good. I'll get your patch applied for
>> 4.19.
>>
>> -- 
>> Jens Axboe
>>
>>
> 
> Hi Jens,
> Thanks for accepting this. There is one small issue, I don't find any emails
> send by me on the lkml mailing list. I am not sure why it didn't land there,
> all I can see is your responses. Do you want one of us to resend the patch
> or will you be able to do it?

That's odd, are you getting rejections on your emails? For reference, the
patch is here:

http://git.kernel.dk/cgit/linux-block/commit/?h=for-4.19/block&id=2887e41b910bb14fd847cf01ab7a5993db989d88

-- 
Jens Axboe

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