Re: [PATCH] blk-wbt: Avoid lock contention and thundering herd issue in wbt_wait

From: Jens Axboe
Date: Mon Aug 20 2018 - 15:29:14 EST

On 8/20/18 1:08 PM, Jens Axboe wrote:
> On 8/20/18 11:34 AM, van der Linden, Frank wrote:
>> On 8/20/18 9:37 AM, Jens Axboe wrote:
>>> On 8/7/18 3:19 PM, Jens Axboe wrote:
>>>> 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
>>> One issue with this, as far as I can tell. Right now we've switched to
>>> waking one task at the time, which is obviously more efficient. But if
>>> we do that with exclusive waits, then we have to ensure that this task
>>> makes progress. If we wake up a task, and then fail to get a queueing
>>> token, then we'll go back to sleep. We need to ensure that someone makes
>>> forward progress at this point. There are two ways I can see that
>>> happening:
>>>
>>> 1) The task woken _always_ gets to queue an IO
>>> 2) If the task woken is NOT allowed to queue an IO, then it must select
>>> a new task to wake up. That new task is then subjected to rule 1 or 2
>>> as well.
>>>
>>> For #1, it could be as simple as:
>>>
>>> if (slept || !rwb_enabled(rwb)) {
>>> atomic_inc(&rqw->inflight);
>>> break;
>>> }
>>>
>>> but this obviously won't always be fair. Might be good enough however,
>>> instead of having to eg replace the generic wait queues with a priority
>>> list/queue.
>>>
>>> Note that this isn't an entirely new issue, it's just so much easier to
>>> hit with the single wakeups.
>>>
>> Hi Jens,
>>
>> What is the scenario that you see under which the woken up task does not
>> get to run?
>
> That scenario is pretty easy to hit - let's say the next in line task
> has a queue limit of 1, and we currently have 4 pending. Task gets
> woken, goes back to sleep. Which should be totally fine. At some point
> we'll get below the limit, and allow the task to proceed. This will
> ensure forward progress.
>
>> The theory behind leaving the task on the wait queue is that the
>> waitqueue_active check in wbt_wait prevents new tasks from taking up a
>> slot in the queue (e.g. incrementing inflight). So, there should not be
>> a way for inflight to be incremented between the time the wake_up is
>> done and the task at the head of the wait queue runs. That's the idea
>> anyway :-) If we missed something, let us know.
>
> And that's a fine theory, I think it's a good improvement (and how it
> should have worked). I'm struggling to see where the issue is. Perhaps
> it's related to the wq active check. With fewer wakeups, we're more
> likely to hit a race there.
>
> I'll poke at it...

Trying something like this:

http://git.kernel.dk/cgit/linux-block/log/?h=for-4.19/wbt

--
Jens Axboe