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

From: Jens Axboe
Date: Wed Aug 22 2018 - 00:11:09 EST

On 8/21/18 10:01 PM, Anchal Agarwal wrote:
> On Tue, Aug 21, 2018 at 09:20:06PM -0600, Jens Axboe wrote:
>> On 8/20/18 8:58 PM, Jens Axboe wrote:
>>> On 8/20/18 4:42 PM, Balbir Singh wrote:
>>>> On Mon, Aug 20, 2018 at 02:20:59PM -0600, Jens Axboe wrote:
>>>>> On 8/20/18 2:19 PM, van der Linden, Frank wrote:
>>>>>> On 8/20/18 12:29 PM, Jens Axboe wrote:
>>>>>>> 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
>>>>>>>
>>>>>> Ah, now I see what you mean.
>>>>>>
>>>>>> This is the case where a task goes to sleep, not because the inflight
>>>>>> limit has been reached, but simply because it needs to go to the back of
>>>>>> the wait queue.
>>>>>>
>>>>>> In that case, it should, for its first time inside the loop, not try to
>>>>>> decrement inflight - since that means it could still race to overtake a
>>>>>> task that got there earlier and is in the wait queue.
>>>>>>
>>>>>> So what you are doing is keeping track of whether it got in to the loop
>>>>>> only because of queueing, and then you don't try to decrement inflight
>>>>>> the first time around the loop.
>>>>>>
>>>>>> I think that should work to fix that corner case.
>>>>>
>>>>> I hope so, got tests running now and we'll see...
>>>>>
>>>>> Outside of that, getting the matching memory barrier for the wq check
>>>>> could also fix a race on the completion side.
>>>>>
>>>>
>>>> I thought all the wait_* and set_current_* and atomic_* had implicit barriers.
>>>> Are you referring to the rwb->wb_* values we consume on the completion side?
>>>
>>> Not waitqueue_active(), which is the one I was referring to. The additional
>>> helper wq_has_sleeper() does.
>>>
>>>> I was initially concerned about not dequeuing the task, but noticed that
>>>> wake_up_common seems to handle that well. I looked for sources of missed wake
>>>> up as well, notifying the same task twice and missing wakeups, but could
>>>> not hit it.
>>>
>>> It's better not to dequeue, since we want the task to stay at the head.
>>> So I think all that makes sense, yet I can't find where it would be
>>> missing either. The missing barrier _could_ explain it, especially
>>> since the risk of hitting it should be higher now with single wakeups.
>>>
>>>> FYI: We ran lock contention and the waitqueue showed up as having the
>>>> largest contention, which disappeared after this patch.
>>>
>>> Yeah, it's a good change for sure, we don't want everybody to wakeup,
>>> and then hammer on the lock both on wq removal and then again for
>>> most of them going back to sleep.
>>
>> OK, I think I see it. The problem is that if a task gets woken up and
>> doesn't get to queue anything, it goes back to sleep. But the default
>> wake function has already removed it from the wait queue... So once that
>> happens, we're dead in the water. The problem isn't that we're now more
>> likely to hit the deadlock with the above change, it's that the above
>> change introduced this deadlock.
>>
>> I'm testing a fix.
>>
>> --
>> Jens Axboe
>
> Are you talking about default_wake_function? If so then the woken task
> will not be deleted from the waitqueue until after it gets scheduled
> however, the earlier function used in DEFINE_WAIT -
> autoremove_wake_function does delete the woken up task from the
> waitqueue. Am I missing anything?

The problem is actually in a backport of it, since it didn't do the
proper wait queue func change, hence it was still using
autoremove_wake_function. You are right that in mainline it looks fine.

If you have time, please look at the 3 patches I posted earlier today.
Those are for mainline, so should be OK :-)

--
Jens Axboe