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Date:   Tue, 21 Aug 2018 08:42:43 +1000
From:   Balbir Singh <sblbir@amazon.com>
To:     Jens Axboe <axboe@kernel.dk>
CC:     "van der Linden, Frank" <fllinden@amazon.com>,
        "Agarwal, Anchal" <anchalag@amazon.com>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Wilson, Matt" <msw@amazon.com>
Subject: Re: [PATCH] blk-wbt: Avoid lock contention and thundering herd issue
 in wbt_wait
Message-ID: <20180820224241.GA72523@8c8590bceeee.ant.amazon.com>
References: <20180807201247.GA21108@kaos-source-ops-60001.pdx1.amazon.com>
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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?

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.

FYI: We ran lock contention and the waitqueue showed up as having the
largest contention, which disappeared after this patch.

Balbir Singh.