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
