Re: [PATCH] blk-wbt: Avoid lock contention and thundering herd issue in wbt_wait
From: van der Linden, Frank <hidden>
Date: 2018-08-20 20:19:38
Also in:
lkml
On 8/20/18 12:29 PM, Jens Axboe wrote:=0A=
On 8/20/18 1:08 PM, Jens Axboe wrote:=0A=quoted
On 8/20/18 11:34 AM, van der Linden, Frank wrote:=0A=quoted
On 8/20/18 9:37 AM, Jens Axboe wrote:=0A=quoted
On 8/7/18 3:19 PM, Jens Axboe wrote:=0A=quoted
On 8/7/18 3:12 PM, Anchal Agarwal wrote:=0A=quoted
On Tue, Aug 07, 2018 at 02:39:48PM -0600, Jens Axboe wrote:=0A=quoted
On 8/7/18 2:12 PM, Anchal Agarwal wrote:=0A=quoted
On Tue, Aug 07, 2018 at 08:29:44AM -0600, Jens Axboe wrote:=0A=quoted
On 8/1/18 4:09 PM, Jens Axboe wrote:=0A=quoted
On 8/1/18 11:06 AM, Anchal Agarwal wrote:=0A=quoted
On Wed, Aug 01, 2018 at 09:14:50AM -0600, Jens Axboe wrote:=0A=quoted
On 7/31/18 3:34 PM, Anchal Agarwal wrote:=0A=quoted
Hi folks,=0A= =0A= This patch modifies commit e34cbd307477a=0A= (blk-wbt: add general throttling mechanism)=0A= =0A= I am currently running a large bare metal instance (i3.metal)=
=0A=
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on EC2 with 72 cores, 512GB of RAM and NVME drives, with a=0A= 4.18 kernel. I have a workload that simulates a database=0A= workload and I am running into lockup issues when writeback=
=0A=
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throttling is enabled,with the hung task detector also=0A= kicking in.=0A= =0A= Crash dumps show that most CPUs (up to 50 of them) are=0A= all trying to get the wbt wait queue lock while trying to add=
=0A=
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themselves to it in __wbt_wait (see stack traces below).=0A= =0A= [ 0.948118] CPU: 45 PID: 0 Comm: swapper/45 Not tainted 4.=
14.51-62.38.amzn1.x86_64 #1=0A=
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[ 0.948119] Hardware name: Amazon EC2 i3.metal/Not Specifi=
ed, BIOS 1.0 10/16/2017=0A=
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[ 0.948120] task: ffff883f7878c000 task.stack: ffffc9000c6=
9c000=0A=
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[ 0.948124] RIP: 0010:native_queued_spin_lock_slowpath+0xf=
8/0x1a0=0A=
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[ 0.948125] RSP: 0018:ffff883f7fcc3dc8 EFLAGS: 00000046=0A= [ 0.948126] RAX: 0000000000000000 RBX: ffff887f7709ca68 RC=
X: ffff883f7fce2a00=0A=
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[ 0.948128] RDX: 000000000000001c RSI: 0000000000740001 RD=
I: ffff887f7709ca68=0A=
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[ 0.948129] RBP: 0000000000000002 R08: 0000000000b80000 R0=
9: 0000000000000000=0A=
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[ 0.948130] R10: ffff883f7fcc3d78 R11: 000000000de27121 R1=
2: 0000000000000002=0A=
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[ 0.948131] R13: 0000000000000003 R14: 0000000000000000 R1=
5: 0000000000000000=0A=
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[ 0.948132] FS: 0000000000000000(0000) GS:ffff883f7fcc000=
0(0000) knlGS:0000000000000000=0A=
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[ 0.948134] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050=
033=0A=
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[ 0.948135] CR2: 000000c424c77000 CR3: 0000000002010005 CR=
4: 00000000003606e0=0A=
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[ 0.948136] DR0: 0000000000000000 DR1: 0000000000000000 DR=
2: 0000000000000000=0A=
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[ 0.948137] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR=
7: 0000000000000400=0A=
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[ 0.948138] Call Trace:=0A= [ 0.948139] <IRQ>=0A= [ 0.948142] do_raw_spin_lock+0xad/0xc0=0A= [ 0.948145] _raw_spin_lock_irqsave+0x44/0x4b=0A= [ 0.948149] ? __wake_up_common_lock+0x53/0x90=0A= [ 0.948150] __wake_up_common_lock+0x53/0x90=0A= [ 0.948155] wbt_done+0x7b/0xa0=0A= [ 0.948158] blk_mq_free_request+0xb7/0x110=0A= [ 0.948161] __blk_mq_complete_request+0xcb/0x140=0A= [ 0.948166] nvme_process_cq+0xce/0x1a0 [nvme]=0A= [ 0.948169] nvme_irq+0x23/0x50 [nvme]=0A= [ 0.948173] __handle_irq_event_percpu+0x46/0x300=0A= [ 0.948176] handle_irq_event_percpu+0x20/0x50=0A= [ 0.948179] handle_irq_event+0x34/0x60=0A= [ 0.948181] handle_edge_irq+0x77/0x190=0A= [ 0.948185] handle_irq+0xaf/0x120=0A= [ 0.948188] do_IRQ+0x53/0x110=0A= [ 0.948191] common_interrupt+0x87/0x87=0A= [ 0.948192] </IRQ>=0A= ....=0A= [ 0.311136] CPU: 4 PID: 9737 Comm: run_linux_amd64 Not tai=
nted 4.14.51-62.38.amzn1.x86_64 #1=0A=
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[ 0.311137] Hardware name: Amazon EC2 i3.metal/Not Specifi=
ed, BIOS 1.0 10/16/2017=0A=
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[ 0.311138] task: ffff883f6e6a8000 task.stack: ffffc9000f1=
ec000=0A=
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[ 0.311141] RIP: 0010:native_queued_spin_lock_slowpath+0xf=
5/0x1a0=0A=
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[ 0.311142] RSP: 0018:ffffc9000f1efa28 EFLAGS: 00000046=0A= [ 0.311144] RAX: 0000000000000000 RBX: ffff887f7709ca68 RC=
X: ffff883f7f722a00=0A=
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[ 0.311145] RDX: 0000000000000035 RSI: 0000000000d80001 RD=
I: ffff887f7709ca68=0A=
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[ 0.311146] RBP: 0000000000000202 R08: 0000000000140000 R0=
9: 0000000000000000=0A=
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[ 0.311147] R10: ffffc9000f1ef9d8 R11: 000000001a249fa0 R1=
2: ffff887f7709ca68=0A=
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[ 0.311148] R13: ffffc9000f1efad0 R14: 0000000000000000 R1=
5: ffff887f7709ca00=0A=
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[ 0.311149] FS: 000000c423f30090(0000) GS:ffff883f7f70000=
0(0000) knlGS:0000000000000000=0A=
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[ 0.311150] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050=
033=0A=
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[ 0.311151] CR2: 00007feefcea4000 CR3: 0000007f7016e001 CR=
4: 00000000003606e0=0A=
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[ 0.311152] DR0: 0000000000000000 DR1: 0000000000000000 DR=
2: 0000000000000000=0A=
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[ 0.311153] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR=
7: 0000000000000400=0A=
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[ 0.311154] Call Trace:=0A= [ 0.311157] do_raw_spin_lock+0xad/0xc0=0A= [ 0.311160] _raw_spin_lock_irqsave+0x44/0x4b=0A= [ 0.311162] ? prepare_to_wait_exclusive+0x28/0xb0=0A= [ 0.311164] prepare_to_wait_exclusive+0x28/0xb0=0A= [ 0.311167] wbt_wait+0x127/0x330=0A= [ 0.311169] ? finish_wait+0x80/0x80=0A= [ 0.311172] ? generic_make_request+0xda/0x3b0=0A= [ 0.311174] blk_mq_make_request+0xd6/0x7b0=0A= [ 0.311176] ? blk_queue_enter+0x24/0x260=0A= [ 0.311178] ? generic_make_request+0xda/0x3b0=0A= [ 0.311181] generic_make_request+0x10c/0x3b0=0A= [ 0.311183] ? submit_bio+0x5c/0x110=0A= [ 0.311185] submit_bio+0x5c/0x110=0A= [ 0.311197] ? __ext4_journal_stop+0x36/0xa0 [ext4]=0A= [ 0.311210] ext4_io_submit+0x48/0x60 [ext4]=0A= [ 0.311222] ext4_writepages+0x810/0x11f0 [ext4]=0A= [ 0.311229] ? do_writepages+0x3c/0xd0=0A= [ 0.311239] ? ext4_mark_inode_dirty+0x260/0x260 [ext4]=0A= [ 0.311240] do_writepages+0x3c/0xd0=0A= [ 0.311243] ? _raw_spin_unlock+0x24/0x30=0A= [ 0.311245] ? wbc_attach_and_unlock_inode+0x165/0x280=0A= [ 0.311248] ? __filemap_fdatawrite_range+0xa3/0xe0=0A= [ 0.311250] __filemap_fdatawrite_range+0xa3/0xe0=0A= [ 0.311253] file_write_and_wait_range+0x34/0x90=0A= [ 0.311264] ext4_sync_file+0x151/0x500 [ext4]=0A= [ 0.311267] do_fsync+0x38/0x60=0A= [ 0.311270] SyS_fsync+0xc/0x10=0A= [ 0.311272] do_syscall_64+0x6f/0x170=0A= [ 0.311274] entry_SYSCALL_64_after_hwframe+0x42/0xb7=0A= =0A= In the original patch, wbt_done is waking up all the exclusiv=
e=0A=
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processes in the wait queue, which can cause a thundering her=
d=0A=
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if there is a large number of writer threads in the queue. Th=
e=0A=
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original intention of the code seems to be to wake up one thr=
ead=0A=
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only however, it uses wake_up_all() in __wbt_done(), and then=
=0A=
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uses the following check in __wbt_wait to have only one threa=
d=0A=
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actually get out of the wait loop:=0A= =0A= if (waitqueue_active(&rqw->wait) &&=0A= rqw->wait.head.next !=3D &wait->entry)=0A= return false;=0A= =0A= The problem with this is that the wait entry in wbt_wait is=
=0A=
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define with DEFINE_WAIT, which uses the autoremove wakeup fun=
ction.=0A=
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That means that the above check is invalid - the wait entry w=
ill=0A=
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have been removed from the queue already by the time we hit t=
he=0A=
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check in the loop.=0A= =0A= Secondly, auto-removing the wait entries also means that the =
wait=0A=
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queue essentially gets reordered "randomly" (e.g. threads re-=
add=0A=
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themselves in the order they got to run after being woken up)=
.=0A=
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Additionally, new requests entering wbt_wait might overtake r=
equests=0A=
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that were queued earlier, because the wait queue will be=0A= (temporarily) empty after the wake_up_all, so the waitqueue_a=
ctive=0A=
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check will not stop them. This can cause certain threads to s=
tarve=0A=
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under high load.=0A= =0A= The fix is to leave the woken up requests in the queue and re=
move=0A=
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them in finish_wait() once the current thread breaks out of t=
he=0A=
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wait loop in __wbt_wait. This will ensure new requests always=
=0A=
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end up at the back of the queue, and they won't overtake requ=
ests=0A=
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that are already in the wait queue. With that change, the loo=
p=0A=
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in wbt_wait is also in line with many other wait loops in the=
kernel.=0A=
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Waking up just one thread drastically reduces lock contention=
, as=0A=
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does moving the wait queue add/remove out of the loop.=0A= =0A= A significant drop in lockdep's lock contention numbers is se=
en when=0A=
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running the test application on the patched kernel.=0A=I like the patch, and a few weeks ago we independently discove=
red that=0A=
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the waitqueue list checking was bogus as well. My only worry i=
s that=0A=
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changes like this can be delicate, meaning that it's easy to i=
ntroduce=0A=
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stall conditions. What kind of testing did you push this throu=
gh?=0A=
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=0A= -- =0A= Jens Axboe=0A= =0A=I ran the following tests on both real HW with NVME devices att=
ached=0A=
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and emulated NVME too:=0A= =0A= 1. The test case I used to reproduce the issue, spawns a bunch =
of threads =0A=
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to concurrently read and write files with random size and co=
ntent. =0A=
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Files are randomly fsync'd. The implementation is a FIFO que=
ue of files. =0A=
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When the queue fills the test starts to verify and remove th=
e files. This =0A=
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test will fail if there's a read, write, or hash check failu=
re. It tests=0A=
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for file corruption when lots of small files are being read =
and written =0A=
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with high concurrency.=0A= =0A= 2. Fio for random writes with a root NVME device of 200GB=0A= =0A= fio --name=3Drandwrite --ioengine=3Dlibaio --iodepth=3D1 --rw=
=3Drandwrite --bs=3D4k =0A=
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--direct=3D0 --size=3D10G --numjobs=3D2 --runtime=3D60 --grou=
p_reporting=0A=
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=0A= fio --name=3Drandwrite --ioengine=3Dlibaio --iodepth=3D1 --rw=
=3Drandwrite --bs=3D4k=0A=
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--direct=3D0 --size=3D5G --numjobs=3D2 --runtime=3D30 --fsync=
=3D64 --group_reporting=0A=
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=0A= I did see an improvement in the bandwidth numbers reported on=
the patched=0A=
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kernel. =0A= =0A= Do you have any test case/suite in mind that you would suggest =
me to =0A=
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run to be sure that patch does not introduce any stall conditio=
ns?=0A=
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One thing that is always useful is to run xfstest, do a full run=
on=0A=
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the device. If that works, then do another full run, this time l=
imiting=0A=
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the queue depth of the SCSI device to 1. If both of those pass, =
then=0A=
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I'd feel pretty good getting this applied for 4.19.=0A=Did you get a chance to run this full test?=0A= =0A= -- =0A= Jens Axboe=0A= =0A= =0A=Hi Jens,=0A= Yes I did run the tests and was in the process of compiling concre=
te results=0A=
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I tested following environments against xfs/auto group=0A= 1. Vanilla 4.18.rc kernel=0A= 2. 4.18 kernel with the blk-wbt patch=0A= 3. 4.18 kernel with the blk-wbt patch + io_queue_depth=3D2. I =0A= understand you asked for queue depth for SCSI device=3D1 however, =
I have NVME =0A=
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devices in my environment and 2 is the minimum value for io_queue_=
depth allowed =0A=
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according to the NVME driver code. The results pretty much look sa=
me with no =0A=
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stalls or exceptional failures. =0A= xfs/auto ran 296 odd tests with 3 failures and 130 something "no r=
uns". =0A=
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Remaining tests passed. "Skipped tests" were mostly due to missin=
g features=0A=
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(eg: reflink support on scratch filesystem)=0A= The failures were consistent across runs on 3 different environmen=
ts. =0A=
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I am also running full test suite but it is taking long time as I =
am =0A=
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hitting kernel BUG in xfs code in some generic tests. This BUG is =
not =0A=
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related to the patch and I see them in vanilla kernel too. I am i=
n =0A=
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the process of excluding these kind of tests as they come and =0A= re-run the suite however, this proces is time taking. =0A= Do you have any specific tests in mind that you would like me =0A= to run apart from what I have already tested above?=0A=Thanks, I think that looks good. I'll get your patch applied for=0A= 4.19.=0A= =0A= -- =0A= Jens Axboe=0A= =0A= =0A=Hi Jens,=0A= Thanks for accepting this. There is one small issue, I don't find an=
y emails=0A=
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send by me on the lkml mailing list. I am not sure why it didn't lan=
d there,=0A=
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all I can see is your responses. Do you want one of us to resend the=
patch=0A=
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or will you be able to do it?=0A=That's odd, are you getting rejections on your emails? For reference,=
the=0A=
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patch is here:=0A= =0A= http://git.kernel.dk/cgit/linux-block/commit/?h=3Dfor-4.19/block&id=
=3D2887e41b910bb14fd847cf01ab7a5993db989d88=0A=
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One issue with this, as far as I can tell. Right now we've switched to=
=0A=
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waking one task at the time, which is obviously more efficient. But if=
=0A=
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we do that with exclusive waits, then we have to ensure that this task=
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makes progress. If we wake up a task, and then fail to get a queueing=
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token, then we'll go back to sleep. We need to ensure that someone mak=
es=0A=
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forward progress at this point. There are two ways I can see that=0A= happening:=0A= =0A= 1) The task woken _always_ gets to queue an IO=0A= 2) If the task woken is NOT allowed to queue an IO, then it must selec=
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a new task to wake up. That new task is then subjected to rule 1 or=
2=0A=
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as well.=0A= =0A= For #1, it could be as simple as:=0A= =0A= if (slept || !rwb_enabled(rwb)) {=0A= atomic_inc(&rqw->inflight);=0A= break;=0A= }=0A= =0A= but this obviously won't always be fair. Might be good enough however,=
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instead of having to eg replace the generic wait queues with a priorit=
y=0A=
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list/queue.=0A= =0A= Note that this isn't an entirely new issue, it's just so much easier t=
o=0A=
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hit with the single wakeups.=0A= =0A=Hi Jens,=0A= =0A= What is the scenario that you see under which the woken up task does no=
t=0A=
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get to run?=0A=That scenario is pretty easy to hit - let's say the next in line task=0A= has a queue limit of 1, and we currently have 4 pending. Task gets=0A= woken, goes back to sleep. Which should be totally fine. At some point=
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we'll get below the limit, and allow the task to proceed. This will=0A= ensure forward progress.=0A= =0A=quoted
The theory behind leaving the task on the wait queue is that the=0A= waitqueue_active check in wbt_wait prevents new tasks from taking up a=
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slot in the queue (e.g. incrementing inflight). So, there should not be=
=0A=
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a way for inflight to be incremented between the time the wake_up is=0A= done and the task at the head of the wait queue runs. That's the idea=
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anyway :-) If we missed something, let us know.=0A=And that's a fine theory, I think it's a good improvement (and how it=0A= should have worked). I'm struggling to see where the issue is. Perhaps=
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it's related to the wq active check. With fewer wakeups, we're more=0A= likely to hit a race there.=0A= =0A= I'll poke at it...=0A=Trying something like this:=0A= =0A= http://git.kernel.dk/cgit/linux-block/log/?h=3Dfor-4.19/wbt=0A= =0A=
Ah, now I see what you mean.=0A= =0A= This is the case where a task goes to sleep, not because the inflight=0A= limit has been reached, but simply because it needs to go to the back of=0A= the wait queue.=0A= =0A= In that case, it should, for its first time inside the loop, not try to=0A= decrement inflight - since that means it could still race to overtake a=0A= task that got there earlier and is in the wait queue.=0A= =0A= So what you are doing is keeping track of whether it got in to the loop=0A= only because of queueing, and then you don't try to decrement inflight=0A= the first time around the loop.=0A= =0A= I think that should work to fix that corner case.=0A= =0A= Frank=0A= =0A=