Messages in this thread Patch in this message | | | Subject | FIO: kjournald blocked for more than 120 seconds | From | Lin Ming <> | Date | Mon, 16 Jun 2008 10:04:35 +0800 |
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Hi, Jens
When runnig FIO benchmark, kjournald blocked for more than 120 seconds. Detailed root cause analysis and proposed solutions as below.
Any comment is appreciated.
Hardware Environment --------------------- 13 SEAGATE ST373307FC disks in a JBOD, connected by a Qlogic ISP2312 Fibe Channel HBA.
Bug description ---------------- fio vsync random read 4K in 13 disks, 4 processes per disk, fio global paramter as below, [global] direct=0 ioengine=vsync iodepth=256 iodepth_batch=32 size=2G bs=4k numjobs=2 loops=5 runtime=1200 group_reporting
Tested 4 IO schedulers, issue is only seen in CFQ.
INFO: task kjournald:20558 blocked for more than 120 seconds. "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. kjournald D ffff810010820978 6712 20558 2 ffff81022ddb1d10 0000000000000046 ffff81022e7baa10 ffffffff803ba6f2 ffff81022ecd0000 ffff8101e6dc9160 ffff81022ecd0348 000000008048b6cb 0000000000000086 ffff81022c4e8d30 0000000000000000 ffffffff80247537 Call Trace: [<ffffffff803ba6f2>] kobject_get+0x12/0x17 [<ffffffff80247537>] getnstimeofday+0x2f/0x83 [<ffffffff8029c1ac>] sync_buffer+0x0/0x3f [<ffffffff8066d195>] io_schedule+0x5d/0x9f [<ffffffff8029c1e7>] sync_buffer+0x3b/0x3f [<ffffffff8066d3f0>] __wait_on_bit+0x40/0x6f [<ffffffff8029c1ac>] sync_buffer+0x0/0x3f [<ffffffff8066d48b>] out_of_line_wait_on_bit+0x6c/0x78 [<ffffffff80243909>] wake_bit_function+0x0/0x23 [<ffffffff8029e3ad>] sync_dirty_buffer+0x98/0xcb [<ffffffff8030056b>] journal_commit_transaction+0x97d/0xcb6 [<ffffffff8023a676>] lock_timer_base+0x26/0x4b [<ffffffff8030300a>] kjournald+0xc1/0x1fb [<ffffffff802438db>] autoremove_wake_function+0x0/0x2e [<ffffffff80302f49>] kjournald+0x0/0x1fb [<ffffffff802437bb>] kthread+0x47/0x74 [<ffffffff8022de51>] schedule_tail+0x28/0x5d [<ffffffff8020cac8>] child_rip+0xa/0x12 [<ffffffff80243774>] kthread+0x0/0x74 [<ffffffff8020cabe>] child_rip+0x0/0x12
Other people reported the similar issue on LKML. http://lkml.org/lkml/2008/3/26/494
I did lots of kernel instrumentation and testing, and located the root cause.
Root cause analysis -------------------
kjournald is blocked at: journal_write_commit_record -> sync_dirty_buffer(bh) -> wait_on_buffer(bh)
Buffer is submitted but it never finished.
git bisect to below patch,
commit cc19747977824ece6aa1c56a29e974fef5ec2b32 Author: Jens Axboe <jens.axboe@oracle.com> Date: Fri Apr 20 20:45:39 2007 +0200
cfq-iosched: tighten queue request overlap condition For tagged devices, allow overlap of requests if the idle window isn't enabled on the current active queue. Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c index a8237be..e859b49 100644 --- a/block/cfq-iosched.c +++ b/block/cfq-iosched.c @@ -989,7 +989,8 @@ static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd) * flight or is idling for a new request, allow either of these * conditions to happen (or time out) before selecting a new queue. */ - if (cfqq->dispatched || timer_pending(&cfqd->idle_slice_timer)) { + if (timer_pending(&cfqd->idle_slice_timer) || + (cfqq->dispatched && cfq_cfqq_idle_window(cfqq))) { cfqq = NULL; goto keep_queue; }
The disks of my testing machine are tagged devices, so the CFQ idle window is disabled. In other words, the active queue of tagged devices(cfqd->hw_tag=1) never idle for a new request.
This causes active queue be expired immediately if it's empty, although it has not run out of time. CFQ will select next queue as active queue. In this testcase, there are thousands of FIO read requests in sync queues, only a few write requests by journal_write_commit_record in async queues.
In the other hand, all processes use the default io class and priority. They share the async queue for the same device, but have their own sync queue, so the sync queue number is 4 while asyn queue number is just 1 for the same device.
So sync queue has much more chances be selected as new active queue than async queue.
Sync queues do not idle and they are dispatched all the time. This leads to many unfinished requests in external queue, namely, cfqd->sync_flight > 0.
static int cfq_dispatch_requests (...) { .... while ((cfqq = cfq_select_queue(cfqd)) != NULL) { .... if (cfqd->sync_flight && !cfq_cfqq_sync(cfqq)) break; .... __cfq_dispatch_requests(cfqq) } .... } When cfq_select_queue selects the async queue which includes kjournald's write request, this selected async queue will never be dispatched since cfqd->sync_flight > 0, so kjournald is blocked. Proposed 3 solutions ------------------ 1. Do not check cfqd->sync_flight - if (cfqd->sync_flight && !cfq_cfqq_sync(cfqq)) - break; 2. If we do need to check cfqd->sync_flight, then for tagged devices, we should give a little more chances to async queue to be dispatched.
@@ -1102,7 +1102,7 @@ static int cfq_dispatch_requests(struct request_queue *q, int force) break; } - if (cfqd->sync_flight && !cfq_cfqq_sync(cfqq)) + if (cfqd->sync_flight && !cfq_cfqq_sync(cfqq) && ! cfqd->hw_tag) break; 3. Force write request issued by journal_write_commit_record as sync request. As a matter of fact, it looks like most write requests submitted by kjournald is async request. We need convert them to sync requests.
Cheers, Lin Ming
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