On 08/19/2016 07:28 AM, Gabriel Krisman Bertazi wrote:
Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxxxxxxx> writes:
We, IBM, have been experiencing eventual Oops when stressing IO at the
same time we add/remove processors. The Oops happens in the IRQ path,
when we try to complete a request that was apparently meant for another
queue.
In __nvme_process_cq, the driver will use the cqe.command_id and the
nvmeq->tags to find out, via blk_mq_tag_to_rq, the request that
initiated the IO. Eventually, it happens that the request returned by
that function is not initialized, and we crash inside
__blk_mq_complete_request, as shown below.
[ 2679.050701] Faulting instruction address: 0xc000000000549500
[ 2679.050711] Oops: Kernel access of bad area, sig: 11 [#1]
[ 2679.050716] SMP NR_CPUS=2048 NUMA pSeries
[ 2679.050727] Modules linked in: minix nls_iso8859_1 xfs libcrc32c
rpcsec_gss_krb5 auth_rpcgss
nfsv4 nfs lockd grace fscache binfmt_misc pseries_rng rtc_generic sunrpc
autofs4 btrfs xor
raid6_pq ibmvscsi ibmveth nvme
[ 2679.050771] CPU: 21 PID: 45045 Comm: ppc64_cpu Not tainted 4.4.0-22-generic #40-Ubuntu
[ 2679.050780] task: c000000497b07e60 ti: c0000004fff2c000 task.ti: c000000451bc0000
[ 2679.050786] NIP: c000000000549500 LR: d0000000046b5e84 CTR: c000000000549680
[ 2679.050803] REGS: c0000004fff2fa80 TRAP: 0300 Not tainted (4.4.0-22-generic)
[ 2679.050807] MSR: 8000000100009033 <SF,EE,ME,IR,DR,RI,LE> CR: 28022428 XER: 00000000
[ 2679.050824] CFAR: c000000000008468 DAR: 00000000000000b0 DSISR: 40000000 SOFTE: 0
GPR00: d0000000046b5e84 c0000004fff2fd00 c0000000015b4300 c0000002799a0a00
GPR04: 0000000000000000 0000000000000105 0000000000000004 0000000000000001
GPR08: c0000002799a0a50 0000000000000000 0000000000000000 d0000000046bdc68
GPR12: c000000000549680 c000000007b1c780 0000000000000008 0000000000000001
GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000010005b50
GPR20: 0000000010005dc8 c0000000015eaa60 0000000000000001 0000000000000002
GPR24: 00000000000001e3 0000000000000000 0000000000000000 ffffffffffffffff
GPR28: 0000000000000000 0000000000000000 c0000004f8cd5180 c0000002799a0a00
[ 2679.050908] NIP [c000000000549500] __blk_mq_complete_request+0x30/0x1b0
[ 2679.050924] LR [d0000000046b5e84] __nvme_process_cq+0xf4/0x2c0 [nvme]
[ 2679.050930] Call Trace:
[ 2679.050941] [c0000004fff2fd00] [c0000004fff2fd40] 0xc0000004fff2fd40 (unreliable)
[ 2679.050952] [c0000004fff2fd40] [d0000000046b5e84] __nvme_process_cq+0xf4/0x2c0 [nvme]
[ 2679.050961] [c0000004fff2fde0] [d0000000046b613c] nvme_irq+0x3c/0xb0 [nvme]
[ 2679.050972] [c0000004fff2fe10] [c000000000130660] handle_irq_event_percpu+0xa0/0x320
[ 2679.050981] [c0000004fff2fed0] [c000000000130948] handle_irq_event+0x68/0xc0
[ 2679.050989] [c0000004fff2ff00] [c000000000135c2c] handle_fasteoi_irq+0xec/0x2b0
[ 2679.050997] [c0000004fff2ff30] [c00000000012f844] generic_handle_irq+0x54/0x80
[ 2679.051007] [c0000004fff2ff60] [c000000000011320] __do_irq+0x80/0x1d0
[ 2679.051020] [c0000004fff2ff90] [c000000000024800] call_do_irq+0x14/0x24
[ 2679.051037] [c000000451bc3350] [c000000000011508] do_IRQ+0x98/0x140
[ 2679.051054] [c000000451bc33a0] [c000000000002594]
hardware_interrupt_common+0x114/0x180
I added some debugging which skipped the Oops and made me think that we
are fetching the wrong request because we are actually looking at the
wrong set of tags.
The log below exemplifies what I am saying. Instead of crashing in the
Oops, I made the execution skip the request completion and simply
consider that odd CQE as handled. The first line of the log does that.
nvme nvme0: Skip completion of I/O 309 on queue 35 due to missing q
nvme nvme0: I/O 309 QID 63 timeout, aborting
nvme nvme0: Abort status: 0x0
nvme nvme0: I/O 309 QID 63 timeout, reset controller
nvme nvme0: completing aborted command with status: fffffffc
nvme 0003:03:00.0: Using 64-bit DMA iommu bypass
blk_update_request: I/O error, dev nvme0n1, sector 2105537536
In the first line, we found the request 309 for queue 35, which would
have crashed the execution. My debug patch just ignores it.
Then, we can see that eventually, the same IO, 309, will timeout in
another QID, which was actually expecting for it. The Abort request
gets sent and completed, but we never receive the completion of the
aborted request, thus we timeout again and restart the device.
This only happens when we are changing the SMT settings very
frequently, which points back to the way we correlate the hctx->tags to
the nvmeq structure, but I failed to find the exact code spot were
things get messed up.
I've put a some effort on this code lately, but I definitely need some
assistance on this one. Since, as far as I understand, this issue is
much more likely to reproduce on boxes with a lot of hotpluggable
processors, I'm willing to test any patches you may find suitable in our
systems.
Hi,
[Adding linux-block, linux-scsi and Jens in CC for blk-mq input]
Made some progress on tracking down the issue. It's looking more of a
blk-mq bug than nvme to me now. Let me know if I made any wrong
assumption below:
I started to wonder whether blk-mq is requesting the IO in a wrong queue
or if the request is being moved to another queue during hotplug, as it
would happen in blk_mq_hctx_cpu_offline.
I verified that when the fail occurs, the request never was never
touched in blk_mq_hctx_cpu_offline. Also, and more interesting, I added
the following BUG_ON to nvme_queue_req:
BUG_ON(blk_mq_tag_to_rq(*nvmeq->tags, req->tag) != req);
From my experiments, I could never hit this during normal operation
(without SMT). But, as soon as I start triggering hotplug changes, I hit
it pretty fast. By changing to WARN_ON, I see that the Oops will happen
almost immediately after this request is submitted, once __process_cq
tries to complete it.
Also, when adding the patch below, we send a few -EIOs back to upper
layers, but it prevents the crash.
I also think i saw a similar hang when running on a virtio_scsi disk. I
couldn't reproduce it a second time, so we still lack confirmation, but
if confirmed, it indeed points away from nvme and back to the block
layer.
Any suggestions to further track this down?
Some good detective work so far! I agree, this looks like a blk-mq core
bug. Do you have a trace of a BUG() triggering in nvme_queue_rq(), when
req->tag != nvmeq->tags? I don't immediately see how this could happen,
the freezing should protect us from this, unless it's broken somehow.
I'll take a look at this.
--
Jens Axboe
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