Hi.
I'd like to resurrect previous discussion [1] regarding slow kernel boot
inside QEMU with virtio-scsi disks attached and blk_mq enabled.
Symptom:
[ 2.830857] ata1: SATA max UDMA/133 abar m4096@0x98002000 port
0x98002100 irq 36
[ 2.834559] ata2: SATA max UDMA/133 abar m4096@0x98002000 port
0x98002180 irq 36
[ 2.837746] ata3: SATA max UDMA/133 abar m4096@0x98002000 port
0x98002200 irq 36
[ 2.841861] ata4: SATA max UDMA/133 abar m4096@0x98002000 port
0x98002280 irq 36
[ 2.847899] ata5: SATA max UDMA/133 abar m4096@0x98002000 port
0x98002300 irq 36
[ 2.853229] ata6: SATA max UDMA/133 abar m4096@0x98002000 port
0x98002380 irq 36
[ 3.172159] ata1: SATA link down (SStatus 0 SControl 300)
[ 3.183552] ata5: SATA link down (SStatus 0 SControl 300)
[ 3.189925] ata3: SATA link down (SStatus 0 SControl 300)
[ 3.196156] ata6: SATA link down (SStatus 0 SControl 300)
[ 3.201136] ata2: SATA link down (SStatus 0 SControl 300)
[ 3.208559] ata4: SATA link down (SStatus 0 SControl 300)
[ 16.480972] sd 0:0:1:0: Power-on or device reset occurred
[ 16.481591] sd 0:0:0:0: [sda] 16777216 512-byte logical blocks: (8.59
GB/8.00 GiB)
[ 16.481671] sd 0:0:0:0: [sda] Write Protect is off
[ 16.481815] sd 0:0:0:0: [sda] Write cache: disabled, read cache:
enabled, doesn't support DPO or FUA
[ 16.491325] sda: sda1 sda2
[ 16.517532] sd 0:0:1:0: [sdb] 16777216 512-byte logical blocks: (8.59
GB/8.00 GiB)
[ 16.525131] sr 0:0:2:0: Power-on or device reset occurred
[ 16.525974] sd 0:0:1:0: [sdb] Write Protect is off
[ 16.530946] sr 0:0:2:0: [sr0] scsi3-mmc drive: 16x/50x cd/rw xa/form2
cdda tray
[ 16.543592] cdrom: Uniform CD-ROM driver Revision: 3.20
[ 16.549815] sd 0:0:1:0: [sdb] Write cache: disabled, read cache:
enabled, doesn't support DPO or FUA
[ 16.549833] sd 0:0:0:0: [sda] Attached SCSI disk
[ 16.572055] sdb: sdb1 sdb2
[ 16.580463] sd 0:0:1:0: [sdb] Attached SCSI disk
(note the hang that lasts for 13 seconds)
The disks are attached to the VM in the following manner:
-device virtio-scsi,id=scsi -device scsi-hd,drive=hd1 -drive
if=none,media=disk,id=hd1,file=sda.img,format=raw
What I've tested so far:
* 4.14.62 + virtio-scsi + blk_mq == slow boot
* 4.14.62 + virtio-scsi + no blk_mq == fast boot
* 4.17.13 + virtio-scsi + blk_mq == slow boot
* 4.18-rc8 + virtio-scsi + blk_mq == slow boot
QEMU is of v2.12.1, runs with "-machine q35,accel=kvm -cpu host". Also,
if virtio-scsi disks are replaced with SATA disks, the hang does not
occur (although, QEMU has other issues with SATA, but that's another
story [3]).
Apparently, the commit that was mentioned in [2],
b5b6e8c8d3b4cbeb447a0f10c7d5de3caa573299, forces blk_mq for virtio_scsi,
so it cannot be disabled for new kernels.
Any hint on how to avoid this hang while still having virtio-scsi disks
and blk_mq enabled please?
Thanks.
--
Oleksandr Natalenko (post-factum)
[1]
https://lists.gnu.org/archive/html/qemu-discuss/2018-07/msg00022.html
[2]
https://lists.gnu.org/archive/html/qemu-discuss/2018-07/msg00037.html
[3]
https://lists.nongnu.org/archive/html/qemu-devel/2018-05/msg06942.html
