Hi Jens, hi Tejun, I've seen multi-second boot stalls in one of my KVM setups during the initial scsi scan: [ 0.949892] scsi host0: Virtio SCSI HBA [ 1.007864] scsi 0:0:0:0: Direct-Access QEMU QEMU HARDDISK 1.1. PQ: 0 ANSI: 5 [ 1.021299] scsi 0:0:1:0: Direct-Access QEMU QEMU HARDDISK 1.1. PQ: 0 ANSI: 5 [ 1.520356] tsc: Refined TSC clocksource calibration: 2491.910 MHz <stall> [ 16.186549] sd 0:0:0:0: Attached scsi generic sg0 type 0 [ 16.190478] sd 0:0:1:0: Attached scsi generic sg1 type 0 [ 16.194099] osd: LOADED open-osd 0.2.1 [ 16.203202] sd 0:0:0:0: [sda] 31457280 512-byte logical blocks: (16.1 GB/15.0 GiB) [ 16.208478] sd 0:0:0:0: [sda] Write Protect is off [ 16.211439] sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA [ 16.218771] sd 0:0:1:0: [sdb] 31457280 512-byte logical blocks: (16.1 GB/15.0 GiB) [ 16.223264] sd 0:0:1:0: [sdb] Write Protect is off [ 16.225682] sd 0:0:1:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA I've tracked this down to "blk-mq: use percpu_ref for mq usage count" in a rather painful way as that one introduced enough other regressions to mess up bisect. If I revert the following commits: dd840087086f3b93ac20f7472b4fca59aff7b79f cddd5d17642cc6881352732693c2ae6930e9ce65 add703fda981b9719d37f371498b9f129acbd997 which are the above mentioned commit and two fixes to it the problem goes away. My qemu command line is below: kvm \ -m 2048 \ -smp 1 \ -kernel arch/x86/boot/bzImage \ -append "root=/dev/vda console=tty0 console=ttyS0,115200n8 scsi_mod.use_blk_mq=Y" \ -nographic \ -drive if=virtio,file=/work/images/debian.qcow2,cache=none,serial="test1234" \ -drive if=none,id=test,file=/work/images/test.img,cache=none,aio=native \ -drive if=none,id=scratch,file=/work/images/scratch.img,cache=none,aio=native \ -device virtio-scsi-pci,id=scsi \ -device scsi-hd,drive=test \ -device scsi-hd,drive=scratch \ -drive file=/work/images/debian-7.3.0-amd64-netinst.iso,index=2,media=cdrom -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
