On Fri, Nov 03, 2023 at 01:56 PM +0300, Dmitrii Gavrilov <ds-g...@yandex-team.ru> wrote:
Original goal of addition of drain_call_rcu to qmp_device_add was to cover
the failure case of qdev_device_add. It seems call of drain_call_rcu was
misplaced in 7bed89958bfbf40df what led to waiting for pending RCU callbacks
under happy path too. What led to overall performance degradation of
qmp_device_add.
In this patch call of drain_call_rcu moved under handling of failure of
qdev_device_add.
Signed-off-by: Dmitrii Gavrilov <ds-g...@yandex-team.ru>
I don't know the exact reason, but this commit caused udev events to
show up much slower than before (~3s vs. ~23s) when a virtio-scsi device
is hotplugged (I’ve tested this only on s390x). Importantly, this only
happens when asynchronous SCSI scanning is disabled in the *guest*
kernel (scsi_mod.scan=sync or CONFIG_SCSI_SCAN_ASYNC=n).
The `udevadm monitor` output captured while hotplugging the device
(using QEMU 012b170173bc ("system/qdev-monitor: move drain_call_rcu call
under if (!dev) in qmp_device_add()")):
…
KERNEL[2.166575] add /devices/css0/0.0.0002/0.0.0002 (ccw)
KERNEL[2.166594] bind /devices/css0/0.0.0002/0.0.0002 (ccw)
KERNEL[2.166826] add /devices/css0/0.0.0002/0.0.0002/virtio2 (virtio)
UDEV [2.166846] add /devices/css0/0.0.0002/0.0.0002 (ccw)
UDEV [2.167013] bind /devices/css0/0.0.0002/0.0.0002 (ccw)
KERNEL[2.167560] add /devices/virtual/workqueue/scsi_tmf_0 (workqueue)
UDEV [2.167977] add /devices/virtual/workqueue/scsi_tmf_0 (workqueue)
KERNEL[2.167987] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0 (scsi)
KERNEL[2.167996] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/scsi_host/host0 (scsi_host)
KERNEL[2.169113] change /0:0:0:0 (scsi)
UDEV [2.169212] change /0:0:0:0 (scsi)
KERNEL[2.199500] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0 (scsi)
KERNEL[2.199513] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0 (scsi)
KERNEL[2.199523] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/scsi_device/0:0:0:0 (scsi_device)
KERNEL[2.199532] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/scsi_disk/0:0:0:0 (scsi_disk)
KERNEL[2.199564] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/scsi_generic/sg0 (scsi_generic)
KERNEL[2.199586] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/bsg/0:0:0:0 (bsg)
KERNEL[2.280482] add /devices/virtual/bdi/8:0 (bdi)
UDEV [2.280634] add /devices/virtual/bdi/8:0 (bdi)
KERNEL[3.060145] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/block/sda (block)
KERNEL[3.060160] bind /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0 (scsi)
KERNEL[22.160147] bind /devices/css0/0.0.0002/0.0.0002/virtio2 (virtio)
KERNEL[22.160161] add /bus/virtio/drivers/virtio_scsi (drivers)
KERNEL[22.160169] add /module/virtio_scsi (module)
UDEV [22.161078] add /devices/css0/0.0.0002/0.0.0002/virtio2 (virtio)
UDEV [22.161339] add /bus/virtio/drivers/virtio_scsi (drivers)
UDEV [22.161860] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0 (scsi)
UDEV [22.161869] add /module/virtio_scsi (module)
UDEV [22.161880] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0 (scsi)
UDEV [22.161890] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/scsi_host/host0 (scsi_host)
UDEV [22.161901] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0 (scsi)
UDEV [22.161911] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/scsi_disk/0:0:0:0 (scsi_disk)
UDEV [22.161924] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/bsg/0:0:0:0 (bsg)
UDEV [22.161937] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/scsi_generic/sg0 (scsi_generic)
UDEV [22.162123] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/scsi_device/0:0:0:0 (scsi_device)
UDEV [22.468924] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/block/sda (block)
UDEV [22.473955] bind /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0 (scsi)
UDEV [22.473970] bind /devices/css0/0.0.0002/0.0.0002/virtio2 (virtio)
The `udevadm monitor` output without this commit (QEMU 9876359990dd ("hw/scsi/lsi53c895a: add timer to scripts processing")):
…
KERNEL[2.091114] add /devices/virtual/workqueue/scsi_tmf_0 (workqueue)
UDEV [2.091218] add /devices/virtual/workqueue/scsi_tmf_0 (workqueue)
KERNEL[2.091408] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0 (scsi)
KERNEL[2.091418] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/scsi_host/host0 (scsi_host)
KERNEL[2.200461] bind /devices/css0/0.0.0002/0.0.0002/virtio2 (virtio)
KERNEL[2.200473] add /bus/virtio/drivers/virtio_scsi (drivers)
KERNEL[2.200481] add /module/virtio_scsi (module)
UDEV [2.200634] add /module/virtio_scsi (module)
UDEV [2.200678] add /devices/css0/0.0.0002/0.0.0002/virtio2 (virtio)
UDEV [2.200746] add /bus/virtio/drivers/virtio_scsi (drivers)
UDEV [2.200830] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0 (scsi)
UDEV [2.200972] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/scsi_host/host0 (scsi_host)
UDEV [2.201148] bind /devices/css0/0.0.0002/0.0.0002/virtio2 (virtio)
KERNEL[2.201699] change /0:0:0:0 (scsi)
KERNEL[2.201734] change /0:0:0:0 (scsi)
UDEV [2.201815] change /0:0:0:0 (scsi)
UDEV [2.201888] change /0:0:0:0 (scsi)
KERNEL[2.222062] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0 (scsi)
KERNEL[2.222074] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0 (scsi)
KERNEL[2.222083] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/scsi_device/0:0:0:0 (scsi_device)
KERNEL[2.222092] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/scsi_disk/0:0:0:0 (scsi_disk)
KERNEL[2.222104] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/scsi_generic/sg0 (scsi_generic)
KERNEL[2.222127] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/bsg/0:0:0:0 (bsg)
UDEV [2.222241] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0 (scsi)
UDEV [2.222486] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0 (scsi)
UDEV [2.222667] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/scsi_disk/0:0:0:0 (scsi_disk)
UDEV [2.222715] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/bsg/0:0:0:0 (bsg)
UDEV [2.222877] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/scsi_device/0:0:0:0 (scsi_device)
UDEV [2.223116] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/scsi_generic/sg0 (scsi_generic)
KERNEL[2.303063] add /devices/virtual/bdi/8:0 (bdi)
UDEV [2.303197] add /devices/virtual/bdi/8:0 (bdi)
KERNEL[2.394175] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/block/sda (block)
KERNEL[2.394186] bind /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0 (scsi)
UDEV [2.706054] add /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0/block/sda (block)
UDEV [2.706075] bind /devices/css0/0.0.0002/0.0.0002/virtio2/host0/target0:0:0/0:0:0:0 (scsi)
I’ve used as host kernel 6.7.0-rc3-00033-ge72f947b4f0d and guest kernel
v6.5.0.
QEMU 'info qtree' output when the device was hotplugged:
bus: main-system-bus
type System
dev: s390-pcihost, id ""
x-config-reg-migration-enabled = true
bypass-iommu = false
bus: s390-pcibus.0
type s390-pcibus
bus: pci.0
type PCI
dev: virtual-css-bridge, id ""
css_dev_path = true
bus: virtual-css
type virtual-css-bus
dev: virtio-scsi-ccw, id "scsi0"
ioeventfd = true
max_revision = 2 (0x2)
devno = "fe.0.0002"
dev_id = "fe.0.0002"
subch_id = "fe.0.0002"
bus: virtio-bus
type virtio-ccw-bus
dev: virtio-scsi-device, id ""
num_queues = 1 (0x1)
virtqueue_size = 256 (0x100)
seg_max_adjust = true
max_sectors = 65535 (0xffff)
cmd_per_lun = 128 (0x80)
hotplug = true
param_change = true
indirect_desc = true
event_idx = true
notify_on_empty = true
any_layout = true
iommu_platform = false
packed = false
queue_reset = true
use-started = true
use-disabled-flag = true
x-disable-legacy-check = false
bus: scsi0.0
type SCSI
dev: scsi-generic, id "hostdev0"
drive = "libvirt-1-backend"
share-rw = false
io_timeout = 30 (0x1e)
channel = 0 (0x0)
scsi-id = 0 (0x0)
lun = 0 (0x0)
…
Any ideas?
Thanks in advance.
Kind regards,
Marc
26.04.2024, 11:17, "Marc Hartmayer" <mhart...@linux.ibm.com>:
Hello!
Thank you for mentioning this.
At first glance it seems that using scsi in synchonous mode caues the global
QEMU mutex lock until the scanning phase is complete. Prior to 012b170173bc
("system/qdev-monitor: move drain_call_rcu call under if (!dev) in
qmp_device_add()") on each device adition the lock would be forcibly removed
allowing callbacks (including UDEV ones) to be processed after a new device
is added.
I`ll try to investigate this furter. But currently it appears to me like
performance or observability dilemma.
Is behaviour you mentioning consistant?
Best regards,
Dmitrii
- Re: [PATCH] system/qdev-monitor: move drain_call_rcu cal... Marc Hartmayer
- Re: [PATCH] system/qdev-monitor: move drain_call_rc... Dmitrii Gavrilov
- Re: [PATCH] system/qdev-monitor: move drain_cal... Marc Hartmayer
- Re: [PATCH] system/qdev-monitor: move drain_call_rc... Igor Mammedov
- Re: [PATCH] system/qdev-monitor: move drain_cal... boris . ostrovsky
