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| From: | Daniel Henrique Barboza |
| Subject: | Re: [Qemu-ppc] [RFC drcVI PATCH] spapr: reset DRCs on migration pre_load |
| Date: | Mon, 10 Jul 2017 17:37:31 -0300 |
| User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.1.0 |
On 07/10/2017 03:39 AM, David Gibson wrote:
On Fri, Jul 07, 2017 at 06:20:37PM -0300, Daniel Henrique Barboza wrote:"spapr: Remove 'awaiting_allocation' DRC flag" removed the flag that was originally was being used to prevent a race condition between hot unplug and hotplug. The DRC code base got simplified and more robust over time, eliminating the conditions that led to this race. Thus the awaiting_allocation existence wasn't justifiable anymore. A side effect of the flag removal was seen when testing the Libvirt hotplug-migration-unplug scenario, where a device is hotplugged in both source and target using device_add prior to the migration, then the device is removed after migration in the target. Before that cleanup, the hot unplug at the target fails in both QEMU and guest kernel because the DRC state at the target is inconsistent. After removing that flag, the hot unplug works at QEMU but the guest kernel hungs on the middle of the unplug process. It turns out that the awaiting_allocation logic was preventing the hot unplug from happening at the target because the DRC state, at this specific hot unplug scenario, was matching the race condition the flag was originally designed to avoid. Removing the flag allowed the device to be removed from QEMU, leading to this new behavior. The root cause of those problems is, in fact, the inconsistent state of the target DRCs after migration is completed. Doing device_add in the INMIGRATE status leaves the DRC in a state that isn't recognized as a valid hotplugged device in the guest OS. This patch fixes the problem by using the recently modified 'drc_reset' function, that now forces the DRC to a known state by checking its device status, to reset all DRCs in the pre_load hook of the migration. Resetting the DRCs in pre_load allows the DRCs to be in a predictable state when we load the migration at the target, allowing for hot unplugs to work as expected. Signed-off-by: Daniel Henrique Barboza <address@hidden>Ok, so the fact this works is pretty promising. However, I'm still trying to fully understand what's going on here. I have a suspicion that this is only necessary because something isn't quite right with the reset / inmigrate sequencing in the generic code, which we should fix instead of hacking around.
Agreed.
IIUC, in the problem case, on the source the hotplug has fully
completed, so the DRC will be in CONFIGURED state. Since the device
is CONFIGURED and attached, no DRC info is sent in the migration
stream. On the destination what seems to be happening is:
1. qemu is started with "-incoming defer", and cpu *not* present
DRC is uninitialized
2. qemu_system_reset() is called in vl.c
DRC is in UNALLOCATED / detached state
3. libvirt device_adds the cpu
DRC is in UNALLOCATED / attached state
4. libvirt initiates incoming migration
DRC remains in UNALLOCATED / attached state
5. Guest resumes on the destination
DRC still in UNALLOCATED / attached state
Which mismatches what we had on the source so => bug.
BUT, AFAIK the libvirt coldplug case below *is* working. Which
tracing through the code I'd expect:
1. qemu is started with -S and cpu not present
DRC is uninitialized
2. qemu_system_reset() is called in vl.c
DRC is in UNALLOCATED / detached state
3. libvirt device_adds in prelaunch phase
DRC is in UNALLOCATED / attached state
4. Guest is started
DRC is in UNALLOCATED / attached state
Which is also incorrect: the device was present when the guest
started, so it should be in CONFIGURED state. IIUC this case is
working, so I think it is must actually be in CONFIGURED state.
Just did a test here and the device isn't present when the guest starts in the second example you mentioned, Tested with current qemu master. QEMU shows the extra
CPU as 'halted' always, even after the guest starts and OS boots up:address@hidden:~/qemu/build/ppc64-softmmu$ sudo ./qemu-system-ppc64 -name migrate_qemu -boot strict=on --enable-kvm -device nec-usb-xhci,id=usb,bus=pci.0,addr=0xf -device spapr-vscsi,id=scsi0,reg=0x2000 -smp 1,maxcpus=4,sockets=4,cores=1,threads=1 --machine pseries,accel=kvm,usb=off,dump-guest-core=off -m 4G,slots=32,maxmem=32G -drive file=/home/danielhb/vm_imgs/ubuntu1704.qcow2,format=qcow2,if=none,id=drive-virtio-disk0,cache=none -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x2,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -nographic -S
QEMU 2.9.50 monitor - type 'help' for more information <<<<< at this point qemu_system_reset is called, as expected >>>>> (qemu) device_add host-spapr-cpu-core,id=core1,core-id=1 (qemu) info cpus * CPU #0: nip=0x0000000000000100 thread_id=16523 CPU #1: nip=0x0000000000000000 (halted) thread_id=16598 (qemu) cont --- guest boots up ---- (qemu) info cpus * CPU #0: nip=0xc0000000000a3e0c thread_id=16523 CPU #1: nip=0x0000000000000000 (halted) thread_id=16598 address@hidden:~$ lscpu Architecture: ppc64le Byte Order: Little Endian CPU(s): 1 On-line CPU(s) list: 0 Thread(s) per core: 1 Core(s) per socket: 1 Socket(s): 1 NUMA node(s): 1 Model: 2.1 (pvr 004b 0201) Model name: POWER8E (raw), altivec supported Hypervisor vendor: horizontal Virtualization type: full L1d cache: 64K L1i cache: 32K NUMA node0 CPU(s): 0 address@hidden:~$ (qemu) (qemu) device_del core1 (qemu) info cpus * CPU #0: nip=0xc0000000000a3e0c thread_id=16523 CPU #1: nip=0x0000000000000000 (halted) thread_id=16598 address@hidden:~$ lscpu Architecture: ppc64le Byte Order: Little Endian CPU(s): 1 On-line CPU(s) list: 0 Thread(s) per core: 1 Core(s) per socket: 1 Socket(s): 1 NUMA node(s): 1 Model: 2.1 (pvr 004b 0201) Model name: POWER8E (raw), altivec supported Hypervisor vendor: horizontal Virtualization type: full L1d cache: 64K L1i cache: 32K NUMA node0 CPU(s): 0 address@hidden:~$ dmesg | tail -n 5[ 6.307988] audit: type=1400 audit(1499705034.060:10): apparmor="STATUS" operation="profile_load" profile="unconfined" name="/usr/bin/lxc-start" pid=2212 comm="apparmor_parser" [ 6.318556] audit: type=1400 audit(1499705034.068:11): apparmor="STATUS" operation="profile_load" profile="unconfined" name="/usr/lib/snapd/snap-confine" pid=2213 comm="apparmor_parser" [ 7.087170] cgroup: new mount options do not match the existing superblock, will be ignored [ 88.093598] pseries-hotplug-cpu: Failed to acquire DRC, rc: -22, drc index: 10000008 [ 88.093606] pseries-hotplug-cpu: Cannot find CPU (drc index 10000008) to remove
address@hidden:~$Debugging it a little I see that device_adding a CPU while the VM isn't started yet is being considered "hotplugged" by spapr_core_plug (dev->hotplugged is True). Also, there is a note in 'spapr_cpu_reset'
saying:
/* All CPUs start halted. CPU0 is unhalted from the machine level
* reset code and the rest are explicitly started up by the guest
* using an RTAS call */
cs->halted = 1;
And yeah, the guest isn't calling 'start-cpu' and the CPU remains
halted. When comparing to
a scenario where I start the VM with 2 cpus in the command line, the
first one is started by the
machine reset and the other one by the RTAS call 'start-cpu', as
expected I'll investigate why this
is happening - starting with 2 coldplugged CPUs versus one coldplugged
CPU and a second one
attached with device_add with while on -S should yield the same outcome.All this said, I am not sure if this behavior has the same root cause as the migration problem this patch solves with the reset on pre_load though. Hopefully I'll know more in these next days.
So, I'm trying to understand why we get different results in these two cases. [Aside: If we do need to manually trigger a reset, code like the below is basically what we need. Just putting a reset in each DRC's pre_load looks neater, but won't work - the pre_load won't be called unless the object actually appears in the migration stream, and IIUC the case which isn't working is exactly the case where the object is omitted from the migration stream.]
Yeah, I have thought about it before sending this RFC patch and I couldn't find anywhere else to hook this reset code, given that the pre_load hook requires the object to be migrated. And, as you said, if the DRC is being migrated the problem doesn't
happen anyway. Daniel
--- hw/ppc/spapr.c | 7 +++++++ hw/ppc/spapr_drc.c | 17 +++++++++++++++++ include/hw/ppc/spapr_drc.h | 1 + 3 files changed, 25 insertions(+) diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c index 089d41d..aea85b0 100644 --- a/hw/ppc/spapr.c +++ b/hw/ppc/spapr.c @@ -1473,6 +1473,12 @@ static bool spapr_vga_init(PCIBus *pci_bus, Error **errp) } }+static int spapr_pre_load(void *opaque)+{ + spapr_reset_all_drcs(); + return 0; +} + static int spapr_post_load(void *opaque, int version_id) { sPAPRMachineState *spapr = (sPAPRMachineState *)opaque; @@ -1598,6 +1604,7 @@ static const VMStateDescription vmstate_spapr = { .name = "spapr", .version_id = 3, .minimum_version_id = 1, + .pre_load = spapr_pre_load, .post_load = spapr_post_load, .fields = (VMStateField[]) { /* used to be @next_irq */ diff --git a/hw/ppc/spapr_drc.c b/hw/ppc/spapr_drc.c index 63637d8..74f3957 100644 --- a/hw/ppc/spapr_drc.c +++ b/hw/ppc/spapr_drc.c @@ -449,6 +449,23 @@ static void drc_reset(void *opaque) drc->ccs_depth = -1; }+void spapr_reset_all_drcs(void)+{ + Object *drc_container, *obj; + ObjectProperty *prop; + ObjectPropertyIterator iter; + + drc_container = container_get(object_get_root(), DRC_CONTAINER_PATH); + object_property_iter_init(&iter, drc_container); + while ((prop = object_property_iter_next(&iter))) { + if (!strstart(prop->type, "link<", NULL)) { + continue; + } + obj = object_property_get_link(drc_container, prop->name, NULL); + drc_reset(SPAPR_DR_CONNECTOR(obj)); + } +} + static bool spapr_drc_needed(void *opaque) { sPAPRDRConnector *drc = (sPAPRDRConnector *)opaque; diff --git a/include/hw/ppc/spapr_drc.h b/include/hw/ppc/spapr_drc.h index 4c54864..c7553e6 100644 --- a/include/hw/ppc/spapr_drc.h +++ b/include/hw/ppc/spapr_drc.h @@ -250,4 +250,5 @@ static inline bool spapr_drc_unplug_requested(sPAPRDRConnector *drc) return drc->unplug_requested; }+void spapr_reset_all_drcs(void);#endif /* HW_SPAPR_DRC_H */
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