Re: [Qemu-devel] [PATCH] migration: optimize the downtime

[Paolo Bonzini]

On 24/07/2017 17:35, Dr. David Alan Gilbert wrote:
> * Jay Zhou (address@hidden) wrote:
>> Hi Dave,
>>
>> On 2017/7/21 17:49, Dr. David Alan Gilbert wrote:
>>> * Jay Zhou (address@hidden) wrote:
>>>> Qemu_savevm_state_cleanup() takes about 300ms in my ram migration tests
>>>> with a 8U24G vm(20G is really occupied), the main cost comes from
>>>> KVM_SET_USER_MEMORY_REGION ioctl when mem.memory_size = 0 in
>>>> kvm_set_user_memory_region(). In kmod, the main cost is
>>>> kvm_zap_obsolete_pages(), which traverses the active_mmu_pages list to
>>>> zap the unsync sptes.
>>>
>>> Hi Jay,
>>>    Is this actually increasing the real downtime when the guest isn't
>>> running, or is it just the reported time? I see that the s->downtime
>>> value is calculated right after where we currently call
>>> qemu_savevm_state_cleanup.
>>
>> It actually increased the real downtime, I used the "ping" command to
>> test. Reason is that the source side libvirt sends qmp to qemu to query
>> the status of migration, which needs the BQL. qemu_savevm_state_cleanup
>> is done with BQL, qemu can not handle the qmp if qemu_savevm_state_cleanup
>> has not finished. And the source side libvirt delays about 300ms to notify
>> the destination side libvirt to send the "cont" command to start the vm.
>>
>> I think the value of s->downtime is not accurate enough, maybe we could
>> move the calculation of end_time after qemu_savevm_state_cleanup has done.
> 
> I'm copying in Paolo, Radim and Andrea- is there anyway we can make the
> teardown of KVMs dirty tracking not take so long? 300ms is a silly long time
> on only a small VM.

Xiao Guangrong is working on something vaguely related (but different
and simpler because it's entirely contained within KVM), which is to
make log_sync faster.

The Intel folks working on clear containers also would like
MemoryListeners to have a better complexity, but that's again separate
from the "zapping" of SPTEs.

> Can you tell me which version of libvirt you're using?
> I thought the newer ones were supposed to use events so they did't
> have to poll qemu.
> 
>  If we move qemu_savevm_state_cleanup is it still safe? Are there
> some things we're supposed to do at that point which are wrong if
> we don't.
> 
> I wonder about something like;  take a mutex in
> memory_global_dirty_log_start, release it in
> memory_global_dirty_log_stop.  Then make ram_save_cleanup start
> a new thread that does the call to memory_global_dirty_log_stop.

I don't like having such a long-lived mutex (it seems like a recipe for
deadlocks with the BQL), plus memory_region_transaction_commit (the
expensive part of memory_global_dirty_log_stop) needs to be under the
BQL itself because it calls MemoryListeners.

Maybe memory_global_dirty_log_stop can delay itself to the next vm_start
if it's called while runstate_running() returns false (which should be
always the case)?

It could even be entirely enclosed within memory.c if you do it with a
VMChangeStateHandler.

Thanks,

Paolo

> Dave
> 
>> Thanks,
>> Jay
>>
>>> However, we would need to be a bit careful of anything that needs
>>> cleaning up before the source restarts on failure; I'm not sure of
>>> the semantics of all the current things wired into save_cleanup.
>>>
>>> Dave
>>>
>>>
>>>> Signed-off-by: Jay Zhou <address@hidden>
>>>> ---
>>>>   migration/migration.c | 16 +++++++++-------
>>>>   qmp.c                 | 10 ++++++++++
>>>>   2 files changed, 19 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/migration/migration.c b/migration/migration.c
>>>> index a0db40d..72832be 100644
>>>> --- a/migration/migration.c
>>>> +++ b/migration/migration.c
>>>> @@ -1877,6 +1877,15 @@ static void *migration_thread(void *opaque)
>>>>           if (qemu_file_get_error(s->to_dst_file)) {
>>>>               migrate_set_state(&s->state, current_active_state,
>>>>                                 MIGRATION_STATUS_FAILED);
>>>> +            /*
>>>> +             * The resource has been allocated by migration will be 
>>>> reused in
>>>> +             * COLO process, so don't release them.
>>>> +             */
>>>> +            if (!enable_colo) {
>>>> +                qemu_mutex_lock_iothread();
>>>> +                qemu_savevm_state_cleanup();
>>>> +                qemu_mutex_unlock_iothread();
>>>> +            }
>>>>               trace_migration_thread_file_err();
>>>>               break;
>>>>           }
>>>> @@ -1916,13 +1925,6 @@ static void *migration_thread(void *opaque)
>>>>       end_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
>>>>
>>>>       qemu_mutex_lock_iothread();
>>>> -    /*
>>>> -     * The resource has been allocated by migration will be reused in COLO
>>>> -     * process, so don't release them.
>>>> -     */
>>>> -    if (!enable_colo) {
>>>> -        qemu_savevm_state_cleanup();
>>>> -    }
>>>>       if (s->state == MIGRATION_STATUS_COMPLETED) {
>>>>           uint64_t transferred_bytes = qemu_ftell(s->to_dst_file);
>>>>           s->total_time = end_time - s->total_time;
>>>> diff --git a/qmp.c b/qmp.c
>>>> index b86201e..0e68eaa 100644
>>>> --- a/qmp.c
>>>> +++ b/qmp.c
>>>> @@ -37,6 +37,8 @@
>>>>   #include "qom/object_interfaces.h"
>>>>   #include "hw/mem/pc-dimm.h"
>>>>   #include "hw/acpi/acpi_dev_interface.h"
>>>> +#include "migration/migration.h"
>>>> +#include "migration/savevm.h"
>>>>
>>>>   NameInfo *qmp_query_name(Error **errp)
>>>>   {
>>>> @@ -200,6 +202,14 @@ void qmp_cont(Error **errp)
>>>>       if (runstate_check(RUN_STATE_INMIGRATE)) {
>>>>           autostart = 1;
>>>>       } else {
>>>> +        /*
>>>> +         * Delay the cleanup to reduce the downtime of migration.
>>>> +         * The resource has been allocated by migration will be reused
>>>> +         * in COLO process, so don't release them.
>>>> +         */
>>>> +        if (runstate_check(RUN_STATE_POSTMIGRATE) && 
>>>> !migrate_colo_enabled()) {
>>>> +            qemu_savevm_state_cleanup();
>>>> +        }
>>>>           vm_start();
>>>>       }
>>>>   }
>>>> --
>>>> 1.8.3.1
>>>>
>>>>
>>> --
>>> Dr. David Alan Gilbert / address@hidden / Manchester, UK
>>>
>>> .
>>>
>>
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK
>