| Message ID | 20230530144821.1557-1-avihaih@nvidia.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | migration: Add switchover ack capability and VFIO precopy support | expand |
Tested-by: YangHang Liu <yanghliu@redhat.com> On Wed, May 31, 2023 at 1:01 AM Avihai Horon <avihaih@nvidia.com> wrote: > > Hello everyone, > > This is v5 of the switchover ack series. > > Changes from v4 [6]: > * Removed superfluous '"' in vfio_save_iterate() trace. (Cedric) > * Removed VFIOMigration->switchover_ack_needed and computed it locally > when needed. (Cedric) > * Added R-bs. > > Changes from v3 [5]: > * Rebased on latest master branch. > * Simplified switchover ack logic (call switchover_ack_needed only in > destination). (Peter) > * Moved caching of VFIO migration flags to a separate patch. (Cedric) > * Moved adding of x-allow-pre-copy property to a separate patch. (Cedric) > * Reset VFIOMigration->precopy_{init,dirty}_size in vfio_query_precopy_size() > and in vfio_save_cleanup(). (Cedric) > * Added a reference to VFIO uAPI in vfio_save_block() ENOMSG comment. (Cedric) > * Added VFIOMigration->precopy_{init,dirty}_size to trace_vfio_save_iterate(). (Cedric) > * Adapted VFIO migration to switchover ack logic simplification: > - Checked migrate_switchover_ack() in vfio_{save,load}_setup() and set > VFIOMigration->switchover_ack_needed accordingly. > - vfio_switchover_ack_needed() doesn't set VFIOMigration->switchover_ack_needed > and only returns its value. > * Move VFIOMigration->switchover_ack_needed = false to vfio_migration_cleanup() > so it will be set to false both in src and dest. > * Fixed a few typos/coding style. (Peter/Cedric) > * Added R-b/A-b (didn't add Cedric's R-b on patch #7 as switchover ack > changes in patch #2 introduced some changes to patch #7 as well). > > Changes from v2 [4]: > * Rebased on latest master branch. > * Changed the capability name to "switchover-ack" and the related > code/docs accordingly. (Peter) > * Added a counter for the number of switchover ack users in the source > and used it to skip switchover ack if there are no users (instead of > setting the switchover acked flag to true). (Peter) > * Added R-bs. > > Changes from v1 [3]: > * Rebased on latest master branch. > * Updated to latest QAPI doc comment conventions and refined > QAPI docs and capability error message. (Markus) > * Followed Peter/Juan suggestion and removed the handshake between > source and destination. > Now the capability must be set on both source and destination. > Compatibility of this feature between different QEMU versions or > different host capabilities (i.e., kernel) is achieved in the regular > way of device properties and hw_comapt_x_y. > * Replaced is_initial_data_active() and initial_data_loaded() > SaveVMHandlers handlers with a notification mechanism. (Peter) > * Set the capability also in destination in the migration test. > * Added VFIO device property x-allow-pre-copy to be able to preserve > compatibility between different QEMU versions or different host > capabilities (i.e., kernel). > * Changed VFIO precopy initial data implementation according to the > above changes. > * Documented VFIO precopy initial data support in VFIO migration > documentation. > * Added R-bs. > > === > > This series adds a new migration capability called "switchover ack". The > purpose of this capability is to reduce migration downtime in cases > where loading of migration data in the destination can take a lot of > time, such as with VFIO migration data. > > The series then moves to add precopy support and switchover ack support > for VFIO migration. > > Switchover ack is used by VFIO migration, but other migrated devices can > add support for it and use it as well. > > === Background === > > Migration downtime estimation is calculated based on bandwidth and > remaining migration data. This assumes that loading of migration data in > the destination takes a negligible amount of time and that downtime > depends only on network speed. > > While this may be true for RAM, it's not necessarily true for other > migrated devices. For example, loading the data of a VFIO device in the > destination might require from the device to allocate resources and > prepare internal data structures which can take a significant amount of > time to do. > > This poses a problem, as the source may think that the remaining > migration data is small enough to meet the downtime limit, so it will > stop the VM and complete the migration, but in fact sending and loading > the data in the destination may take longer than the downtime limit. > > To solve this, VFIO migration uAPI defines "initial bytes" as part of > its precopy stream [1]. Initial bytes can be used in various ways to > improve VFIO migration performance. For example, it can be used to > transfer device metadata to pre-allocate resources in the destination. > However, for this to work we need to make sure that all initial bytes > are sent and loaded in the destination before the source VM is stopped. > > The new switchover ack migration capability helps us achieve this. > It prevents the source from stopping the VM and completing the migration > until an ACK is received from the destination that it's OK to do so. > Thus, a VFIO device can make sure that its initial bytes were sent > and loaded in the destination before the source VM is stopped. > > Note that this relies on the return path capability to communicate from > the destination back to the source. > > === Flow of operation === > > To use switchover ack, the capability must be enabled in both the source > and the destination. > > During migration setup, migration code in the destination calls the > switchover_ack_needed() SaveVMHandlers handler of the migrated devices > to check if switchover ack is used by them. > A "switchover_ack_pending_num" counter is increased for each migrated > device that supports this feature. It will be used later to mark when an > ACK should be sent to the source. > > Migration is active and the source starts to send precopy data as usual. > In the destination, when a migrated device thinks it's OK to do > switchover, it notifies the migration code about it and the > "switchover_ack_pending_num" counter is decreased. For example, for a > VFIO device it's when the device receives and loads its initial bytes. > > When the "switchover_ack_pending_num" counter reaches zero, it means > that all devices agree to do switchover and an ACK is sent to the > source, which will now be able to complete the migration when > appropriate. > > === Test results === > > The below table shows the downtime of two identical migrations. In the > first migration swithcover ack is disabled and in the second it is > enabled. The migrated VM is assigned with a mlx5 VFIO device which has > 300MB of device data to be migrated. > > +----------------------+-----------------------+----------+ > | Switchover ack | VFIO device data size | Downtime | > +----------------------+-----------------------+----------+ > | Disabled | 300MB | 1900ms | > | Enabled | 300MB | 420ms | > +----------------------+-----------------------+----------+ > > Switchover ack gives a roughly 4.5 times improvement in downtime. > The 1480ms difference is time that is used for resource allocation for > the VFIO device in the destination. Without switchover ack, this time is > spent when the source VM is stopped and thus the downtime is much > higher. With switchover ack, the time is spent when the source VM is > still running. > > === Patch breakdown === > > - Patches 1-4 add the switchover ack capability. > - Patches 5-8 add VFIO migration precopy support. Similar version of > them was previously sent here [2]. > - Patch 9 adds switchover ack support for VFIO migration. > > Thanks for reviewing! > > [1] > https://elixir.bootlin.com/linux/latest/source/include/uapi/linux/vfio.h#L1048 > > [2] > https://lore.kernel.org/qemu-devel/20230222174915.5647-3-avihaih@nvidia.com/ > > [3] > https://lore.kernel.org/qemu-devel/20230501140141.11743-1-avihaih@nvidia.com/ > > [4] > https://lore.kernel.org/qemu-devel/20230517155219.10691-1-avihaih@nvidia.com/ > > [5] > https://lore.kernel.org/qemu-devel/20230521151808.24804-1-avihaih@nvidia.com/ > > [6] > https://lore.kernel.org/qemu-devel/20230528140652.8693-1-avihaih@nvidia.com/ > > Avihai Horon (9): > migration: Add switchover ack capability > migration: Implement switchover ack logic > migration: Enable switchover ack capability > tests: Add migration switchover ack capability test > vfio/migration: Refactor vfio_save_block() to return saved data size > vfio/migration: Store VFIO migration flags in VFIOMigration > vfio/migration: Add VFIO migration pre-copy support > vfio/migration: Add x-allow-pre-copy VFIO device property > vfio/migration: Add support for switchover ack capability > > docs/devel/vfio-migration.rst | 45 +++++-- > qapi/migration.json | 12 +- > include/hw/vfio/vfio-common.h | 5 + > include/migration/register.h | 2 + > migration/migration.h | 14 +++ > migration/options.h | 1 + > migration/savevm.h | 1 + > hw/core/machine.c | 1 + > hw/vfio/common.c | 6 +- > hw/vfio/migration.c | 221 +++++++++++++++++++++++++++++++--- > hw/vfio/pci.c | 2 + > migration/migration.c | 32 ++++- > migration/options.c | 17 +++ > migration/savevm.c | 54 +++++++++ > tests/qtest/migration-test.c | 26 ++++ > hw/vfio/trace-events | 4 +- > migration/trace-events | 3 + > 17 files changed, 413 insertions(+), 33 deletions(-) > > -- > 2.26.3 > >
Hello everyone, This is v5 of the switchover ack series. Changes from v4 [6]: * Removed superfluous '"' in vfio_save_iterate() trace. (Cedric) * Removed VFIOMigration->switchover_ack_needed and computed it locally when needed. (Cedric) * Added R-bs. Changes from v3 [5]: * Rebased on latest master branch. * Simplified switchover ack logic (call switchover_ack_needed only in destination). (Peter) * Moved caching of VFIO migration flags to a separate patch. (Cedric) * Moved adding of x-allow-pre-copy property to a separate patch. (Cedric) * Reset VFIOMigration->precopy_{init,dirty}_size in vfio_query_precopy_size() and in vfio_save_cleanup(). (Cedric) * Added a reference to VFIO uAPI in vfio_save_block() ENOMSG comment. (Cedric) * Added VFIOMigration->precopy_{init,dirty}_size to trace_vfio_save_iterate(). (Cedric) * Adapted VFIO migration to switchover ack logic simplification: - Checked migrate_switchover_ack() in vfio_{save,load}_setup() and set VFIOMigration->switchover_ack_needed accordingly. - vfio_switchover_ack_needed() doesn't set VFIOMigration->switchover_ack_needed and only returns its value. * Move VFIOMigration->switchover_ack_needed = false to vfio_migration_cleanup() so it will be set to false both in src and dest. * Fixed a few typos/coding style. (Peter/Cedric) * Added R-b/A-b (didn't add Cedric's R-b on patch #7 as switchover ack changes in patch #2 introduced some changes to patch #7 as well). Changes from v2 [4]: * Rebased on latest master branch. * Changed the capability name to "switchover-ack" and the related code/docs accordingly. (Peter) * Added a counter for the number of switchover ack users in the source and used it to skip switchover ack if there are no users (instead of setting the switchover acked flag to true). (Peter) * Added R-bs. Changes from v1 [3]: * Rebased on latest master branch. * Updated to latest QAPI doc comment conventions and refined QAPI docs and capability error message. (Markus) * Followed Peter/Juan suggestion and removed the handshake between source and destination. Now the capability must be set on both source and destination. Compatibility of this feature between different QEMU versions or different host capabilities (i.e., kernel) is achieved in the regular way of device properties and hw_comapt_x_y. * Replaced is_initial_data_active() and initial_data_loaded() SaveVMHandlers handlers with a notification mechanism. (Peter) * Set the capability also in destination in the migration test. * Added VFIO device property x-allow-pre-copy to be able to preserve compatibility between different QEMU versions or different host capabilities (i.e., kernel). * Changed VFIO precopy initial data implementation according to the above changes. * Documented VFIO precopy initial data support in VFIO migration documentation. * Added R-bs. === This series adds a new migration capability called "switchover ack". The purpose of this capability is to reduce migration downtime in cases where loading of migration data in the destination can take a lot of time, such as with VFIO migration data. The series then moves to add precopy support and switchover ack support for VFIO migration. Switchover ack is used by VFIO migration, but other migrated devices can add support for it and use it as well. === Background === Migration downtime estimation is calculated based on bandwidth and remaining migration data. This assumes that loading of migration data in the destination takes a negligible amount of time and that downtime depends only on network speed. While this may be true for RAM, it's not necessarily true for other migrated devices. For example, loading the data of a VFIO device in the destination might require from the device to allocate resources and prepare internal data structures which can take a significant amount of time to do. This poses a problem, as the source may think that the remaining migration data is small enough to meet the downtime limit, so it will stop the VM and complete the migration, but in fact sending and loading the data in the destination may take longer than the downtime limit. To solve this, VFIO migration uAPI defines "initial bytes" as part of its precopy stream [1]. Initial bytes can be used in various ways to improve VFIO migration performance. For example, it can be used to transfer device metadata to pre-allocate resources in the destination. However, for this to work we need to make sure that all initial bytes are sent and loaded in the destination before the source VM is stopped. The new switchover ack migration capability helps us achieve this. It prevents the source from stopping the VM and completing the migration until an ACK is received from the destination that it's OK to do so. Thus, a VFIO device can make sure that its initial bytes were sent and loaded in the destination before the source VM is stopped. Note that this relies on the return path capability to communicate from the destination back to the source. === Flow of operation === To use switchover ack, the capability must be enabled in both the source and the destination. During migration setup, migration code in the destination calls the switchover_ack_needed() SaveVMHandlers handler of the migrated devices to check if switchover ack is used by them. A "switchover_ack_pending_num" counter is increased for each migrated device that supports this feature. It will be used later to mark when an ACK should be sent to the source. Migration is active and the source starts to send precopy data as usual. In the destination, when a migrated device thinks it's OK to do switchover, it notifies the migration code about it and the "switchover_ack_pending_num" counter is decreased. For example, for a VFIO device it's when the device receives and loads its initial bytes. When the "switchover_ack_pending_num" counter reaches zero, it means that all devices agree to do switchover and an ACK is sent to the source, which will now be able to complete the migration when appropriate. === Test results === The below table shows the downtime of two identical migrations. In the first migration swithcover ack is disabled and in the second it is enabled. The migrated VM is assigned with a mlx5 VFIO device which has 300MB of device data to be migrated. +----------------------+-----------------------+----------+ | Switchover ack | VFIO device data size | Downtime | +----------------------+-----------------------+----------+ | Disabled | 300MB | 1900ms | | Enabled | 300MB | 420ms | +----------------------+-----------------------+----------+ Switchover ack gives a roughly 4.5 times improvement in downtime. The 1480ms difference is time that is used for resource allocation for the VFIO device in the destination. Without switchover ack, this time is spent when the source VM is stopped and thus the downtime is much higher. With switchover ack, the time is spent when the source VM is still running. === Patch breakdown === - Patches 1-4 add the switchover ack capability. - Patches 5-8 add VFIO migration precopy support. Similar version of them was previously sent here [2]. - Patch 9 adds switchover ack support for VFIO migration. Thanks for reviewing! [1] https://elixir.bootlin.com/linux/latest/source/include/uapi/linux/vfio.h#L1048 [2] https://lore.kernel.org/qemu-devel/20230222174915.5647-3-avihaih@nvidia.com/ [3] https://lore.kernel.org/qemu-devel/20230501140141.11743-1-avihaih@nvidia.com/ [4] https://lore.kernel.org/qemu-devel/20230517155219.10691-1-avihaih@nvidia.com/ [5] https://lore.kernel.org/qemu-devel/20230521151808.24804-1-avihaih@nvidia.com/ [6] https://lore.kernel.org/qemu-devel/20230528140652.8693-1-avihaih@nvidia.com/ Avihai Horon (9): migration: Add switchover ack capability migration: Implement switchover ack logic migration: Enable switchover ack capability tests: Add migration switchover ack capability test vfio/migration: Refactor vfio_save_block() to return saved data size vfio/migration: Store VFIO migration flags in VFIOMigration vfio/migration: Add VFIO migration pre-copy support vfio/migration: Add x-allow-pre-copy VFIO device property vfio/migration: Add support for switchover ack capability docs/devel/vfio-migration.rst | 45 +++++-- qapi/migration.json | 12 +- include/hw/vfio/vfio-common.h | 5 + include/migration/register.h | 2 + migration/migration.h | 14 +++ migration/options.h | 1 + migration/savevm.h | 1 + hw/core/machine.c | 1 + hw/vfio/common.c | 6 +- hw/vfio/migration.c | 221 +++++++++++++++++++++++++++++++--- hw/vfio/pci.c | 2 + migration/migration.c | 32 ++++- migration/options.c | 17 +++ migration/savevm.c | 54 +++++++++ tests/qtest/migration-test.c | 26 ++++ hw/vfio/trace-events | 4 +- migration/trace-events | 3 + 17 files changed, 413 insertions(+), 33 deletions(-)