Re: [PATCH v4 00/19] migration: Postcopy Preemption

[Dr. David Alan Gilbert]

* Peter Xu (peterx@redhat.com) wrote:
> This is v4 of postcopy preempt series.  It can also be found here:
> 
>   https://github.com/xzpeter/qemu/tree/postcopy-preempt
> 
> RFC: 
> https://lore.kernel.org/qemu-devel/20220119080929.39485-1-peterx@redhat.com
> V1:  
> https://lore.kernel.org/qemu-devel/20220216062809.57179-1-peterx@redhat.com
> V2:  
> https://lore.kernel.org/qemu-devel/20220301083925.33483-1-peterx@redhat.com
> V3:  
> https://lore.kernel.org/qemu-devel/20220330213908.26608-1-peterx@redhat.com

I've queued:
migration: Allow migrate-recover to run multiple times
migration: Move channel setup out of postcopy_try_recover()
migration: Export ram_load_postcopy()
migration: Move migrate_allow_multifd and helpers into migration.c
migration: Add pss.postcopy_requested status
migration: Drop multifd tls_hostname cache
migration: Postpone releasing MigrationState.hostname

> v4:
> - Fix a double-free on params.tls-creds when quitting qemu
> - Reorder patches to satisfy per-commit builds
> 
> v3:
> - Rebased to master since many patches landed
> - Fixed one bug on postcopy recovery when preempt enabled, this is only
>   found when I test with TLS+recovery, because TLS changed the timing.
> - Dropped patch:
>   "migration: Fail postcopy preempt with TLS for now"
> - Added patches for TLS:
>   - "migration: Postpone releasing MigrationState.hostname"
>   - "migration: Drop multifd tls_hostname cache"
>   - "migration: Enable TLS for preempt channel"
>   - "migration: Export tls-[creds|hostname|authz] params to cmdline too"
>   - "tests: Add postcopy tls migration test"
>   - "tests: Add postcopy tls recovery migration test"
> - Added two more tests to the preempt test patch (tls, tls+recovery)
> 
> Abstract
> ========
> 
> This series added a new migration capability called "postcopy-preempt".  It 
> can
> be enabled when postcopy is enabled, and it'll simply (but greatly) speed up
> postcopy page requests handling process.
> 
> Below are some initial postcopy page request latency measurements after the
> new series applied.
> 
> For each page size, I measured page request latency for three cases:
> 
>   (a) Vanilla:                the old postcopy
>   (b) Preempt no-break-huge:  preempt enabled, 
> x-postcopy-preempt-break-huge=off
>   (c) Preempt full:           preempt enabled, 
> x-postcopy-preempt-break-huge=on
>                               (this is the default option when preempt 
> enabled)
> 
> Here x-postcopy-preempt-break-huge parameter is just added in v2 so as to
> conditionally disable the behavior to break sending a precopy huge page for
> debugging purpose.  So when it's off, postcopy will not preempt precopy
> sending a huge page, but still postcopy will use its own channel.
> 
> I tested it separately to give a rough idea on which part of the change
> helped how much of it.  The overall benefit should be the comparison
> between case (a) and (c).
> 
>   |-----------+---------+-----------------------+--------------|
>   | Page size | Vanilla | Preempt no-break-huge | Preempt full |
>   |-----------+---------+-----------------------+--------------|
>   | 4K        |   10.68 |               N/A [*] |         0.57 |
>   | 2M        |   10.58 |                  5.49 |         5.02 |
>   | 1G        | 2046.65 |               933.185 |      649.445 |
>   |-----------+---------+-----------------------+--------------|
>   [*]: This case is N/A because 4K page does not contain huge page at all
> 
> [1] 
> https://github.com/xzpeter/small-stuffs/blob/master/tools/huge_vm/uffd-latency.bpf
> 
> TODO List
> =========
> 
> Avoid precopy write() blocks postcopy
> -------------------------------------
> 
> I didn't prove this, but I always think the write() syscalls being blocked
> for precopy pages can affect postcopy services.  If we can solve this
> problem then my wild guess is we can further reduce the average page
> latency.
> 
> Two solutions at least in mind: (1) we could have made the write side of
> the migration channel NON_BLOCK too, or (2) multi-threads on send side,
> just like multifd, but we may use lock to protect which page to send too
> (e.g., the core idea is we should _never_ rely anything on the main thread,
> multifd has that dependency on queuing pages only on main thread).
> 
> That can definitely be done and thought about later.
> 
> Multi-channel for preemption threads
> ------------------------------------
> 
> Currently the postcopy preempt feature use only one extra channel and one
> extra thread on dest (no new thread on src QEMU).  It should be mostly good
> enough for major use cases, but when the postcopy queue is long enough
> (e.g. hundreds of vCPUs faulted on different pages) logically we could
> still observe more delays in average.  Whether growing threads/channels can
> solve it is debatable, but sounds worthwhile a try.  That's yet another
> thing we can think about after this patchset lands.
> 
> Logically the design provides space for that - the receiving postcopy
> preempt thread can understand all ram-layer migration protocol, and for
> multi channel and multi threads we could simply grow that into multile
> threads handling the same protocol (with multiple PostcopyTmpPage).  The
> source needs more thoughts on synchronizations, though, but it shouldn't
> affect the whole protocol layer, so should be easy to keep compatible.
> 
> Please review, thanks.
> 
> Peter Xu (19):
>   migration: Postpone releasing MigrationState.hostname
>   migration: Drop multifd tls_hostname cache
>   migration: Add pss.postcopy_requested status
>   migration: Move migrate_allow_multifd and helpers into migration.c
>   migration: Export ram_load_postcopy()
>   migration: Move channel setup out of postcopy_try_recover()
>   migration: Allow migrate-recover to run multiple times
>   migration: Add postcopy-preempt capability
>   migration: Postcopy preemption preparation on channel creation
>   migration: Postcopy preemption enablement
>   migration: Postcopy recover with preempt enabled
>   migration: Create the postcopy preempt channel asynchronously
>   migration: Parameter x-postcopy-preempt-break-huge
>   migration: Add helpers to detect TLS capability
>   migration: Export tls-[creds|hostname|authz] params to cmdline too
>   migration: Enable TLS for preempt channel
>   tests: Add postcopy tls migration test
>   tests: Add postcopy tls recovery migration test
>   tests: Add postcopy preempt tests
> 
>  migration/channel.c          |  11 +-
>  migration/migration.c        | 218 ++++++++++++++++++++------
>  migration/migration.h        |  52 ++++++-
>  migration/multifd.c          |  36 +----
>  migration/multifd.h          |   4 -
>  migration/postcopy-ram.c     | 190 ++++++++++++++++++++++-
>  migration/postcopy-ram.h     |  11 ++
>  migration/qemu-file.c        |  27 ++++
>  migration/qemu-file.h        |   1 +
>  migration/ram.c              | 288 +++++++++++++++++++++++++++++++++--
>  migration/ram.h              |   3 +
>  migration/savevm.c           |  49 ++++--
>  migration/socket.c           |  22 ++-
>  migration/socket.h           |   1 +
>  migration/trace-events       |  15 +-
>  qapi/migration.json          |   8 +-
>  tests/qtest/migration-test.c | 113 ++++++++++++--
>  17 files changed, 918 insertions(+), 131 deletions(-)
> 
> -- 
> 2.32.0
> 
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK