Re: [PATCH v14 0/5] UFFD write-tracking migration/snapshots

[Dr. David Alan Gilbert]

* Dr. David Alan Gilbert (dgilbert@redhat.com) wrote:
> * Andrey Gruzdev (andrey.gruzdev@virtuozzo.com) wrote:
> > This patch series is a kind of 'rethinking' of Denis Plotnikov's ideas he's
> > implemented in his series '[PATCH v0 0/4] migration: add background 
> > snapshot'.
> > 
> > Currently the only way to make (external) live VM snapshot is using existing
> > dirty page logging migration mechanism. The main problem is that it tends to
> > produce a lot of page duplicates while running VM goes on updating already
> > saved pages. That leads to the fact that vmstate image size is commonly 
> > several
> > times bigger then non-zero part of virtual machine's RSS. Time required to
> > converge RAM migration and the size of snapshot image severely depend on the
> > guest memory write rate, sometimes resulting in unacceptably long snapshot
> > creation time and huge image size.
> > 
> > This series propose a way to solve the aforementioned problems. This is done
> > by using different RAM migration mechanism based on UFFD write protection
> > management introduced in v5.7 kernel. The migration strategy is to 'freeze'
> > guest RAM content using write-protection and iteratively release protection
> > for memory ranges that have already been saved to the migration stream.
> > At the same time we read in pending UFFD write fault events and save those
> > pages out-of-order with higher priority.
> 
> Queued
> 
Andrey:
  I've fixed up some 32bit build casts in the pull.
Please check them.

Dave

> > How to use:
> > 1. Enable write-tracking migration capability
> >    virsh qemu-monitor-command <domain> --hmp migrate_set_capability
> >    background-snapshot on
> > 
> > 2. Start the external migration to a file
> >    virsh qemu-monitor-command <domain> --hmp migrate exec:'cat > ./vm_state'
> > 
> > 3. Wait for the migration finish and check that the migration has completed.
> > state.
> > 
> > 
> > Changes v13->v14:
> > 
> > * 1. Removed unneeded '#ifdef CONFIG_LINUX' from [PATCH 1/5] where #ifdef'ed
> > *    code was originally introduced. In v13 removed #ifdef's appeared to be
> > *    a diff in [PATCH 4/5] on top of previous patches.
> > 
> > Changes v12->v13:
> > 
> > * 1. Fixed codestyle problem for checkpatch.
> > 
> > Changes v11->v12:
> > 
> > * 1. Consolidated UFFD-related code under single #if defined(__linux__).
> > * 2. Abandoned use of pre/post hooks in ram_find_and_save_block() in favour
> > *    of more compact code fragment in ram_save_host_page().
> > * 3. Refactored/simplified eBPF code in userfaultfd-wrlat.py script.
> > 
> > Changes v10->v11:
> > 
> > * 1. Updated commit messages.
> > 
> > Changes v9->v10:
> > 
> > * 1. Fixed commit message for [PATCH v9 1/5].
> > 
> > Changes v8->v9:
> > 
> > * 1. Fixed wrong cover letter subject.
> > 
> > Changes v7->v8:
> > 
> > * 1. Fixed coding style problems to pass checkpatch.
> > 
> > Changes v6->v7:
> > 
> > * 1. Fixed background snapshot on suspended guest: call 
> > qemu_system_wakeup_request()
> > *    before stopping VM to make runstate transition valid.
> > * 2. Disabled dirty page logging and log syn when 'background-snapshot' is 
> > enabled.
> > * 3. Introduced 'userfaultfd-wrlat.py' script to analyze UFFD write fault 
> > latencies.
> > 
> > Changes v5->v6:
> > 
> > * 1. Consider possible hot pluggin/unpluggin of memory device - don't use 
> > static
> > *    for write-tracking support level in migrate_query_write_tracking(), 
> > check
> > *    each time when one tries to enable 'background-snapshot' capability.
> > 
> > Changes v4->v5:
> > 
> > * 1. Refactored util/userfaultfd.c code to support features required by 
> > postcopy.
> > * 2. Introduced checks for host kernel and guest memory backend 
> > compatibility
> > *    to 'background-snapshot' branch in migrate_caps_check().
> > * 3. Switched to using trace_xxx instead of info_report()/error_report() for
> > *    cases when error message must be hidden (probing UFFD-IO) or info may 
> > be
> > *    really littering output if goes to stderr.
> > * 4  Added RCU_READ_LOCK_GUARDs to the code dealing with RAM block list.
> > * 5. Added memory_region_ref() for each RAM block being wr-protected.
> > * 6. Reused qemu_ram_block_from_host() instead of custom RAM block lookup 
> > routine.
> > * 7. Refused from using specific hwaddr/ram_addr_t in favour of void 
> > */uint64_t.
> > * 8. Currently dropped 'linear-scan-rate-limiting' patch. The reason is that
> > *    that choosen criteria for high-latency fault detection (i.e. timestamp 
> > of
> > *    UFFD event fetch) is not representative enough for this task.
> > *    At the moment it looks somehow like premature optimization effort.
> > * 8. Dropped some unnecessary/unused code.
> > 
> > Andrey Gruzdev (5):
> >   migration: introduce 'background-snapshot' migration capability
> >   migration: introduce UFFD-WP low-level interface helpers
> >   migration: support UFFD write fault processing in ram_save_iterate()
> >   migration: implementation of background snapshot thread
> >   migration: introduce 'userfaultfd-wrlat.py' script
> > 
> >  include/exec/memory.h        |   8 +
> >  include/qemu/userfaultfd.h   |  35 ++++
> >  migration/migration.c        | 357 ++++++++++++++++++++++++++++++++++-
> >  migration/migration.h        |   4 +
> >  migration/ram.c              | 303 ++++++++++++++++++++++++++++-
> >  migration/ram.h              |   6 +
> >  migration/savevm.c           |   1 -
> >  migration/savevm.h           |   2 +
> >  migration/trace-events       |   2 +
> >  qapi/migration.json          |   7 +-
> >  scripts/userfaultfd-wrlat.py | 122 ++++++++++++
> >  util/meson.build             |   1 +
> >  util/trace-events            |   9 +
> >  util/userfaultfd.c           | 345 +++++++++++++++++++++++++++++++++
> >  14 files changed, 1190 insertions(+), 12 deletions(-)
> >  create mode 100644 include/qemu/userfaultfd.h
> >  create mode 100755 scripts/userfaultfd-wrlat.py
> >  create mode 100644 util/userfaultfd.c
> > 
> > -- 
> > 2.25.1
> > 
> > 
> -- 
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> 
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK