[v12,0/5] UFFD write-tracking migration/snapshots

Message

Zhijian Li (Fujitsu)" via Jan. 21, 2021, 3:02 p.m. UTC

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.

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 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

Comments

[e-mail] Jan. 21, 2021, 3:15 p.m. UTC | #1

Patchew URL: https://patchew.org/QEMU/[e-mail]/



Hi,

This series seems to have some coding style problems. See output below for
more information:

Type: series
Message-id: [e-mail]
Subject: [PATCH v12 0/5] UFFD write-tracking migration/snapshots

=== TEST SCRIPT BEGIN ===
#!/bin/bash
git rev-parse base > /dev/null || exit 0
git config --local diff.renamelimit 0
git config --local diff.renames True
git config --local diff.algorithm histogram
./scripts/checkpatch.pl --mailback base..
=== TEST SCRIPT END ===

Updating 3c8cf5a9c21ff8782164d1def7f44bd888713384
From https://github.com/patchew-project/qemu
 * [new tag]         patchew/[e-mail] -> patchew/[e-mail]
 * [new tag]         patchew/[e-mail] -> patchew/[e-mail]
 * [new tag]         patchew/[e-mail] -> patchew/[e-mail]
Switched to a new branch 'test'
b373acd migration: introduce 'userfaultfd-wrlat.py' script
65d19c7 migration: implementation of background snapshot thread
0dec39d migration: support UFFD write fault processing in ram_save_iterate()
ce2b6b8 migration: introduce UFFD-WP low-level interface helpers
d60a40d migration: introduce 'background-snapshot' migration capability

=== OUTPUT BEGIN ===
1/5 Checking commit d60a40d7561e (migration: introduce 'background-snapshot' migration capability)
WARNING: line over 80 characters
#123: FILE: migration/migration.c:1231:
+            error_setg(errp, "Background-snapshot is not supported by host kernel");

WARNING: Block comments use a leading /* on a separate line
#200: FILE: migration/ram.c:3791:
+/* ram_write_tracking_available: check if kernel supports required UFFD features

WARNING: Block comments use a leading /* on a separate line
#210: FILE: migration/ram.c:3801:
+/* ram_write_tracking_compatible: check if guest configuration is

total: 0 errors, 3 warnings, 207 lines checked

Patch 1/5 has style problems, please review.  If any of these errors
are false positives report them to the maintainer, see
CHECKPATCH in MAINTAINERS.
2/5 Checking commit ce2b6b8922a3 (migration: introduce UFFD-WP low-level interface helpers)
WARNING: added, moved or deleted file(s), does MAINTAINERS need updating?
#35: 
new file mode 100644

WARNING: line over 80 characters
#272: FILE: util/userfaultfd.c:163:
+ * uffd_change_protection: protect/un-protect memory range for writes via UFFD-IO

WARNING: line over 80 characters
#331: FILE: util/userfaultfd.c:222:
+        error_report("uffd_copy_page() failed: dst_addr=%p src_addr=%p length=%" PRIu64

WARNING: line over 80 characters
#371: FILE: util/userfaultfd.c:262:
+ * uffd_wakeup: wake up threads waiting on page UFFD-managed page fault resolution

WARNING: line over 80 characters
#392: FILE: util/userfaultfd.c:283:
+        error_report("uffd_wakeup() failed: addr=%p length=%" PRIu64 " errno=%i",

total: 0 errors, 5 warnings, 406 lines checked

Patch 2/5 has style problems, please review.  If any of these errors
are false positives report them to the maintainer, see
CHECKPATCH in MAINTAINERS.
3/5 Checking commit 0dec39ddcad4 (migration: support UFFD write fault processing in ram_save_iterate())
WARNING: architecture specific defines should be avoided
#61: FILE: migration/ram.c:61:
+#if defined(__linux__)

WARNING: architecture specific defines should be avoided
#80: FILE: migration/ram.c:1444:
+#if defined(__linux__)

WARNING: Block comments should align the * on each line
#122: FILE: migration/ram.c:1486:
+ * Returns 0 on success, negative value in case of an error
+*/

WARNING: line over 80 characters
#130: FILE: migration/ram.c:1494:
+        void *page_address = pss->block->host + (start_page << TARGET_PAGE_BITS);

WARNING: Block comments use a leading /* on a separate line
#143: FILE: migration/ram.c:1507:
+/* ram_write_tracking_available: check if kernel supports required UFFD features

WARNING: Block comments use a leading /* on a separate line
#157: FILE: migration/ram.c:1521:
+/* ram_write_tracking_compatible: check if guest configuration is

WARNING: line over 80 characters
#246: FILE: migration/ram.c:1610:
+    error_report("ram_write_tracking_start() failed: restoring initial memory state");

WARNING: line over 80 characters
#256: FILE: migration/ram.c:1620:
+        uffd_change_protection(rs->uffdio_fd, bs->host, bs->max_length, false, false);

WARNING: line over 80 characters
#283: FILE: migration/ram.c:1647:
+        uffd_change_protection(rs->uffdio_fd, bs->host, bs->max_length, false, false);

WARNING: Block comments use a leading /* on a separate line
#394: FILE: migration/ram.c:2166:
+        /* caller have hold iothread lock or is in a bh, so there is

total: 0 errors, 10 warnings, 408 lines checked

Patch 3/5 has style problems, please review.  If any of these errors
are false positives report them to the maintainer, see
CHECKPATCH in MAINTAINERS.
4/5 Checking commit 65d19c777763 (migration: implementation of background snapshot thread)
WARNING: line over 80 characters
#200: FILE: migration/migration.c:3824:
+ * mechanism, which effectively results in that saved snapshot is the state of VM

total: 0 errors, 1 warnings, 348 lines checked

Patch 4/5 has style problems, please review.  If any of these errors
are false positives report them to the maintainer, see
CHECKPATCH in MAINTAINERS.
5/5 Checking commit b373acd62b81 (migration: introduce 'userfaultfd-wrlat.py' script)
WARNING: added, moved or deleted file(s), does MAINTAINERS need updating?
#17: 
new file mode 100755

ERROR: trailing whitespace
#65: FILE: scripts/userfaultfd-wrlat.py:44:
+    $

WARNING: line over 80 characters
#122: FILE: scripts/userfaultfd-wrlat.py:101:
+b.attach_kretprobe(event="handle_userfault", fn_name="retprobe_handle_userfault")

total: 1 errors, 2 warnings, 122 lines checked

Patch 5/5 has style problems, please review.  If any of these errors
are false positives report them to the maintainer, see
CHECKPATCH in MAINTAINERS.

=== OUTPUT END ===

Test command exited with code: 1


The full log is available at
http://patchew.org/logs/[e-mail]/testing.checkpatch/?type=message.
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