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[v6,00/10] KVM: Dirty ring support (QEMU part)

Message ID 20210324183954.345629-1-peterx@redhat.com (mailing list archive)
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Series KVM: Dirty ring support (QEMU part) | expand

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Peter Xu March 24, 2021, 6:39 p.m. UTC
This is v6 of the qemu dirty ring interface support.

v6:
- Fix slots_lock init [Keqian, Paolo]
- Comment above KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2 on todo (to enable
  KVM_CLEAR_DIRTY_LOG for dirty ring too) [Keqian, Paolo]
- Fix comment for CPUState [Keqian]

v5:
- rebase
- dropped patch "update-linux-headers: Include const.h" after rebase
- dropped patch "KVM: Fixup kvm_log_clear_one_slot() ioctl return check" since
  similar patch got merged recently (38e0b7904eca7cd32f8953c3)

========= v4 cover letter below =============

It is merely the same as v3 content-wise, but there're a few things to mention
besides the rebase itself:

  - I picked up two patches from Eric Farman for the linux-header updates (from
    Eric's v3 series) for convenience just in case any of the series would got
    queued by any maintainer.

  - One more patch is added as "KVM: Disable manual dirty log when dirty ring
    enabled".  I found this when testing the branch after rebasing to latest
    qemu, that not only the manual dirty log capability is not needed for kvm
    dirty ring, but more importantly INITIALLY_ALL_SET is totally against kvm
    dirty ring and it could silently crash the guest after migration.  For this
    new commit, I touched up "KVM: Add dirty-gfn-count property" a bit.

  - A few more documentation lines in qemu-options.hx.

  - I removed the RFC tag after kernel series got merged.

Again, this is only the 1st step to support dirty ring.  Ideally dirty ring
should grant QEMU the possibility to remove the whole layered dirty bitmap so
that dirty ring will work similarly as auto-converge enabled but should better;
we will just throttle vcpus with the dirty ring kvm exit rather than explicitly
adding a timer to stop the vcpu thread from entering the guest again (like what
we did with current migration auto-converge).  Some more information could also
be found in the kvm forum 2020 talk regarding kvm dirty ring (slides 21/22 [1]).

That next step (to remove all the dirty bitmaps, as mentioned above) is still
discussable: firstly I don't know whether there's anything I've overlooked in
there.  Meanwhile that's also only services huge VM cases, may not be extremely
helpful with a lot major scenarios where VMs are not that huge.

There's probably other ways to fix huge VM migration issues, majorly focusing
on responsiveness and convergence.  For example, Google has proposed some new
userfaultfd kernel capability called "minor modes" [2] to track page minor
faults and that could be finally served for that purpose too using postcopy.
That's another long story so I'll stop here, but just as a marker along with
the dirty ring series so there'll still be a record to reference.

Said that, I still think this series is very worth merging even if we don't
persue the next steps yet, since dirty ring is disabled by default, and we can
always work upon this series.

Please review, thanks.

V3: https://lore.kernel.org/qemu-devel/20200523232035.1029349-1-peterx@redhat.com/
    (V3 contains all the pre-v3 changelog)

QEMU branch for testing (requires kernel version 5.11-rc1+):
    https://github.com/xzpeter/qemu/tree/kvm-dirty-ring

[1] https://static.sched.com/hosted_files/kvmforum2020/97/kvm_dirty_ring_peter.pdf
[2] https://lore.kernel.org/lkml/20210107190453.3051110-1-axelrasmussen@google.com/

---------------------------8<---------------------------------

Overview
========

KVM dirty ring is a new interface to pass over dirty bits from kernel
to the userspace.  Instead of using a bitmap for each memory region,
the dirty ring contains an array of dirtied GPAs to fetch, one ring
per vcpu.

There're a few major changes comparing to how the old dirty logging
interface would work:

- Granularity of dirty bits

  KVM dirty ring interface does not offer memory region level
  granularity to collect dirty bits (i.e., per KVM memory
  slot). Instead the dirty bit is collected globally for all the vcpus
  at once.  The major effect is on VGA part because VGA dirty tracking
  is enabled as long as the device is created, also it was in memory
  region granularity.  Now that operation will be amplified to a VM
  sync.  Maybe there's smarter way to do the same thing in VGA with
  the new interface, but so far I don't see it affects much at least
  on regular VMs.

- Collection of dirty bits

  The old dirty logging interface collects KVM dirty bits when
  synchronizing dirty bits.  KVM dirty ring interface instead used a
  standalone thread to do that.  So when the other thread (e.g., the
  migration thread) wants to synchronize the dirty bits, it simply
  kick the thread and wait until it flushes all the dirty bits to the
  ramblock dirty bitmap.

A new parameter "dirty-ring-size" is added to "-accel kvm".  By
default, dirty ring is still disabled (size==0).  To enable it, we
need to be with:

  -accel kvm,dirty-ring-size=65536

This establishes a 64K dirty ring buffer per vcpu.  Then if we
migrate, it'll switch to dirty ring.

I gave it a shot with a 24G guest, 8 vcpus, using 10g NIC as migration
channel.  When idle or dirty workload small, I don't observe major
difference on total migration time.  When with higher random dirty
workload (800MB/s dirty rate upon 20G memory, worse for kvm dirty
ring). Total migration time is (ping pong migrate for 6 times, in
seconds):

|-------------------------+---------------|
| dirty ring (4k entries) | dirty logging |
|-------------------------+---------------|
|                      70 |            58 |
|                      78 |            70 |
|                      72 |            48 |
|                      74 |            52 |
|                      83 |            49 |
|                      65 |            54 |
|-------------------------+---------------|

Summary:

dirty ring average:    73s
dirty logging average: 55s

The KVM dirty ring will be slower in above case.  The number may show
that the dirty logging is still preferred as a default value because
small/medium VMs are still major cases, and high dirty workload
happens frequently too.  And that's what this series did.

TODO:

- Consider to drop the BQL dependency: then we can run the reaper thread in
  parallel of main thread.  Needs some thought around the race conditions.

- Consider to drop the kvmslot bitmap: logically this can be dropped with kvm
  dirty ring, not only for space saving, but also it's still another layer
  linear to guest mem size which is against the whole idea of kvm dirty ring.
  This should make above number (of kvm dirty ring) even smaller (but still may
  not be as good as dirty logging when with such high workload).

Please refer to the code and comment itself for more information.

Thanks,

Peter Xu (10):
  memory: Introduce log_sync_global() to memory listener
  KVM: Use a big lock to replace per-kml slots_lock
  KVM: Create the KVMSlot dirty bitmap on flag changes
  KVM: Provide helper to get kvm dirty log
  KVM: Provide helper to sync dirty bitmap from slot to ramblock
  KVM: Simplify dirty log sync in kvm_set_phys_mem
  KVM: Cache kvm slot dirty bitmap size
  KVM: Add dirty-gfn-count property
  KVM: Disable manual dirty log when dirty ring enabled
  KVM: Dirty ring support

 accel/kvm/kvm-all.c      | 593 +++++++++++++++++++++++++++++++++------
 accel/kvm/trace-events   |   7 +
 include/exec/memory.h    |  12 +
 include/hw/core/cpu.h    |   7 +
 include/sysemu/kvm_int.h |   7 +-
 qemu-options.hx          |  12 +
 softmmu/memory.c         |  33 ++-
 7 files changed, 572 insertions(+), 99 deletions(-)

Comments

Peter Xu April 29, 2021, 9:07 p.m. UTC | #1
On Wed, Mar 24, 2021 at 02:39:44PM -0400, Peter Xu wrote:
> This is v6 of the qemu dirty ring interface support.
> 
> v6:
> - Fix slots_lock init [Keqian, Paolo]
> - Comment above KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2 on todo (to enable
>   KVM_CLEAR_DIRTY_LOG for dirty ring too) [Keqian, Paolo]
> - Fix comment for CPUState [Keqian]

Ping after v6.0 released (seems still ok to apply).