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Re: [RFC PATCH] migration: reduce time of loading non-iterable vmstate

From: Chuang Xu
Subject: Re: [RFC PATCH] migration: reduce time of loading non-iterable vmstate
Date: Thu, 8 Dec 2022 23:32:19 -0800
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.4.2


On 2022/12/9 上午12:00, Peter Xu wrote:
On Thu, Dec 08, 2022 at 10:39:11PM +0800, Chuang Xu wrote:

On 2022/12/8 上午6:08, Peter Xu wrote:

On Thu, Dec 08, 2022 at 12:07:03AM +0800, Chuang Xu wrote:

On 2022/12/6 上午12:28, Peter Xu wrote:

Chuang,

No worry on the delay; you're faster than when I read yours. :)

On Mon, Dec 05, 2022 at 02:56:15PM +0800, Chuang Xu wrote:

As a start, maybe you can try with poison address_space_to_flatview() (by
e.g. checking the start_pack_mr_change flag and assert it is not set)
during this process to see whether any call stack can even try to
dereference a flatview.

It's just that I didn't figure a good way to "prove" its validity, even if
I think this is an interesting idea worth thinking to shrink the downtime.

Thanks for your sugguestions!
I used a thread local variable to identify whether the current thread is a
migration thread(main thread of target qemu) and I modified the code of
qemu_coroutine_switch to make sure the thread local variable true only in
process_incoming_migration_co call stack. If the target qemu detects that
start_pack_mr_change is set and address_space_to_flatview() is called in
non-migrating threads or non-migrating coroutine, it will crash.

Are you using the thread var just to avoid the assert triggering in the
migration thread when commiting memory changes?

I think _maybe_ another cleaner way to sanity check this is directly upon
the depth:

static inline FlatView *address_space_to_flatview(AddressSpace *as)
{
      /*
       * Before using any flatview, sanity check we're not during a memory
       * region transaction or the map can be invalid.  Note that this can
       * also be called during commit phase of memory transaction, but that
       * should also only happen when the depth decreases to 0 first.
       */
      assert(memory_region_transaction_depth == 0);
      return qatomic_rcu_read(&as->current_map);
}

That should also cover the safe cases of memory transaction commits during
migration.


Peter, I tried this way and found that the target qemu will crash.

Here is the gdb backtrace:

#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
#1  0x00007ff2929d851a in __GI_abort () at abort.c:118
#2  0x00007ff2929cfe67 in __assert_fail_base (fmt=<optimized out>, assertion=assertion@entry=0x55a32578cdc0 "memory_region_transaction_depth == 0", file=file@entry=0x55a32575d9b0 "/data00/migration/qemu-5.2.0/include/exec/memory.h",
     line=line@entry=766, function=function@entry=0x55a32578d6e0 <__PRETTY_FUNCTION__.20463> "address_space_to_flatview") at assert.c:92
#3  0x00007ff2929cff12 in __GI___assert_fail (assertion=assertion@entry=0x55a32578cdc0 "memory_region_transaction_depth == 0", file=file@entry=0x55a32575d9b0 "/data00/migration/qemu-5.2.0/include/exec/memory.h", line=line@entry=766,
     function=function@entry=0x55a32578d6e0 <__PRETTY_FUNCTION__.20463> "address_space_to_flatview") at assert.c:101
#4  0x000055a324b2ed5e in address_space_to_flatview (as=0x55a326132580 <address_space_memory>) at /data00/migration/qemu-5.2.0/include/exec/memory.h:766
#5  0x000055a324e79559 in address_space_to_flatview (as=0x55a326132580 <address_space_memory>) at ../softmmu/memory.c:811
#6  address_space_get_flatview (as=0x55a326132580 <address_space_memory>) at ../softmmu/memory.c:805
#7  0x000055a324e96474 in address_space_cache_init (cache=cache@entry=0x55a32a4fb000, as=<optimized out>, addr=addr@entry=68404985856, len=len@entry=4096, is_write=false) at ../softmmu/physmem.c:3307
#8  0x000055a324ea9cba in virtio_init_region_cache (vdev=0x55a32985d9a0, n=0) at ../hw/virtio/virtio.c:185
#9  0x000055a324eaa615 in virtio_load (vdev=0x55a32985d9a0, f=<optimized out>, version_id=<optimized out>) at ../hw/virtio/virtio.c:3203
#10 0x000055a324c6ab96 in vmstate_load_state (f=f@entry=0x55a329dc0c00, vmsd=0x55a325fc1a60 <vmstate_virtio_scsi>, opaque=0x55a32985d9a0, version_id=1) at ../migration/vmstate.c:143
#11 0x000055a324cda138 in vmstate_load (f=0x55a329dc0c00, se=0x55a329941c90) at ../migration/savevm.c:913
#12 0x000055a324cdda34 in qemu_loadvm_section_start_full (mis=0x55a3284ef9e0, f=0x55a329dc0c00) at ../migration/savevm.c:2741
#13 qemu_loadvm_state_main (f=f@entry=0x55a329dc0c00, mis=mis@entry=0x55a3284ef9e0) at ../migration/savevm.c:2939
#14 0x000055a324cdf66a in qemu_loadvm_state (f=0x55a329dc0c00) at ../migration/savevm.c:3021
#15 0x000055a324d14b4e in process_incoming_migration_co (opaque=<optimized out>) at ../migration/migration.c:574
#16 0x000055a32501ae3b in coroutine_trampoline (i0=<optimized out>, i1=<optimized out>) at ../util/coroutine-ucontext.c:173
#17 0x00007ff2929e8000 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#18 0x00007ffed80dc2a0 in ?? ()
#19 0x0000000000000000 in ?? ()

address_space_cache_init() is the only caller of address_space_to_flatview
I can find in vmstate_load call stack so far. Although I think the mr used
by address_space_cache_init() won't be affected by the delay of
memory_region_transaction_commit(), we really need a mechanism to prevent
the modified mr from being used.

Maybe we can build a stale list:
If a subregion is added, add its parent to the stale list(considering that
new subregion's priority has uncertain effects on flatviews).
If a subregion is deleted, add itself to the stale list.
When memory_region_transaction_commit() regenerates flatviews, clear the
stale list.
when address_space_translate_internal() is called, check whether the mr
looked up matches one of mrs（or its child）in the stale list. If yes, a
crash will be triggered.

I'm not sure that'll work, though.  Consider this graph:

                             A
                            / \
                           B   C
                        (p=1) (p=0)

A,B,C are MRs, B&C are subregions to A.  When B's priority is higher (p=1),
any access to A will go upon B, so far so good.

Then, let's assume D comes under C with even higher priority:

                             A
                            / \
                           B   C
                        (p=1) (p=0)
                               |
                               D
                              (p=2)


Adding C into stale list won't work because when with the old flatview
it'll point to B instead, while B is not in the stale list. The AS
operation will carry out without noticing it's already wrong.

Peter, I think our understanding of priority is different.

In the qemu docs
(https://qemu.readthedocs.io/en/stable-6.1/devel/memory.html#overlapping-regions-and-priority),
it says 'Priority values are local to a container, because the priorities of
two regions are only compared when they are both children of the same container.'
And as I read in code, when doing render_memory_region() operation on A, qemu
will firstly insert B's FlatRanges and its children's FlatRanges recursively
because B's priority is higher than C. After B's FlatRanges and its children's
FlatRanges are all inserted into flatviews, C's FlatRanges and its children's
FlatRanges will be inserted into gaps left by B if B and C overlaps.

So I think adding D as C's subregion has no effect on B in your second case.
The old FlatRange pointing to B is still effective. C and C'children with lower
priority than D will be affected, but we have flagged them as stale.

I hope I have no misunderstanding of the flatview's construction code. If I
understand wrong, please forgive my ignorance..😭

No I think you're right.. thanks, I should read the code/doc first rather
than trusting myself. :)

But still, the whole point is that the parent may not even be visible to
the flatview, so I still don't know how it could work.

My 2nd attempt:

                                  A
                                  |
                                  B
                                (p=1)

Adding C with p=2:

                                  A
                                 / \
                                B   C
                             (p=1) (p=2)

IIUC the flatview to access the offset A resides should point to B, then
after C plugged we'll still lookup and find B.  Even if A is in the stale
list, B is not?

Sorry I forgot to describe my latest ideas about this mechanism in detail.

we can add A, B and B's children to the stale list（If there is D, E and 
other mrs have lower priority than C, we can also add them and their children 
to the stale list recursively). Or we can add a stale flag to mr structure 
to avoid cost of searching mr in the stale list..

The other thing I didn't mention is that I don't think the address space
translation is the solo consumer of the flat view.  Some examples:

common_semi_find_bases() walks the flatview without translations.

memory_region_update_coalesced_range() (calls address_space_get_flatview()
first) notifies kvm coalesced mmio regions without translations.

So at least hooking up address_space_translate_internal() itself may not be
enough too.

This is really a problem. Maybe we should check whether the mr is stale on 
all critical paths, or find other more genneral ways.. 


      

        

        

          

          

            
There may be many details to consider in this mechanism. Hope you can give
some suggestions on its feasibility.


          

          
For this specific case, I'm wildly thinking whether we can just postpone
the init of the vring cache until migration completes.

One thing to mention from what I read it: we'll need to update all the
caches in virtio_memory_listener_commit() anyway, when the batched commit()
happens when migration completes with your approach, so we'll rebuild the
vring cache once and for all which looks also nice if possible.

There's some details to consider. E.g. the commit() happens only when
memory_region_update_pending==true.  We may want to make sure the cache is
initialized unconditionally, at least.  Not sure whether that's doable,
though.

Thanks,



        

        
Good idea! We can try it in the new patches! And with the delay of
virtio_init_region_cache(), we can still use assert in address_space_to_flatview().
However, I think the stale list can be used as a retention scheme for further
discussion in the future, because the stale list may adapt to more complex scenarios.


      

      
If the assert will work that'll be even better.  I'm actually worried this
can trigger like what you mentioned in the virtio path, I didn't expect it
comes that soon.  So if there's a minimum cases and we can fixup easily
that'll be great.  Hopefully there aren't so much or we'll need to revisit
the whole idea.

Thanks,


    

    
+1.. Hope this is the only case that will trigger crash.

I'll upload the second version as soon as possible.

Thanks.


  














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