Re: [PATCH 5/6] migration/qemu-file.c: Don't ratelimit a shutdown fd

[Lukas Straub <lukasstraub2@web.de>]

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On Mon, 18 May 2020 12:55:34 +0100
"Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:

> * Zhanghailiang (zhang.zhanghailiang@huawei.com) wrote:
> > > This causes the migration thread to hang if we failover during checkpoint. A
> > > shutdown fd won't cause network traffic anyway.
> > >   
> > 
> > I'm not quite sure if this modification can take side effect on normal migration process or not,
> > There are several places calling it.
> > 
> > Maybe Juan and Dave can help ;)
> >   
> > > Signed-off-by: Lukas Straub <lukasstraub2@web.de>
> > > ---
> > >  migration/qemu-file.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/migration/qemu-file.c b/migration/qemu-file.c index
> > > 1c3a358a14..0748b5810f 100644
> > > --- a/migration/qemu-file.c
> > > +++ b/migration/qemu-file.c
> > > @@ -660,7 +660,7 @@ int64_t qemu_ftell(QEMUFile *f)  int
> > > qemu_file_rate_limit(QEMUFile *f)  {
> > >      if (f->shutdown) {
> > > -        return 1;
> > > +        return 0;
> > >      }  
> 
> This looks wrong to me; I'd be curious to understand how it's hanging
> for you.
> '1' means 'stop what you're doing', 0 means carry on; carrying on with a
> shutdown fd sounds wrong.
> 
> If we look at ram.c we have:
> 
>         while ((ret = qemu_file_rate_limit(f)) == 0 ||
>                 !QSIMPLEQ_EMPTY(&rs->src_page_requests)) {
>             int pages;
>         ....
> 
> so if it returns '1', as it does at the moment it should cause it to
> exit the ram_save_iterate loop - which is what we want if it's failing.
> Thus I think you need to find the actual place it's stuck in this case -
> I suspect it's repeatedly calling ram_save_iterate and then exiting it,
> but if that's happening perhaps we're missing a qemu_file_get_error
> check somewhere.

Hi,
the problem is in ram_save_host_page and migration_rate_limit, here is a backtrace:

#0  0x00007f7b502921a8 in futex_abstimed_wait_cancelable (private=0, abstime=0x7f7ada7fb3f0, clockid=0, expected=0, futex_word=0x55bc358b9908) at ../sysdeps/unix/sysv/linux/futex-internal.h:208
#1  do_futex_wait (sem=sem@entry=0x55bc358b9908, abstime=abstime@entry=0x7f7ada7fb3f0, clockid=0) at sem_waitcommon.c:112
#2  0x00007f7b502922d3 in __new_sem_wait_slow (sem=0x55bc358b9908, abstime=0x7f7ada7fb3f0, clockid=0) at sem_waitcommon.c:184
#3  0x000055bc3382b6c1 in qemu_sem_timedwait (sem=0x55bc358b9908, ms=100) at util/qemu-thread-posix.c:306
#4  0x000055bc3363950b in migration_rate_limit () at migration/migration.c:3365
#5  0x000055bc332b70d3 in ram_save_host_page (rs=0x7f7acc001a70, pss=0x7f7ada7fb4b0, last_stage=true) at /home/lukas/qemu/migration/ram.c:1696
#6  0x000055bc332b71fa in ram_find_and_save_block (rs=0x7f7acc001a70, last_stage=true) at /home/lukas/qemu/migration/ram.c:1750
#7  0x000055bc332b8bbd in ram_save_complete (f=0x55bc36661330, opaque=0x55bc33fbc678 <ram_state>) at /home/lukas/qemu/migration/ram.c:2606
#8  0x000055bc3364112c in qemu_savevm_state_complete_precopy_iterable (f=0x55bc36661330, in_postcopy=false) at migration/savevm.c:1344
#9  0x000055bc33641556 in qemu_savevm_state_complete_precopy (f=0x55bc36661330, iterable_only=true, inactivate_disks=false) at migration/savevm.c:1442
#10 0x000055bc33641982 in qemu_savevm_live_state (f=0x55bc36661330) at migration/savevm.c:1569
#11 0x000055bc33645407 in colo_do_checkpoint_transaction (s=0x55bc358b9840, bioc=0x7f7acc059990, fb=0x7f7acc4627b0) at migration/colo.c:464
#12 0x000055bc336457ca in colo_process_checkpoint (s=0x55bc358b9840) at migration/colo.c:589
#13 0x000055bc336459e4 in migrate_start_colo_process (s=0x55bc358b9840) at migration/colo.c:666
#14 0x000055bc336393d7 in migration_iteration_finish (s=0x55bc358b9840) at migration/migration.c:3312
#15 0x000055bc33639753 in migration_thread (opaque=0x55bc358b9840) at migration/migration.c:3477
#16 0x000055bc3382bbb5 in qemu_thread_start (args=0x55bc357c27c0) at util/qemu-thread-posix.c:519
#17 0x00007f7b50288f27 in start_thread (arg=<optimized out>) at pthread_create.c:479
#18 0x00007f7b501ba31f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

It hangs in ram_save_host_page for at least 10 Minutes.

Regards,
Lukas Straub

> Dave
> 
> > >      if (qemu_file_get_error(f)) {
> > >          return 1;
> > > --
> > > 2.20.1  
> >   
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> 


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