Re: [PATCH v11 4/5] migration: implementation of background snapshot thread

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Andrey Gruzdev (andrey.gruzdev@virtuozzo.com) wrote:
> On 21.01.2021 12:56, Dr. David Alan Gilbert wrote:
> > * Andrey Gruzdev (andrey.gruzdev@virtuozzo.com) wrote:
> > > On 19.01.2021 21:49, Dr. David Alan Gilbert wrote:
> > > > * Andrey Gruzdev (andrey.gruzdev@virtuozzo.com) wrote:
> > > > > Introducing implementation of 'background' snapshot thread
> > > > > which in overall follows the logic of precopy migration
> > > > > while internally utilizes completely different mechanism
> > > > > to 'freeze' vmstate at the start of snapshot creation.
> > > > > 
> > > > > This mechanism is based on userfault_fd with wr-protection
> > > > > support and is Linux-specific.
> > > > I noticed there weren't any bdrv_ calls in here; I guess with a snapshot
> > > > you still have the source running so still have it accessing the disk;
> > > > do you do anything to try and wire the ram snapshotting up to disk
> > > > snapshotting?
> > > Block-related manipulations should be done externally, I think.
> > > So create backing images for RW nodes, then stop VM, switch block graph
> > > and start background snapshot. Something like create 'virsh snapshot-create-as'
> > > does, but in other sequence.
> > If you get a chance it would be great if you could put together an
> > example of doing the combination RAM+block; that way we find out if there's
> > anything silly missing.
> > 
> > Dave
> 
> Yep, I'll take a look at the QMP command sequences, how it should look
> like in our case and prepare an example, hope we are not missing something serious.
> At least we know that block setup data won't go to snapshot.
> I've also checked starting background snapshot from the stopped VM state -
> looks OK, VM resumes operation, snapshot is saved, no apparent problems.
> 
> Maybe it will take some time, since now I'm on task to create tool to store
> snapshots with RAM indexable by GPFNs, together with the rest of VMSTATE.

If you want to make it indexable, why not just do a simple write(2) call
for the whole of RAM rather than doing the thing like normal migration?

Dave

> Based on QCOW2 format. Also it should support snapshot revert in postcopy mode.
> 
> Andrey
> 
> > > //
> > > 
> > > > > Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
> > > > > Acked-by: Peter Xu <peterx@redhat.com>
> > > > > ---
> > > > >    migration/migration.c | 255 +++++++++++++++++++++++++++++++++++++++++-
> > > > >    migration/migration.h |   3 +
> > > > >    migration/ram.c       |   2 +
> > > > >    migration/savevm.c    |   1 -
> > > > >    migration/savevm.h    |   2 +
> > > > >    5 files changed, 260 insertions(+), 3 deletions(-)
> > > > > 
> > > > > diff --git a/migration/migration.c b/migration/migration.c
> > > > > index 2c2cb9ef01..0901a15ac5 100644
> > > > <snip>
> > > > 
> > > > > -    qemu_thread_create(&s->thread, "live_migration", migration_thread, s,
> > > > > -                       QEMU_THREAD_JOINABLE);
> > > > > +
> > > > > +    if (migrate_background_snapshot()) {
> > > > > +        qemu_thread_create(&s->thread, "background_snapshot",
> > > > Unfortunately that wont work - there's a 14 character limit on
> > > > the thread name length; I guess we just shorten that to bg_snapshot
> > > Yep, missed that pthread_set_name_np() has a length limit)
> > > 
> > > > Other than that,
> > > > 
> > > > 
> > > > 
> > > > Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > > > 
> > > > > +                bg_migration_thread, s, QEMU_THREAD_JOINABLE);
> > > > > +    } else {
> > > > > +        qemu_thread_create(&s->thread, "live_migration",
> > > > > +                migration_thread, s, QEMU_THREAD_JOINABLE);
> > > > > +    }
> > > > >        s->migration_thread_running = true;
> > > > >    }
> > > > > diff --git a/migration/migration.h b/migration/migration.h
> > > > > index f40338cfbf..0723955cd7 100644
> > > > > --- a/migration/migration.h
> > > > > +++ b/migration/migration.h
> > > > > @@ -20,6 +20,7 @@
> > > > >    #include "qemu/thread.h"
> > > > >    #include "qemu/coroutine_int.h"
> > > > >    #include "io/channel.h"
> > > > > +#include "io/channel-buffer.h"
> > > > >    #include "net/announce.h"
> > > > >    #include "qom/object.h"
> > > > > @@ -147,8 +148,10 @@ struct MigrationState {
> > > > >        /*< public >*/
> > > > >        QemuThread thread;
> > > > > +    QEMUBH *vm_start_bh;
> > > > >        QEMUBH *cleanup_bh;
> > > > >        QEMUFile *to_dst_file;
> > > > > +    QIOChannelBuffer *bioc;
> > > > >        /*
> > > > >         * Protects to_dst_file pointer.  We need to make sure we won't
> > > > >         * yield or hang during the critical section, since this lock will
> > > > > diff --git a/migration/ram.c b/migration/ram.c
> > > > > index 5707382db1..05fe0c8592 100644
> > > > > --- a/migration/ram.c
> > > > > +++ b/migration/ram.c
> > > > > @@ -1471,6 +1471,7 @@ static RAMBlock *poll_fault_page(RAMState *rs, ram_addr_t *offset)
> > > > >        page_address = (void *) uffd_msg.arg.pagefault.address;
> > > > >        bs = qemu_ram_block_from_host(page_address, false, offset);
> > > > >        assert(bs && (bs->flags & RAM_UF_WRITEPROTECT) != 0);
> > > > > +
> > > > >        return bs;
> > > > >    }
> > > > >    #endif /* CONFIG_LINUX */
> > > > > @@ -1836,6 +1837,7 @@ static void ram_save_host_page_post(RAMState *rs, PageSearchStatus *pss,
> > > > >            /* Un-protect memory range. */
> > > > >            res = uffd_change_protection(rs->uffdio_fd, page_address, run_length,
> > > > >                    false, false);
> > > > > +
> > > > >            /* We don't want to override existing error from ram_save_host_page(). */
> > > > >            if (res < 0 && *res_override >= 0) {
> > > > >                *res_override = res;
> > > > > diff --git a/migration/savevm.c b/migration/savevm.c
> > > > > index 27e842812e..dd4ad0aaaf 100644
> > > > > --- a/migration/savevm.c
> > > > > +++ b/migration/savevm.c
> > > > > @@ -1354,7 +1354,6 @@ int qemu_savevm_state_complete_precopy_iterable(QEMUFile *f, bool in_postcopy)
> > > > >        return 0;
> > > > >    }
> > > > > -static
> > > > >    int qemu_savevm_state_complete_precopy_non_iterable(QEMUFile *f,
> > > > >                                                        bool in_postcopy,
> > > > >                                                        bool inactivate_disks)
> > > > > diff --git a/migration/savevm.h b/migration/savevm.h
> > > > > index ba64a7e271..aaee2528ed 100644
> > > > > --- a/migration/savevm.h
> > > > > +++ b/migration/savevm.h
> > > > > @@ -64,5 +64,7 @@ int qemu_loadvm_state(QEMUFile *f);
> > > > >    void qemu_loadvm_state_cleanup(void);
> > > > >    int qemu_loadvm_state_main(QEMUFile *f, MigrationIncomingState *mis);
> > > > >    int qemu_load_device_state(QEMUFile *f);
> > > > > +int qemu_savevm_state_complete_precopy_non_iterable(QEMUFile *f,
> > > > > +        bool in_postcopy, bool inactivate_disks);
> > > > >    #endif
> > > > > -- 
> > > > > 2.25.1
> > > > > 
> > > -- 
> > > Andrey Gruzdev, Principal Engineer
> > > Virtuozzo GmbH  +7-903-247-6397
> > >                  virtuzzo.com
> > > 
> -- 
> Andrey Gruzdev, Principal Engineer
> Virtuozzo GmbH  +7-903-247-6397
>                 virtuzzo.com
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK