Re: [Qemu-devel] [PATCH v3 22/29] vhost+postcopy: Call wakeups

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

* Peter Xu (peterx@redhat.com) wrote:
> On Fri, Feb 16, 2018 at 01:16:18PM +0000, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > Cause the vhost-user client to be woken up whenever:
> >   a) We place a page in postcopy mode
> >   b) We get a fault and the page has already been received
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> >  migration/postcopy-ram.c | 14 ++++++++++----
> >  migration/trace-events   |  1 +
> >  2 files changed, 11 insertions(+), 4 deletions(-)
> > 
> > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> > index 879711968c..13561703b5 100644
> > --- a/migration/postcopy-ram.c
> > +++ b/migration/postcopy-ram.c
> > @@ -566,7 +566,11 @@ int postcopy_request_shared_page(struct PostCopyFD *pcfd, RAMBlock *rb,
> >  
> >      trace_postcopy_request_shared_page(pcfd->idstr, qemu_ram_get_idstr(rb),
> >                                         rb_offset);
> > -    /* TODO: Check bitmap to see if we already have the page */
> > +    if (ramblock_recv_bitmap_test_byte_offset(rb, aligned_rbo)) {
> > +        trace_postcopy_request_shared_page_present(pcfd->idstr,
> > +                                        qemu_ram_get_idstr(rb), rb_offset);
> > +        return postcopy_wake_shared(pcfd, client_addr, rb);
> > +    }
> >      if (rb != mis->last_rb) {
> >          mis->last_rb = rb;
> >          migrate_send_rp_req_pages(mis, qemu_ram_get_idstr(rb),
> > @@ -863,7 +867,8 @@ int postcopy_place_page(MigrationIncomingState *mis, void *host, void *from,
> >      }
> >  
> >      trace_postcopy_place_page(host);
> > -    return 0;
> > +    return postcopy_notify_shared_wake(rb,
> > +                                       qemu_ram_block_host_offset(rb, host));
> >  }
> >  
> >  /*
> > @@ -887,6 +892,9 @@ int postcopy_place_page_zero(MigrationIncomingState *mis, void *host,
> >  
> >              return -e;
> >          }
> > +        return postcopy_notify_shared_wake(rb,
> > +                                           qemu_ram_block_host_offset(rb,
> > +                                                                      host));
> >      } else {
> >          /* The kernel can't use UFFDIO_ZEROPAGE for hugepages */
> >          if (!mis->postcopy_tmp_zero_page) {
> > @@ -906,8 +914,6 @@ int postcopy_place_page_zero(MigrationIncomingState *mis, void *host,
> >          return postcopy_place_page(mis, host, mis->postcopy_tmp_zero_page,
> >                                     rb);
> >      }
> > -
> > -    return 0;
> >  }
> 
> Could there be race?  E.g.:
> 
>               ram_load_thread             page_fault_thread
>              -----------------           -------------------
> 
>                                           if (recv_bitmap_set())
>                                               wake()
>              copy_page()
>              recv_bitmap_set()
>              wake()
>                                           request_page()
> 
> Then the last requested page may never be serviced?

The postcopy finishes when the last page is received, and thus when that
also performs the wake() (from the load thread); so that's not a
problem.
You can get the case where a page that qemu has already received, still
needs to be woken for the shared users (which is why we have the wake in
the fault_thread).
When the postcopy finishes, the client is sent a POSTCOPY_END, at which
point it closes it's userfaultfd and it should wake everything remaining
up; so any late requests shouldn't be a problem (the END is sent
before the fault-thread quits).

Dave


> Thanks,
> 
> -- 
> Peter Xu
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK