Re: [External] Re: [PATCH v2 4/7] migration/multifd: Enable zero page checking from multifd threads.

[Hao Xiang]

On Wed, Feb 21, 2024 at 1:06 PM Fabiano Rosas <farosas@suse.de> wrote:
>
> Hao Xiang <hao.xiang@bytedance.com> writes:
>
> > This change adds a dedicated handler for MigrationOps::ram_save_target_page 
> > in
>
> nit: Add a dedicated handler...
>
> Usually "this patch/change" is used only when necessary to avoid
> ambiguity.

Will do.

>
> > multifd live migration. Now zero page checking can be done in the multifd 
> > threads
> > and this becomes the default configuration. We still provide backward 
> > compatibility
> > where zero page checking is done from the migration main thread.
> >
> > Signed-off-by: Hao Xiang <hao.xiang@bytedance.com>
> > ---
> >  migration/multifd.c |  1 +
> >  migration/options.c |  2 +-
> >  migration/ram.c     | 53 ++++++++++++++++++++++++++++++++++-----------
> >  3 files changed, 42 insertions(+), 14 deletions(-)
> >
> > diff --git a/migration/multifd.c b/migration/multifd.c
> > index fbb40ea10b..ef5dad1019 100644
> > --- a/migration/multifd.c
> > +++ b/migration/multifd.c
> > @@ -13,6 +13,7 @@
> >  #include "qemu/osdep.h"
> >  #include "qemu/cutils.h"
>
> This include...
>
> >  #include "qemu/rcu.h"
> > +#include "qemu/cutils.h"
>
> is there already.
>
> >  #include "exec/target_page.h"
> >  #include "sysemu/sysemu.h"
> >  #include "exec/ramblock.h"
> > diff --git a/migration/options.c b/migration/options.c
> > index 3c603391b0..3c79b6ccd4 100644
> > --- a/migration/options.c
> > +++ b/migration/options.c
> > @@ -181,7 +181,7 @@ Property migration_properties[] = {
> >                        MIG_MODE_NORMAL),
> >      DEFINE_PROP_ZERO_PAGE_DETECTION("zero-page-detection", MigrationState,
> >                         parameters.zero_page_detection,
> > -                       ZERO_PAGE_DETECTION_LEGACY),
> > +                       ZERO_PAGE_DETECTION_MULTIFD),
>
> I think we'll need something to avoid a 9.0 -> 8.2 migration with this
> enabled. Otherwise it will go along happily until we get data corruption
> because the new QEMU didn't send any zero pages on the migration thread
> and the old QEMU did not look for them in the multifd packet.
>
> Perhaps bumping the MULTIFD_VERSION when ZERO_PAGE_DETECTION_MULTIFD is
> in use. We'd just need to fix the test in the new QEMU to check
> (msg.version > MULTIFD_VERSION) instead of (msg.version != MULTIFD_VERSION).
>
> >
> >      /* Migration capabilities */
> >      DEFINE_PROP_MIG_CAP("x-xbzrle", MIGRATION_CAPABILITY_XBZRLE),
> > diff --git a/migration/ram.c b/migration/ram.c
> > index 5ece9f042e..b088c5a98c 100644
> > --- a/migration/ram.c
> > +++ b/migration/ram.c
> > @@ -1123,10 +1123,6 @@ static int save_zero_page(RAMState *rs, 
> > PageSearchStatus *pss,
> >      QEMUFile *file = pss->pss_channel;
> >      int len = 0;
> >
> > -    if (migrate_zero_page_detection() != ZERO_PAGE_DETECTION_LEGACY) {
> > -        return 0;
> > -    }
>
> How does 'none' work now?

I tested it and all pages are transferred with payload (including the
zero pages).

>
> > -
> >      if (!buffer_is_zero(p, TARGET_PAGE_SIZE)) {
> >          return 0;
> >      }
> > @@ -1256,6 +1252,10 @@ static int ram_save_page(RAMState *rs, 
> > PageSearchStatus *pss)
> >
> >  static int ram_save_multifd_page(RAMBlock *block, ram_addr_t offset)
> >  {
> > +    assert(migrate_multifd());
> > +    assert(!migrate_compress());
> > +    assert(!migration_in_postcopy());
>
> Drop these, please. Keep only the asserts that are likely to trigger
> during development, such as the existing ones at multifd_send_pages.

I think I have got enough feedback regarding too many asserts. I will
drop these. assert is not compiled into the free build, correct?

>
> > +
> >      if (!multifd_queue_page(block, offset)) {
> >          return -1;
> >      }
> > @@ -2046,7 +2046,6 @@ static bool save_compress_page(RAMState *rs, 
> > PageSearchStatus *pss,
> >   */
> >  static int ram_save_target_page_legacy(RAMState *rs, PageSearchStatus *pss)
> >  {
> > -    RAMBlock *block = pss->block;
> >      ram_addr_t offset = ((ram_addr_t)pss->page) << TARGET_PAGE_BITS;
> >      int res;
> >
> > @@ -2062,17 +2061,40 @@ static int ram_save_target_page_legacy(RAMState 
> > *rs, PageSearchStatus *pss)
> >          return 1;
> >      }
> >
> > +    return ram_save_page(rs, pss);
>
> Look at where git put this! Are you using the default diff algorithm? If
> not try using --patience to see if it improves the diff.

I used the default diff algorithm.

>
> > +}
> > +
> > +/**
> > + * ram_save_target_page_multifd: save one target page
> > + *
> > + * Returns the number of pages written
>
> We could be more precise here:
>
>  ram_save_target_page_multifd: send one target page to multifd workers
>
>  Returns 1 if the page was queued, -1 otherwise.

Will do.

>
> > + *
> > + * @rs: current RAM state
> > + * @pss: data about the page we want to send
> > + */
> > +static int ram_save_target_page_multifd(RAMState *rs, PageSearchStatus 
> > *pss)
> > +{
> > +    RAMBlock *block = pss->block;
> > +    ram_addr_t offset = ((ram_addr_t)pss->page) << TARGET_PAGE_BITS;
> > +
> > +    /* Multifd is not compatible with old compression. */
> > +    assert(!migrate_compress());
>
> This should already be enforced at options.c.
>
> > +
> > +    /* Multifd is not compabible with postcopy. */
> > +    assert(!migration_in_postcopy());
>
> Same here.
>
> > +
> >      /*
> > -     * Do not use multifd in postcopy as one whole host page should be
> > -     * placed.  Meanwhile postcopy requires atomic update of pages, so even
> > -     * if host page size == guest page size the dest guest during run may
> > -     * still see partially copied pages which is data corruption.
> > +     * Backward compatibility support. While using multifd live
> > +     * migration, we still need to handle zero page checking on the
> > +     * migration main thread.
> >       */
> > -    if (migrate_multifd() && !migration_in_postcopy()) {
> > -        return ram_save_multifd_page(block, offset);
> > +    if (migrate_zero_page_detection() == ZERO_PAGE_DETECTION_LEGACY) {
> > +        if (save_zero_page(rs, pss, offset)) {
> > +            return 1;
> > +        }
> >      }
> >
> > -    return ram_save_page(rs, pss);
> > +    return ram_save_multifd_page(block, offset);
> >  }
> >
> >  /* Should be called before sending a host page */
> > @@ -2984,7 +3006,12 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
> >      }
> >
> >      migration_ops = g_malloc0(sizeof(MigrationOps));
> > -    migration_ops->ram_save_target_page = ram_save_target_page_legacy;
> > +
> > +    if (migrate_multifd()) {
> > +        migration_ops->ram_save_target_page = ram_save_target_page_multifd;
> > +    } else {
> > +        migration_ops->ram_save_target_page = ram_save_target_page_legacy;
> > +    }
> >
> >      bql_unlock();
> >      ret = multifd_send_sync_main();