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

[Fabiano Rosas]

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.

> 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?

> -
>      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.

> +
>      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.

> +}
> +
> +/**
> + * 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.

> + *
> + * @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();