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

[Hao Xiang]

On Wed, Feb 21, 2024 at 8:11 AM Elena Ufimtseva <ufimtseva@gmail.com> wrote:
>
>
>
> On Fri, Feb 16, 2024 at 2:42 PM Hao Xiang <hao.xiang@bytedance.com> wrote:
>>
>> This change adds a dedicated handler for MigrationOps::ram_save_target_page 
>> in
>> 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"
>>  #include "qemu/rcu.h"
>> +#include "qemu/cutils.h"
>>  #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),
>>
>>      /* 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;
>> -    }
>> -
>>      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());
>
> We only call ram_save_multifd_page() if:
>  if (migrate_multifd()) {
>         migration_ops->ram_save_target_page = ram_save_target_page_multifd;
> So this assert is not needed.

The point of an assert is to ensure the current function is called
with the correct assumptions. In the future, if someone moves this
function to a different place, we can catch the potential issues.

>
>> +    assert(!migrate_compress());
>>
>> +    assert(!migration_in_postcopy());
>
> These two are redundant and done before we call in here.
>
>> +
>>      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);
>> +}
>> +
>> +/**
>> + * ram_save_target_page_multifd: save one target page
>> + *
>> + * Returns the number of pages written
>> + *
>> + * @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());
>
> Do we need to check this for every page?
>
>>
>> +    /* Multifd is not compabible with postcopy. */
>> +    assert(!migration_in_postcopy());
>> +
>>      /*
>> -     * 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();
>> --
>> 2.30.2
>>
>>
>
>
> --
> Elena