Re: [PATCH 04/20] migration: Introduce postcopy channels on dest node

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

* Peter Xu (peterx@redhat.com) wrote:
> Postcopy handles huge pages in a special way that currently we can only have
> one "channel" to transfer the page.
> 
> It's because when we install pages using UFFDIO_COPY, we need to have the whole
> huge page ready, it also means we need to have a temp huge page when trying to
> receive the whole content of the page.
> 
> Currently all maintainance around this tmp page is global: firstly we'll
> allocate a temp huge page, then we maintain its status mostly within
> ram_load_postcopy().
> 
> To enable multiple channels for postcopy, the first thing we need to do is to
> prepare N temp huge pages as caching, one for each channel.
> 
> Meanwhile we need to maintain the tmp huge page status per-channel too.
> 
> To give some example, some local variables maintained in ram_load_postcopy()
> are listed; they are responsible for maintaining temp huge page status:
> 
>   - all_zero:     this keeps whether this huge page contains all zeros
>   - target_pages: this counts how many target pages have been copied
>   - host_page:    this keeps the host ptr for the page to install
> 
> Move all these fields to be together with the temp huge pages to form a new
> structure called PostcopyTmpPage.  Then for each (future) postcopy channel, we
> need one structure to keep the state around.
> 
> For vanilla postcopy, obviously there's only one channel.  It contains both
> precopy and postcopy pages.
> 
> This patch teaches the dest migration node to start realize the possible number
> of postcopy channels by introducing the "postcopy_channels" variable.  Its
> value is calculated when setup postcopy on dest node (during POSTCOPY_LISTEN
> phase).
> 
> Vanilla postcopy will have channels=1, but when postcopy-preempt capability is
> enabled (in the future), we will boost it to 2 because even during partial
> sending of a precopy huge page we still want to preempt it and start sending
> the postcopy requested page right away (so we start to keep two temp huge
> pages; more if we want to enable multifd).  In this patch there's a TODO marked
> for that; so far the channels is always set to 1.
> 
> We need to send one "host huge page" on one channel only and we cannot split
> them, because otherwise the data upon the same huge page can locate on more
> than one channel so we need more complicated logic to manage.  One temp host
> huge page for each channel will be enough for us for now.
> 
> Postcopy will still always use the index=0 huge page even after this patch.
> However it prepares for the latter patches where it can start to use multiple
> channels (which needs src intervention, because only src knows which channel we
> should use).
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
>  migration/migration.h    | 36 +++++++++++++++++++++++-
>  migration/postcopy-ram.c | 60 ++++++++++++++++++++++++++++++----------
>  migration/ram.c          | 43 ++++++++++++++--------------
>  migration/savevm.c       | 12 ++++++++
>  4 files changed, 113 insertions(+), 38 deletions(-)
> 
> diff --git a/migration/migration.h b/migration/migration.h
> index 8130b703eb..42c7395094 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -45,6 +45,24 @@ struct PostcopyBlocktimeContext;
>   */
>  #define CLEAR_BITMAP_SHIFT_MAX            31
>  
> +/* This is an abstraction of a "temp huge page" for postcopy's purpose */
> +typedef struct {
> +    /*
> +     * This points to a temporary huge page as a buffer for UFFDIO_COPY.  It's
> +     * mmap()ed and needs to be freed when cleanup.
> +     */
> +    void *tmp_huge_page;
> +    /*
> +     * This points to the host page we're going to install for this temp page.
> +     * It tells us after we've received the whole page, where we should put it.
> +     */
> +    void *host_addr;
> +    /* Number of small pages copied (in size of TARGET_PAGE_SIZE) */
> +    unsigned int target_pages;
> +    /* Whether this page contains all zeros */
> +    bool all_zero;
> +} PostcopyTmpPage;
> +
>  /* State for the incoming migration */
>  struct MigrationIncomingState {
>      QEMUFile *from_src_file;
> @@ -81,7 +99,22 @@ struct MigrationIncomingState {
>      QemuMutex rp_mutex;    /* We send replies from multiple threads */
>      /* RAMBlock of last request sent to source */
>      RAMBlock *last_rb;
> -    void     *postcopy_tmp_page;
> +    /*
> +     * Number of postcopy channels including the default precopy channel, so
> +     * vanilla postcopy will only contain one channel which contain both
> +     * precopy and postcopy streams.
> +     *
> +     * This is calculated when the src requests to enable postcopy but before
> +     * it starts.  Its value can depend on e.g. whether postcopy preemption is
> +     * enabled.
> +     */
> +    unsigned int postcopy_channels;
> +    /*
> +     * An array of temp host huge pages to be used, one for each postcopy
> +     * channel.
> +     */
> +    PostcopyTmpPage *postcopy_tmp_pages;
> +    /* This is shared for all postcopy channels */
>      void     *postcopy_tmp_zero_page;
>      /* PostCopyFD's for external userfaultfds & handlers of shared memory */
>      GArray   *postcopy_remote_fds;
> @@ -391,5 +424,6 @@ bool migration_rate_limit(void);
>  void migration_cancel(const Error *error);
>  
>  void populate_vfio_info(MigrationInfo *info);
> +void postcopy_temp_page_reset(PostcopyTmpPage *tmp_page);
>  
>  #endif
> diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> index e662dd05cc..315f784965 100644
> --- a/migration/postcopy-ram.c
> +++ b/migration/postcopy-ram.c
> @@ -525,9 +525,18 @@ int postcopy_ram_incoming_init(MigrationIncomingState *mis)
>  
>  static void postcopy_temp_pages_cleanup(MigrationIncomingState *mis)
>  {
> -    if (mis->postcopy_tmp_page) {
> -        munmap(mis->postcopy_tmp_page, mis->largest_page_size);
> -        mis->postcopy_tmp_page = NULL;
> +    int i;
> +
> +    if (mis->postcopy_tmp_pages) {
> +        for (i = 0; i < mis->postcopy_channels; i++) {
> +            if (mis->postcopy_tmp_pages[i].tmp_huge_page) {
> +                munmap(mis->postcopy_tmp_pages[i].tmp_huge_page,
> +                       mis->largest_page_size);
> +                mis->postcopy_tmp_pages[i].tmp_huge_page = NULL;
> +            }
> +        }
> +        g_free(mis->postcopy_tmp_pages);
> +        mis->postcopy_tmp_pages = NULL;
>      }
>  
>      if (mis->postcopy_tmp_zero_page) {
> @@ -1091,17 +1100,30 @@ retry:
>  
>  static int postcopy_temp_pages_setup(MigrationIncomingState *mis)
>  {
> -    int err;
> -
> -    mis->postcopy_tmp_page = mmap(NULL, mis->largest_page_size,
> -                                  PROT_READ | PROT_WRITE,
> -                                  MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> -    if (mis->postcopy_tmp_page == MAP_FAILED) {
> -        err = errno;
> -        mis->postcopy_tmp_page = NULL;
> -        error_report("%s: Failed to map postcopy_tmp_page %s",
> -                     __func__, strerror(err));
> -        return -err;
> +    PostcopyTmpPage *tmp_page;
> +    int err, i, channels;
> +    void *temp_page;
> +
> +    /* TODO: will be boosted when enable postcopy preemption */
> +    mis->postcopy_channels = 1;
> +
> +    channels = mis->postcopy_channels;
> +    mis->postcopy_tmp_pages = g_malloc0_n(sizeof(PostcopyTmpPage), channels);
> +
> +    for (i = 0; i < channels; i++) {
> +        tmp_page = &mis->postcopy_tmp_pages[i];
> +        temp_page = mmap(NULL, mis->largest_page_size, PROT_READ | PROT_WRITE,
> +                         MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> +        if (temp_page == MAP_FAILED) {
> +            err = errno;
> +            error_report("%s: Failed to map postcopy_tmp_pages[%d]: %s",
> +                         __func__, i, strerror(err));
> +            /* Clean up will be done later */
> +            return -err;
> +        }
> +        tmp_page->tmp_huge_page = temp_page;
> +        /* Initialize default states for each tmp page */
> +        postcopy_temp_page_reset(tmp_page);
>      }
>  
>      /*
> @@ -1351,6 +1373,16 @@ int postcopy_wake_shared(struct PostCopyFD *pcfd,
>  #endif
>  
>  /* ------------------------------------------------------------------------- */
> +void postcopy_temp_page_reset(PostcopyTmpPage *tmp_page)
> +{
> +    tmp_page->target_pages = 0;
> +    tmp_page->host_addr = NULL;
> +    /*
> +     * This is set to true when reset, and cleared as long as we received any
> +     * of the non-zero small page within this huge page.
> +     */
> +    tmp_page->all_zero = true;
> +}
>  
>  void postcopy_fault_thread_notify(MigrationIncomingState *mis)
>  {
> diff --git a/migration/ram.c b/migration/ram.c
> index 91ca743ac8..36b0a53afe 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -3640,11 +3640,8 @@ static int ram_load_postcopy(QEMUFile *f)
>      bool place_needed = false;
>      bool matches_target_page_size = false;
>      MigrationIncomingState *mis = migration_incoming_get_current();
> -    /* Temporary page that is later 'placed' */
> -    void *postcopy_host_page = mis->postcopy_tmp_page;
> -    void *host_page = NULL;
> -    bool all_zero = true;
> -    int target_pages = 0;
> +    /* Currently we only use channel 0.  TODO: use all the channels */
> +    PostcopyTmpPage *tmp_page = &mis->postcopy_tmp_pages[0];
>  
>      while (!ret && !(flags & RAM_SAVE_FLAG_EOS)) {
>          ram_addr_t addr;
> @@ -3688,7 +3685,7 @@ static int ram_load_postcopy(QEMUFile *f)
>                  ret = -EINVAL;
>                  break;
>              }
> -            target_pages++;
> +            tmp_page->target_pages++;
>              matches_target_page_size = block->page_size == TARGET_PAGE_SIZE;
>              /*
>               * Postcopy requires that we place whole host pages atomically;
> @@ -3700,15 +3697,16 @@ static int ram_load_postcopy(QEMUFile *f)
>               * however the source ensures it always sends all the components
>               * of a host page in one chunk.
>               */
> -            page_buffer = postcopy_host_page +
> +            page_buffer = tmp_page->tmp_huge_page +
>                            host_page_offset_from_ram_block_offset(block, addr);
>              /* If all TP are zero then we can optimise the place */
> -            if (target_pages == 1) {
> -                host_page = host_page_from_ram_block_offset(block, addr);
> -            } else if (host_page != host_page_from_ram_block_offset(block,
> -                                                                    addr)) {
> +            if (tmp_page->target_pages == 1) {
> +                tmp_page->host_addr =
> +                    host_page_from_ram_block_offset(block, addr);
> +            } else if (tmp_page->host_addr !=
> +                       host_page_from_ram_block_offset(block, addr)) {
>                  /* not the 1st TP within the HP */
> -                error_report("Non-same host page %p/%p", host_page,
> +                error_report("Non-same host page %p/%p", tmp_page->host_addr,
>                               host_page_from_ram_block_offset(block, addr));
>                  ret = -EINVAL;
>                  break;
> @@ -3718,10 +3716,11 @@ static int ram_load_postcopy(QEMUFile *f)
>               * If it's the last part of a host page then we place the host
>               * page
>               */
> -            if (target_pages == (block->page_size / TARGET_PAGE_SIZE)) {
> +            if (tmp_page->target_pages ==
> +                (block->page_size / TARGET_PAGE_SIZE)) {
>                  place_needed = true;
>              }
> -            place_source = postcopy_host_page;
> +            place_source = tmp_page->tmp_huge_page;
>          }
>  
>          switch (flags & ~RAM_SAVE_FLAG_CONTINUE) {
> @@ -3735,12 +3734,12 @@ static int ram_load_postcopy(QEMUFile *f)
>                  memset(page_buffer, ch, TARGET_PAGE_SIZE);
>              }
>              if (ch) {
> -                all_zero = false;
> +                tmp_page->all_zero = false;
>              }
>              break;
>  
>          case RAM_SAVE_FLAG_PAGE:
> -            all_zero = false;
> +            tmp_page->all_zero = false;
>              if (!matches_target_page_size) {
>                  /* For huge pages, we always use temporary buffer */
>                  qemu_get_buffer(f, page_buffer, TARGET_PAGE_SIZE);
> @@ -3758,7 +3757,7 @@ static int ram_load_postcopy(QEMUFile *f)
>              }
>              break;
>          case RAM_SAVE_FLAG_COMPRESS_PAGE:
> -            all_zero = false;
> +            tmp_page->all_zero = false;
>              len = qemu_get_be32(f);
>              if (len < 0 || len > compressBound(TARGET_PAGE_SIZE)) {
>                  error_report("Invalid compressed data length: %d", len);
> @@ -3790,16 +3789,14 @@ static int ram_load_postcopy(QEMUFile *f)
>          }
>  
>          if (!ret && place_needed) {
> -            if (all_zero) {
> -                ret = postcopy_place_page_zero(mis, host_page, block);
> +            if (tmp_page->all_zero) {
> +                ret = postcopy_place_page_zero(mis, tmp_page->host_addr, block);
>              } else {
> -                ret = postcopy_place_page(mis, host_page, place_source,
> +                ret = postcopy_place_page(mis, tmp_page->host_addr, place_source,
>                                            block);
>              }
>              place_needed = false;
> -            target_pages = 0;
> -            /* Assume we have a zero page until we detect something different */
> -            all_zero = true;
> +            postcopy_temp_page_reset(tmp_page);
>          }
>      }
>  
> diff --git a/migration/savevm.c b/migration/savevm.c
> index 41e3238798..0ccd7e5e3f 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -2579,6 +2579,18 @@ void qemu_loadvm_state_cleanup(void)
>  /* Return true if we should continue the migration, or false. */
>  static bool postcopy_pause_incoming(MigrationIncomingState *mis)
>  {
> +    int i;
> +
> +    /*
> +     * If network is interrupted, any temp page we received will be useless
> +     * because we didn't mark them as "received" in receivedmap.  After a
> +     * proper recovery later (which will sync src dirty bitmap with receivedmap
> +     * on dest) these cached small pages will be resent again.
> +     */
> +    for (i = 0; i < mis->postcopy_channels; i++) {
> +        postcopy_temp_page_reset(&mis->postcopy_tmp_pages[i]);
> +    }
> +
>      trace_postcopy_pause_incoming();
>  
>      assert(migrate_postcopy_ram());
> -- 
> 2.32.0
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK