Re: [Qemu-devel] [PATCH v7 32/42] Page request: Consume pages off the post-copy queue

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

* Amit Shah (amit.shah@redhat.com) wrote:
> On (Tue) 16 Jun 2015 [11:26:45], Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > When transmitting RAM pages, consume pages that have been queued by
> > MIG_RPCOMM_REQPAGE commands and send them ahead of normal page scanning.
> 
> It's slightly confusing with 'consume': we're /servicing/ requests from
> the dest at the src here rather than /consuming/ pages sent by src at
> the dest.  If you find 'service' better than 'consume', please update
> the commit msg+log.

'consume' is a fairly normal term for taking an item off a queue and
processing it.

> > Note:
> >   a) After a queued page the linear walk carries on from after the
> > unqueued page; there is a reasonable chance that the destination
> > was about to ask for other closeby pages anyway.
> > 
> >   b) We have to be careful of any assumptions that the page walking
> > code makes, in particular it does some short cuts on its first linear
> > walk that break as soon as we do a queued page.
> > 
> >   c) We have to be careful to not break up host-page size chunks, since
> > this makes it harder to place the pages on the destination.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> 
> Reviewed-by: Amit Shah <amit.shah@redhat.com>
> 
> > +static int ram_save_host_page(MigrationState *ms, QEMUFile *f, RAMBlock* block,
> > +                              ram_addr_t *offset, bool last_stage,
> > +                              uint64_t *bytes_transferred,
> > +                              ram_addr_t dirty_ram_abs)
> > +{
> > +    int tmppages, pages = 0;
> > +    do {
> > +        /* Check the pages is dirty and if it is send it */
> > +        if (migration_bitmap_clear_dirty(dirty_ram_abs)) {
> > +            if (compression_switch && migrate_use_compression()) {
> > +                tmppages = ram_save_compressed_page(f, block, *offset,
> > +                                                    last_stage,
> > +                                                    bytes_transferred);
> > +            } else {
> > +                tmppages = ram_save_page(f, block, *offset, last_stage,
> > +                                         bytes_transferred);
> > +            }
> 
> Something for the future: we should just have ram_save_page which does
> compression (or not); and even encryption (or not), and so on.

Yep, in my current world that's now a 'ram_save_host_page' function
that has that buried in it.

> > +
> > +            if (tmppages < 0) {
> > +                return tmppages;
> > +            } else {
> > +                if (ms->sentmap) {
> > +                    set_bit(dirty_ram_abs >> TARGET_PAGE_BITS, ms->sentmap);
> > +                }
> > +            }
> 
> This else could be dropped as the if stmt returns.

Done

> > +            pages += tmppages;
> > +        }
> > +        *offset += TARGET_PAGE_SIZE;
> > +        dirty_ram_abs += TARGET_PAGE_SIZE;
> > +    } while (*offset & (qemu_host_page_size - 1));
> > +
> > +    /* The offset we leave with is the last one we looked at */
> > +    *offset -= TARGET_PAGE_SIZE;
> > +    return pages;
> > +}
> > +
> > +/**
> >   * ram_find_and_save_block: Finds a dirty page and sends it to f
> >   *
> >   * Called within an RCU critical section.
> > @@ -997,65 +1094,102 @@ err:
> >   * @f: QEMUFile where to send the data
> >   * @last_stage: if we are at the completion stage
> >   * @bytes_transferred: increase it with the number of transferred bytes
> > + *
> > + * On systems where host-page-size > target-page-size it will send all the
> > + * pages in a host page that are dirty.
> >   */
> >  
> >  static int ram_find_and_save_block(QEMUFile *f, bool last_stage,
> >                                     uint64_t *bytes_transferred)
> >  {
> > +    MigrationState *ms = migrate_get_current();
> >      RAMBlock *block = last_seen_block;
> > +    RAMBlock *tmpblock;
> >      ram_addr_t offset = last_offset;
> > +    ram_addr_t tmpoffset;
> >      bool complete_round = false;
> >      int pages = 0;
> > -    MemoryRegion *mr;
> >      ram_addr_t dirty_ram_abs; /* Address of the start of the dirty page in
> >                                   ram_addr_t space */
> >  
> > -    if (!block)
> > +    if (!block) {
> >          block = QLIST_FIRST_RCU(&ram_list.blocks);
> > +        last_was_from_queue = false;
> > +    }
> >  
> > -    while (true) {
> > -        mr = block->mr;
> > -        offset = migration_bitmap_find_and_reset_dirty(mr, offset,
> > -                                                       &dirty_ram_abs);
> > -        if (complete_round && block == last_seen_block &&
> > -            offset >= last_offset) {
> > -            break;
> > -        }
> > -        if (offset >= block->used_length) {
> > -            offset = 0;
> > -            block = QLIST_NEXT_RCU(block, next);
> > -            if (!block) {
> > -                block = QLIST_FIRST_RCU(&ram_list.blocks);
> > -                complete_round = true;
> > -                ram_bulk_stage = false;
> > -                if (migrate_use_xbzrle()) {
> > -                    /* If xbzrle is on, stop using the data compression at this
> > -                     * point. In theory, xbzrle can do better than compression.
> > -                     */
> > -                    flush_compressed_data(f);
> > -                    compression_switch = false;
> > -                }
> > +    while (true) { /* Until we send a block or run out of stuff to send */
> > +        tmpblock = ram_save_unqueue_page(ms, &tmpoffset, &dirty_ram_abs);
> > +
> > +        if (tmpblock) {
> > +            /* We've got a block from the postcopy queue */
> > +            trace_ram_find_and_save_block_postcopy(tmpblock->idstr,
> > +                                                   (uint64_t)tmpoffset,
> > +                                                   (uint64_t)dirty_ram_abs);
> > +            /*
> > +             * We're sending this page, and since it's postcopy nothing else
> > +             * will dirty it, and we must make sure it doesn't get sent again
> > +             * even if this queue request was received after the background
> > +             * search already sent it.
> > +             */
> > +            if (!test_bit(dirty_ram_abs >> TARGET_PAGE_BITS,
> > +                          migration_bitmap)) {
> > +                trace_ram_find_and_save_block_postcopy_not_dirty(
> > +                    tmpblock->idstr, (uint64_t)tmpoffset,
> > +                    (uint64_t)dirty_ram_abs,
> > +                    test_bit(dirty_ram_abs >> TARGET_PAGE_BITS, ms->sentmap));
> > +
> > +                continue;
> >              }
> > +            /*
> > +             * As soon as we start servicing pages out of order, then we have
> > +             * to kill the bulk stage, since the bulk stage assumes
> > +             * in (migration_bitmap_find_and_reset_dirty) that every page is
> > +             * dirty, that's no longer true.
> > +             */
> > +            ram_bulk_stage = false;
> > +            /*
> > +             * We want the background search to continue from the queued page
> > +             * since the guest is likely to want other pages near to the page
> > +             * it just requested.
> > +             */
> > +            block = tmpblock;
> > +            offset = tmpoffset;
> >          } else {
> > -            if (compression_switch && migrate_use_compression()) {
> > -                pages = ram_save_compressed_page(f, block, offset, last_stage,
> > -                                                 bytes_transferred);
> > -            } else {
> > -                pages = ram_save_page(f, block, offset, last_stage,
> > -                                      bytes_transferred);
> > +            MemoryRegion *mr;
> > +            /* priority queue empty, so just search for something dirty */
> > +            mr = block->mr;
> > +            offset = migration_bitmap_find_dirty(mr, offset, &dirty_ram_abs);
> > +            if (complete_round && block == last_seen_block &&
> > +                offset >= last_offset) {
> > +                break;
> >              }
> > -
> > -            /* if page is unmodified, continue to the next */
> > -            if (pages > 0) {
> > -                MigrationState *ms = migrate_get_current();
> > -                if (ms->sentmap) {
> > -                    set_bit(dirty_ram_abs >> TARGET_PAGE_BITS, ms->sentmap);
> > +            if (offset >= block->used_length) {
> > +                offset = 0;
> > +                block = QLIST_NEXT_RCU(block, next);
> > +                if (!block) {
> > +                    block = QLIST_FIRST_RCU(&ram_list.blocks);
> > +                    complete_round = true;
> > +                    ram_bulk_stage = false;
> > +                    if (migrate_use_xbzrle()) {
> > +                        /* If xbzrle is on, stop using the data compression at
> > +                         * this point. In theory, xbzrle can do better than
> > +                         * compression.
> > +                         */
> > +                        flush_compressed_data(f);
> > +                        compression_switch = false;
> > +                    }
> >                  }
> > -
> > -                last_sent_block = block;
> > -                break;
> > +                continue; /* pick an offset in the new block */
> >              }
> >          }
> > +
> > +        pages = ram_save_host_page(ms, f, block, &offset, last_stage,
> > +                                   bytes_transferred, dirty_ram_abs);
> > +
> > +        /* if page is unmodified, continue to the next */
> > +        if (pages > 0) {
> > +            break;
> > +        }
> 
> This function could use splitting into multiple ones.

Done, a separate pair of patches is on list to do that split; please review.

Dave

> 
> 
> 		Amit
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK