Re: [External] Re: [PATCH v2 3/7] migration/multifd: Zero page transmission on the multifd thread.

[Hao Xiang <hao.xiang@bytedance.com>]

On Wed, Feb 21, 2024 at 1:04 PM Fabiano Rosas <farosas@suse.de> wrote:
>
> Hao Xiang <hao.xiang@bytedance.com> writes:
>
> > 1. Implements the zero page detection and handling on the multifd
> > threads for non-compression, zlib and zstd compression backends.
> > 2. Added a new value 'multifd' in ZeroPageDetection enumeration.
> > 3. Add proper asserts to ensure pages->normal are used for normal pages
> > in all scenarios.
> >
> > Signed-off-by: Hao Xiang <hao.xiang@bytedance.com>
> > ---
> >  migration/meson.build         |  1 +
> >  migration/multifd-zero-page.c | 59 +++++++++++++++++++++++++++++++++++
> >  migration/multifd-zlib.c      | 26 ++++++++++++---
> >  migration/multifd-zstd.c      | 25 ++++++++++++---
> >  migration/multifd.c           | 50 +++++++++++++++++++++++------
> >  migration/multifd.h           |  7 +++++
> >  qapi/migration.json           |  4 ++-
> >  7 files changed, 151 insertions(+), 21 deletions(-)
> >  create mode 100644 migration/multifd-zero-page.c
> >
> > diff --git a/migration/meson.build b/migration/meson.build
> > index 92b1cc4297..1eeb915ff6 100644
> > --- a/migration/meson.build
> > +++ b/migration/meson.build
> > @@ -22,6 +22,7 @@ system_ss.add(files(
> >    'migration.c',
> >    'multifd.c',
> >    'multifd-zlib.c',
> > +  'multifd-zero-page.c',
> >    'ram-compress.c',
> >    'options.c',
> >    'postcopy-ram.c',
> > diff --git a/migration/multifd-zero-page.c b/migration/multifd-zero-page.c
> > new file mode 100644
> > index 0000000000..f0cd8e2c53
> > --- /dev/null
> > +++ b/migration/multifd-zero-page.c
> > @@ -0,0 +1,59 @@
> > +/*
> > + * Multifd zero page detection implementation.
> > + *
> > + * Copyright (c) 2024 Bytedance Inc
> > + *
> > + * Authors:
> > + *  Hao Xiang <hao.xiang@bytedance.com>
> > + *
> > + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> > + * See the COPYING file in the top-level directory.
> > + */
> > +
> > +#include "qemu/osdep.h"
> > +#include "qemu/cutils.h"
> > +#include "exec/ramblock.h"
> > +#include "migration.h"
> > +#include "multifd.h"
> > +#include "options.h"
> > +#include "ram.h"
> > +
> > +void multifd_zero_page_check_send(MultiFDSendParams *p)
> > +{
> > +    /*
> > +     * QEMU older than 9.0 don't understand zero page
> > +     * on multifd channel. This switch is required to
> > +     * maintain backward compatibility.
> > +     */
> > +    bool use_multifd_zero_page =
> > +        (migrate_zero_page_detection() == ZERO_PAGE_DETECTION_MULTIFD);
> > +    MultiFDPages_t *pages = p->pages;
> > +    RAMBlock *rb = pages->block;
> > +
> > +    assert(pages->num != 0);
> > +    assert(pages->normal_num == 0);
> > +    assert(pages->zero_num == 0);
>
> We can drop these before the final version.

Elena has the same concern. I will drop these.

>
> > +
> > +    for (int i = 0; i < pages->num; i++) {
> > +        uint64_t offset = pages->offset[i];
> > +        if (use_multifd_zero_page &&
> > +            buffer_is_zero(rb->host + offset, p->page_size)) {
> > +            pages->zero[pages->zero_num] = offset;
> > +            pages->zero_num++;
> > +            ram_release_page(rb->idstr, offset);
> > +        } else {
> > +            pages->normal[pages->normal_num] = offset;
> > +            pages->normal_num++;
> > +        }
> > +    }
>
> I don't think it's super clean to have three arrays offset, zero and
> normal, all sized for the full packet size. It might be possible to just
> carry a bitmap of non-zero pages along with pages->offset and operate on
> that instead.
>
> What do you think?
>
> Peter, any ideas? Should we just leave this for another time?
>
> > +}
> > +
> > +void multifd_zero_page_check_recv(MultiFDRecvParams *p)
> > +{
> > +    for (int i = 0; i < p->zero_num; i++) {
> > +        void *page = p->host + p->zero[i];
> > +        if (!buffer_is_zero(page, p->page_size)) {
> > +            memset(page, 0, p->page_size);
> > +        }
> > +    }
> > +}
> > diff --git a/migration/multifd-zlib.c b/migration/multifd-zlib.c
> > index 012e3bdea1..cdfe0fa70e 100644
> > --- a/migration/multifd-zlib.c
> > +++ b/migration/multifd-zlib.c
> > @@ -123,13 +123,20 @@ static int zlib_send_prepare(MultiFDSendParams *p, Error **errp)
> >      int ret;
> >      uint32_t i;
> >
> > +    multifd_zero_page_check_send(p);
> > +
> > +    if (!pages->normal_num) {
> > +        p->next_packet_size = 0;
> > +        goto out;
> > +    }
> > +
> >      multifd_send_prepare_header(p);
> >
> > -    for (i = 0; i < pages->num; i++) {
> > +    for (i = 0; i < pages->normal_num; i++) {
> >          uint32_t available = z->zbuff_len - out_size;
> >          int flush = Z_NO_FLUSH;
> >
> > -        if (i == pages->num - 1) {
> > +        if (i == pages->normal_num - 1) {
> >              flush = Z_SYNC_FLUSH;
> >          }
> >
> > @@ -138,7 +145,7 @@ static int zlib_send_prepare(MultiFDSendParams *p, Error **errp)
> >           * with compression. zlib does not guarantee that this is safe,
> >           * therefore copy the page before calling deflate().
> >           */
> > -        memcpy(z->buf, p->pages->block->host + pages->offset[i], p->page_size);
> > +        memcpy(z->buf, p->pages->block->host + pages->normal[i], p->page_size);
> >          zs->avail_in = p->page_size;
> >          zs->next_in = z->buf;
> >
> > @@ -172,10 +179,10 @@ static int zlib_send_prepare(MultiFDSendParams *p, Error **errp)
> >      p->iov[p->iovs_num].iov_len = out_size;
> >      p->iovs_num++;
> >      p->next_packet_size = out_size;
> > -    p->flags |= MULTIFD_FLAG_ZLIB;
> >
> > +out:
> > +    p->flags |= MULTIFD_FLAG_ZLIB;
> >      multifd_send_fill_packet(p);
> > -
> >      return 0;
> >  }
> >
> > @@ -261,6 +268,14 @@ static int zlib_recv_pages(MultiFDRecvParams *p, Error **errp)
> >                     p->id, flags, MULTIFD_FLAG_ZLIB);
> >          return -1;
> >      }
> > +
> > +    multifd_zero_page_check_recv(p);
> > +
> > +    if (!p->normal_num) {
> > +        assert(in_size == 0);
> > +        return 0;
> > +    }
> > +
> >      ret = qio_channel_read_all(p->c, (void *)z->zbuff, in_size, errp);
> >
> >      if (ret != 0) {
> > @@ -310,6 +325,7 @@ static int zlib_recv_pages(MultiFDRecvParams *p, Error **errp)
> >                     p->id, out_size, expected_size);
> >          return -1;
> >      }
> > +
> >      return 0;
> >  }
> >
> > diff --git a/migration/multifd-zstd.c b/migration/multifd-zstd.c
> > index dc8fe43e94..27a1eba075 100644
> > --- a/migration/multifd-zstd.c
> > +++ b/migration/multifd-zstd.c
> > @@ -118,19 +118,26 @@ static int zstd_send_prepare(MultiFDSendParams *p, Error **errp)
> >      int ret;
> >      uint32_t i;
> >
> > +    multifd_zero_page_check_send(p);
> > +
> > +    if (!pages->normal_num) {
> > +        p->next_packet_size = 0;
> > +        goto out;
> > +    }
> > +
> >      multifd_send_prepare_header(p);
> >
> >      z->out.dst = z->zbuff;
> >      z->out.size = z->zbuff_len;
> >      z->out.pos = 0;
> >
> > -    for (i = 0; i < pages->num; i++) {
> > +    for (i = 0; i < pages->normal_num; i++) {
> >          ZSTD_EndDirective flush = ZSTD_e_continue;
> >
> > -        if (i == pages->num - 1) {
> > +        if (i == pages->normal_num - 1) {
> >              flush = ZSTD_e_flush;
> >          }
> > -        z->in.src = p->pages->block->host + pages->offset[i];
> > +        z->in.src = p->pages->block->host + pages->normal[i];
> >          z->in.size = p->page_size;
> >          z->in.pos = 0;
> >
> > @@ -161,10 +168,10 @@ static int zstd_send_prepare(MultiFDSendParams *p, Error **errp)
> >      p->iov[p->iovs_num].iov_len = z->out.pos;
> >      p->iovs_num++;
> >      p->next_packet_size = z->out.pos;
> > -    p->flags |= MULTIFD_FLAG_ZSTD;
> >
> > +out:
> > +    p->flags |= MULTIFD_FLAG_ZSTD;
> >      multifd_send_fill_packet(p);
> > -
> >      return 0;
> >  }
> >
> > @@ -257,6 +264,14 @@ static int zstd_recv_pages(MultiFDRecvParams *p, Error **errp)
> >                     p->id, flags, MULTIFD_FLAG_ZSTD);
> >          return -1;
> >      }
> > +
> > +    multifd_zero_page_check_recv(p);
> > +
> > +    if (!p->normal_num) {
> > +        assert(in_size == 0);
> > +        return 0;
> > +    }
> > +
> >      ret = qio_channel_read_all(p->c, (void *)z->zbuff, in_size, errp);
> >
> >      if (ret != 0) {
> > diff --git a/migration/multifd.c b/migration/multifd.c
> > index a33dba40d9..fbb40ea10b 100644
> > --- a/migration/multifd.c
> > +++ b/migration/multifd.c
> > @@ -11,6 +11,7 @@
> >   */
> >
> >  #include "qemu/osdep.h"
> > +#include "qemu/cutils.h"
> >  #include "qemu/rcu.h"
> >  #include "exec/target_page.h"
> >  #include "sysemu/sysemu.h"
> > @@ -126,6 +127,8 @@ static int nocomp_send_prepare(MultiFDSendParams *p, Error **errp)
> >      MultiFDPages_t *pages = p->pages;
> >      int ret;
> >
> > +    multifd_zero_page_check_send(p);
> > +
> >      if (!use_zero_copy_send) {
> >          /*
> >           * Only !zerocopy needs the header in IOV; zerocopy will
> > @@ -134,13 +137,13 @@ static int nocomp_send_prepare(MultiFDSendParams *p, Error **errp)
> >          multifd_send_prepare_header(p);
> >      }
> >
> > -    for (int i = 0; i < pages->num; i++) {
> > -        p->iov[p->iovs_num].iov_base = pages->block->host + pages->offset[i];
> > +    for (int i = 0; i < pages->normal_num; i++) {
> > +        p->iov[p->iovs_num].iov_base = pages->block->host + pages->normal[i];
> >          p->iov[p->iovs_num].iov_len = p->page_size;
> >          p->iovs_num++;
> >      }
> >
> > -    p->next_packet_size = pages->num * p->page_size;
> > +    p->next_packet_size = pages->normal_num * p->page_size;
> >      p->flags |= MULTIFD_FLAG_NOCOMP;
> >
> >      multifd_send_fill_packet(p);
> > @@ -202,6 +205,13 @@ static int nocomp_recv_pages(MultiFDRecvParams *p, Error **errp)
> >                     p->id, flags, MULTIFD_FLAG_NOCOMP);
> >          return -1;
> >      }
> > +
> > +    multifd_zero_page_check_recv(p);
> > +
> > +    if (!p->normal_num) {
> > +        return 0;
> > +    }
> > +
> >      for (int i = 0; i < p->normal_num; i++) {
> >          p->iov[i].iov_base = p->host + p->normal[i];
> >          p->iov[i].iov_len = p->page_size;
> > @@ -339,7 +349,7 @@ void multifd_send_fill_packet(MultiFDSendParams *p)
> >
> >      packet->flags = cpu_to_be32(p->flags);
> >      packet->pages_alloc = cpu_to_be32(p->pages->allocated);
> > -    packet->normal_pages = cpu_to_be32(pages->num);
> > +    packet->normal_pages = cpu_to_be32(pages->normal_num);
> >      packet->zero_pages = cpu_to_be32(pages->zero_num);
> >      packet->next_packet_size = cpu_to_be32(p->next_packet_size);
> >
> > @@ -350,18 +360,25 @@ void multifd_send_fill_packet(MultiFDSendParams *p)
> >          strncpy(packet->ramblock, pages->block->idstr, 256);
> >      }
> >
> > -    for (i = 0; i < pages->num; i++) {
> > +    for (i = 0; i < pages->normal_num; i++) {
> >          /* there are architectures where ram_addr_t is 32 bit */
> > -        uint64_t temp = pages->offset[i];
> > +        uint64_t temp = pages->normal[i];
> >
> >          packet->offset[i] = cpu_to_be64(temp);
> >      }
> >
> > +    for (i = 0; i < pages->zero_num; i++) {
> > +        /* there are architectures where ram_addr_t is 32 bit */
> > +        uint64_t temp = pages->zero[i];
> > +
> > +        packet->offset[pages->normal_num + i] = cpu_to_be64(temp);
> > +    }
> > +
> >      p->packets_sent++;
> > -    p->total_normal_pages += pages->num;
> > +    p->total_normal_pages += pages->normal_num;
> >      p->total_zero_pages += pages->zero_num;
> >
> > -    trace_multifd_send(p->id, packet_num, pages->num, pages->zero_num,
> > +    trace_multifd_send(p->id, packet_num, pages->normal_num, pages->zero_num,
> >                         p->flags, p->next_packet_size);
> >  }
> >
> > @@ -451,6 +468,18 @@ static int multifd_recv_unfill_packet(MultiFDRecvParams *p, Error **errp)
> >          p->normal[i] = offset;
> >      }
> >
> > +    for (i = 0; i < p->zero_num; i++) {
> > +        uint64_t offset = be64_to_cpu(packet->offset[p->normal_num + i]);
> > +
> > +        if (offset > (p->block->used_length - p->page_size)) {
> > +            error_setg(errp, "multifd: offset too long %" PRIu64
> > +                       " (max " RAM_ADDR_FMT ")",
> > +                       offset, p->block->used_length);
> > +            return -1;
> > +        }
> > +        p->zero[i] = offset;
> > +    }
> > +
> >      return 0;
> >  }
> >
> > @@ -842,7 +871,7 @@ static void *multifd_send_thread(void *opaque)
> >
> >              stat64_add(&mig_stats.multifd_bytes,
> >                         p->next_packet_size + p->packet_len);
> > -            stat64_add(&mig_stats.normal_pages, pages->num);
> > +            stat64_add(&mig_stats.normal_pages, pages->normal_num);
> >              stat64_add(&mig_stats.zero_pages, pages->zero_num);
> >
> >              multifd_pages_reset(p->pages);
> > @@ -1256,7 +1285,8 @@ static void *multifd_recv_thread(void *opaque)
> >          p->flags &= ~MULTIFD_FLAG_SYNC;
> >          qemu_mutex_unlock(&p->mutex);
> >
> > -        if (p->normal_num) {
> > +        if (p->normal_num + p->zero_num) {
> > +            assert(!(flags & MULTIFD_FLAG_SYNC));
>
> This breaks 8.2 -> 9.0 migration. QEMU 8.2 is still sending the SYNC
> along with the data packet.

I keep missing the compatibility thing. Will remove this.

>
> >              ret = multifd_recv_state->ops->recv_pages(p, &local_err);
> >              if (ret != 0) {
> >                  break;
> > diff --git a/migration/multifd.h b/migration/multifd.h
> > index 9822ff298a..125f0bbe60 100644
> > --- a/migration/multifd.h
> > +++ b/migration/multifd.h
> > @@ -53,6 +53,11 @@ typedef struct {
> >      uint32_t unused32[1];    /* Reserved for future use */
> >      uint64_t unused64[3];    /* Reserved for future use */
> >      char ramblock[256];
> > +    /*
> > +     * This array contains the pointers to:
> > +     *  - normal pages (initial normal_pages entries)
> > +     *  - zero pages (following zero_pages entries)
> > +     */
> >      uint64_t offset[];
> >  } __attribute__((packed)) MultiFDPacket_t;
> >
> > @@ -224,6 +229,8 @@ typedef struct {
> >
> >  void multifd_register_ops(int method, MultiFDMethods *ops);
> >  void multifd_send_fill_packet(MultiFDSendParams *p);
> > +void multifd_zero_page_check_send(MultiFDSendParams *p);
> > +void multifd_zero_page_check_recv(MultiFDRecvParams *p);
> >
> >  static inline void multifd_send_prepare_header(MultiFDSendParams *p)
> >  {
> > diff --git a/qapi/migration.json b/qapi/migration.json
> > index 99843a8e95..e2450b92d4 100644
> > --- a/qapi/migration.json
> > +++ b/qapi/migration.json
> > @@ -660,9 +660,11 @@
> >  #
> >  # @none: Do not perform zero page checking.
> >  #
> > +# @multifd: Perform zero page checking on the multifd sender thread. (since 9.0)
> > +#
> >  ##
> >  { 'enum': 'ZeroPageDetection',
> > -  'data': [ 'legacy', 'none' ] }
> > +  'data': [ 'legacy', 'none', 'multifd' ] }
> >
> >  ##
> >  # @BitmapMigrationBitmapAliasTransform: