Re: [PATCH 04/14] migration/multifd: Postpone reset of MultiFDPages_t

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> On Wed, Jan 31, 2024 at 12:27:51PM -0300, Fabiano Rosas wrote:
>> > +/* Reset a MultiFDPages_t* object for the next use */
>> > +static void multifd_pages_reset(MultiFDPages_t *pages)
>> > +{
>> > +    /*
>> > +     * We don't need to touch offset[] array, because it will be
>> > +     * overwritten later when reused.
>> > +     */
>> > +    pages->num = 0;
>> > +    pages->block = NULL;
>> 
>> Having to do this at all is a huge overloading of this field. This not
>> only resets it, but it also communicates to multifd_queue_page() that
>> the previous payload has been sent. Otherwise, multifd_queue_page()
>> wouldn't know whether the previous call to multifd_queue_page() has
>> called multifd_send_pages() or if it has exited early. So this basically
>> means "the block that was previously here has been sent".
>> 
>> That's why we need the changed=true logic. A
>> multifd_send_state->pages->block still has a few pages left to send, but
>> because it's less than pages->allocated, it skips
>> multifd_send_pages(). The next call to multifd_queue_page() already has
>> the next ramblock. So we set changed=true, call multifd_send_pages() to
>> send the remaining pages of that block and recurse into
>> multifd_queue_page() once more to send the new block.
>
> I agree, the queue page routines are not easy to follow as well.
>
> How do you like a rewrite of the queue logic, like this?
>
> =====
> bool multifd_queue_page(RAMBlock *block, ram_addr_t offset)
> {
>     MultiFDPages_t *pages;
>
> retry:
>     pages = multifd_send_state->pages;
>
>     /* If the queue is empty, we can already enqueue now */
>     if (multifd_queue_empty(pages)) {
>         pages->block = block;
>         multifd_enqueue(pages, offset);
>         return true;
>     }
>
>     /*
>      * Not empty, meanwhile we need a flush.  It can because of either:
>      *
>      * (1) The page is not on the same ramblock of previous ones, or,
>      * (2) The queue is full.
>      *
>      * After flush, always retry.
>      */
>     if (pages->block != block || multifd_queue_full(pages)) {
>         if (!multifd_send_pages()) {
>             return false;
>         }
>         goto retry;
>     }
>
>     /* Not empty, and we still have space, do it! */
>     multifd_enqueue(pages, offset);
>     return true;
> }
> =====
>
> Would this be clearer?  With above, we can drop the ->ramblock reset,
> afaict.
>
> I attached three patches if you agree it's better, then I'll include them
> in v2.

Yes, let's do it.

>
> -- 
> Peter Xu
> From c5dc2052794efd6da6a1e6f4b49f25d5b32879f7 Mon Sep 17 00:00:00 2001
> From: Peter Xu <peterx@redhat.com>
> Date: Thu, 1 Feb 2024 17:50:21 +0800
> Subject: [PATCH 1/3] migration/multifd: Change retval of multifd_queue_page()
>
> Using int is an overkill when there're only two options.  Change it to a
> boolean.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  migration/multifd.h | 2 +-
>  migration/multifd.c | 9 +++++----
>  migration/ram.c     | 2 +-
>  3 files changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/migration/multifd.h b/migration/multifd.h
> index 34a2ecb9f4..a320c53a6f 100644
> --- a/migration/multifd.h
> +++ b/migration/multifd.h
> @@ -22,7 +22,7 @@ bool multifd_recv_all_channels_created(void);
>  void multifd_recv_new_channel(QIOChannel *ioc, Error **errp);
>  void multifd_recv_sync_main(void);
>  int multifd_send_sync_main(void);
> -int multifd_queue_page(RAMBlock *block, ram_addr_t offset);
> +bool multifd_queue_page(RAMBlock *block, ram_addr_t offset);
>  
>  /* Multifd Compression flags */
>  #define MULTIFD_FLAG_SYNC (1 << 0)
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 91be6d2fc4..d0a3b4e062 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -505,7 +505,8 @@ static int multifd_send_pages(void)
>      return 1;
>  }
>  
> -int multifd_queue_page(RAMBlock *block, ram_addr_t offset)
> +/* Returns true if enqueue successful, false otherwise */
> +bool multifd_queue_page(RAMBlock *block, ram_addr_t offset)
>  {
>      MultiFDPages_t *pages = multifd_send_state->pages;
>      bool changed = false;
> @@ -519,21 +520,21 @@ int multifd_queue_page(RAMBlock *block, ram_addr_t offset)
>          pages->num++;
>  
>          if (pages->num < pages->allocated) {
> -            return 1;
> +            return true;
>          }
>      } else {
>          changed = true;
>      }
>  
>      if (multifd_send_pages() < 0) {
> -        return -1;
> +        return false;
>      }
>  
>      if (changed) {
>          return multifd_queue_page(block, offset);
>      }
>  
> -    return 1;
> +    return true;
>  }
>  
>  /* Multifd send side hit an error; remember it and prepare to quit */
> diff --git a/migration/ram.c b/migration/ram.c
> index d5b7cd5ac2..4649a81204 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1252,7 +1252,7 @@ static int ram_save_page(RAMState *rs, PageSearchStatus *pss)
>  
>  static int ram_save_multifd_page(RAMBlock *block, ram_addr_t offset)
>  {
> -    if (multifd_queue_page(block, offset) < 0) {
> +    if (!multifd_queue_page(block, offset)) {
>          return -1;
>      }
>      stat64_add(&mig_stats.normal_pages, 1);
> -- 
> 2.43.0
>
> From f393f1cfe95d79bed72e6043903ee4c4cb298c21 Mon Sep 17 00:00:00 2001
> From: Peter Xu <peterx@redhat.com>
> Date: Thu, 1 Feb 2024 17:51:38 +0800
> Subject: [PATCH 2/3] migration/multifd: Change retval of multifd_send_pages()
>
> Using int is an overkill when there're only two options.  Change it to a
> boolean.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  migration/multifd.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
>
> diff --git a/migration/multifd.c b/migration/multifd.c
> index d0a3b4e062..d2b0f0eda9 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -449,9 +449,10 @@ static void multifd_send_kick_main(MultiFDSendParams *p)
>   * thread is using the channel mutex when changing it, and the channel
>   * have to had finish with its own, otherwise pending_job can't be
>   * false.
> + *
> + * Returns true if succeed, false otherwise.
>   */
> -
> -static int multifd_send_pages(void)
> +static bool multifd_send_pages(void)
>  {
>      int i;
>      static int next_channel;
> @@ -459,7 +460,7 @@ static int multifd_send_pages(void)
>      MultiFDPages_t *pages = multifd_send_state->pages;
>  
>      if (multifd_send_should_exit()) {
> -        return -1;
> +        return false;
>      }
>  
>      /* We wait here, until at least one channel is ready */
> @@ -473,7 +474,7 @@ static int multifd_send_pages(void)
>      next_channel %= migrate_multifd_channels();
>      for (i = next_channel;; i = (i + 1) % migrate_multifd_channels()) {
>          if (multifd_send_should_exit()) {
> -            return -1;
> +            return false;
>          }
>          p = &multifd_send_state->params[i];
>          /*
> @@ -502,7 +503,7 @@ static int multifd_send_pages(void)
>      qemu_mutex_unlock(&p->mutex);
>      qemu_sem_post(&p->sem);
>  
> -    return 1;
> +    return true;
>  }
>  
>  /* Returns true if enqueue successful, false otherwise */
> @@ -526,7 +527,7 @@ bool multifd_queue_page(RAMBlock *block, ram_addr_t offset)
>          changed = true;
>      }
>  
> -    if (multifd_send_pages() < 0) {
> +    if (!multifd_send_pages()) {
>          return false;
>      }
>  
> @@ -666,7 +667,7 @@ int multifd_send_sync_main(void)
>          return 0;
>      }
>      if (multifd_send_state->pages->num) {
> -        if (multifd_send_pages() < 0) {
> +        if (!multifd_send_pages()) {
>              error_report("%s: multifd_send_pages fail", __func__);
>              return -1;
>          }
> -- 
> 2.43.0
>
> From fcddc942cb31bc9d395d67a555d9a2281da452b1 Mon Sep 17 00:00:00 2001
> From: Peter Xu <peterx@redhat.com>
> Date: Thu, 1 Feb 2024 17:55:42 +0800
> Subject: [PATCH 3/3] migration/multifd: Rewrite multifd_queue_page()
>
> The current multifd_queue_page() is not easy to read and follow.  It is not
> good with a few reasons:
>
>   - No helper at all to show what exactly does a condition mean; in short,
>   readability is low.
>
>   - Rely on pages->ramblock being cleared to detect an empty queue.  It's
>   slightly an overload of the ramblock pointer, per Fabiano [1], which I
>   also agree.
>
>   - Contains a self recursion, even if not necessary..
>
> Rewrite this function.  We add some comments to make it even clearer on
> what it does.
>
> [1] https://lore.kernel.org/r/87wmrpjzew.fsf@suse.de
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  migration/multifd.c | 56 ++++++++++++++++++++++++++++++---------------
>  1 file changed, 37 insertions(+), 19 deletions(-)
>
> diff --git a/migration/multifd.c b/migration/multifd.c
> index d2b0f0eda9..5a64a9c2e2 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -506,35 +506,53 @@ static bool multifd_send_pages(void)
>      return true;
>  }
>  
> +static inline bool multifd_queue_empty(MultiFDPages_t *pages)
> +{
> +    return pages->num == 0;
> +}

Good, because we can later switch from pages to something else entirely.

> +
> +static inline bool multifd_queue_full(MultiFDPages_t *pages)
> +{
> +    return pages->num == pages->allocated;
> +}

Pages allocated is nonsense. See if you agree with its removal:
https://gitlab.com/farosas/qemu/-/commit/7cfff1a3e31b271e901a6c08d8b5d8c01b680e4d

---
From 7cfff1a3e31b271e901a6c08d8b5d8c01b680e4d Mon Sep 17 00:00:00 2001
From: Fabiano Rosas <farosas@suse.de>
Date: Tue, 24 Oct 2023 19:03:41 -0300
Subject: [PATCH] multifd: Remove MultiFDPage_t:allocated

When dealing with RAM, having a field called 'allocated' is
confusing. This field simply holds number of pages that fit in a
multifd packet.

Since it is a constant dependent on the size of the multifd packet,
remove it and instead use the page size and MULTIFD_PACKET_SIZE
directly.

This is another step in the direction of having no mentions of 'page'
in the multifd send thread.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 migration/multifd.c | 6 ++----
 migration/multifd.h | 2 --
 2 files changed, 2 insertions(+), 6 deletions(-)

diff --git a/migration/multifd.c b/migration/multifd.c
index bdefce27706..83fb2caab04 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -241,7 +241,6 @@ static MultiFDPages_t *multifd_pages_init(uint32_t n)
 {
     MultiFDPages_t *pages = g_new0(MultiFDPages_t, 1);
 
-    pages->allocated = n;
     pages->offset = g_new0(ram_addr_t, n);
     pages->page_size = qemu_target_page_size();
 
@@ -251,7 +250,6 @@ static MultiFDPages_t *multifd_pages_init(uint32_t n)
 static void multifd_pages_clear(MultiFDPages_t *pages)
 {
     pages->num = 0;
-    pages->allocated = 0;
     pages->block = NULL;
     g_free(pages->offset);
     pages->offset = NULL;
@@ -264,7 +262,7 @@ static void multifd_send_fill_packet(MultiFDSendParams *p)
     int i;
 
     packet->flags = cpu_to_be32(p->flags);
-    packet->pages_alloc = cpu_to_be32(p->pages->allocated);
+    packet->pages_alloc = cpu_to_be32(MULTIFD_PACKET_SIZE / p->pages->page_size);
     packet->normal_pages = cpu_to_be32(p->pages->num);
     packet->next_packet_size = cpu_to_be32(p->next_packet_size);
     packet->packet_num = cpu_to_be64(p->packet_num);
@@ -451,7 +449,7 @@ int multifd_queue_page(RAMBlock *block, ram_addr_t offset)
         pages->offset[pages->num] = offset;
         pages->num++;
 
-        if (pages->num < pages->allocated) {
+        if (pages->num * pages->page_size < MULTIFD_PACKET_SIZE) {
             return 1;
         }
     } else {
diff --git a/migration/multifd.h b/migration/multifd.h
index 655f8d5eeb4..d1342296d63 100644
--- a/migration/multifd.h
+++ b/migration/multifd.h
@@ -56,8 +56,6 @@ typedef struct {
 typedef struct {
     /* number of used pages */
     uint32_t num;
-    /* number of allocated pages */
-    uint32_t allocated;
     /* guest page size */
     uint32_t page_size;
     /* offset of each page */
-- 

> +
> +static inline void multifd_enqueue(MultiFDPages_t *pages, ram_addr_t offset)
> +{
> +    pages->offset[pages->num++] = offset;
> +}
> +
>  /* Returns true if enqueue successful, false otherwise */
>  bool multifd_queue_page(RAMBlock *block, ram_addr_t offset)
>  {
> -    MultiFDPages_t *pages = multifd_send_state->pages;
> -    bool changed = false;
> +    MultiFDPages_t *pages;
> +
> +retry:
> +    pages = multifd_send_state->pages;
>  
> -    if (!pages->block) {
> +    /* If the queue is empty, we can already enqueue now */
> +    if (multifd_queue_empty(pages)) {
>          pages->block = block;
> +        multifd_enqueue(pages, offset);
> +        return true;
>      }
>  
> -    if (pages->block == block) {
> -        pages->offset[pages->num] = offset;
> -        pages->num++;
> -
> -        if (pages->num < pages->allocated) {
> -            return true;
> +    /*
> +     * Not empty, meanwhile we need a flush.  It can because of either:
> +     *
> +     * (1) The page is not on the same ramblock of previous ones, or,
> +     * (2) The queue is full.
> +     *
> +     * After flush, always retry.
> +     */
> +    if (pages->block != block || multifd_queue_full(pages)) {
> +        if (!multifd_send_pages()) {
> +            return false;
>          }
> -    } else {
> -        changed = true;
> -    }
> -
> -    if (!multifd_send_pages()) {
> -        return false;
> -    }
> -
> -    if (changed) {
> -        return multifd_queue_page(block, offset);
> +        goto retry;
>      }
>  
> +    /* Not empty, and we still have space, do it! */
> +    multifd_enqueue(pages, offset);
>      return true;
>  }