Re: [RFC PATCH 5/7] migration/multifd: Isolate ram pages packet data

[Fabiano Rosas <farosas@suse.de>]

Fabiano Rosas <farosas@suse.de> writes:

> While we cannot yet disentangle the multifd packet from page data, we
> can make the code a bit cleaner by setting the page-related fields in
> a separate function.
>
> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> ---
>  migration/multifd.c | 104 +++++++++++++++++++++++++++++---------------
>  1 file changed, 68 insertions(+), 36 deletions(-)
>
> diff --git a/migration/multifd.c b/migration/multifd.c
> index c4a952576d..6fe339b378 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -407,65 +407,64 @@ static void multifd_pages_clear(MultiFDPages_t *pages)
>      g_free(pages);
>  }
>  
> -void multifd_send_fill_packet(MultiFDSendParams *p)
> +static void multifd_ram_fill_packet(MultiFDSendParams *p)
>  {
>      MultiFDPacket_t *packet = p->packet;
>      MultiFDPages_t *pages = p->data->opaque;
> -    uint64_t packet_num;
>      uint32_t zero_num = pages->num - pages->normal_num;
> -    int i;
>  
> -    packet->flags = cpu_to_be32(p->flags);
>      packet->pages_alloc = cpu_to_be32(pages->allocated);
>      packet->normal_pages = cpu_to_be32(pages->normal_num);
>      packet->zero_pages = cpu_to_be32(zero_num);
> -    packet->next_packet_size = cpu_to_be32(p->next_packet_size);
> -
> -    packet_num = qatomic_fetch_inc(&multifd_send_state->packet_num);
> -    packet->packet_num = cpu_to_be64(packet_num);
>  
>      if (pages->block) {
>          strncpy(packet->ramblock, pages->block->idstr, 256);
>      }
>  
> -    for (i = 0; i < pages->num; i++) {
> +    for (int i = 0; i < pages->num; i++) {
>          /* there are architectures where ram_addr_t is 32 bit */
>          uint64_t temp = pages->offset[i];
>  
>          packet->offset[i] = cpu_to_be64(temp);
>      }
>  
> -    p->packets_sent++;
>      p->total_normal_pages += pages->normal_num;
>      p->total_zero_pages += zero_num;
> +}
>  
> -    trace_multifd_send(p->id, packet_num, pages->normal_num, zero_num,
> +void multifd_send_fill_packet(MultiFDSendParams *p)
> +{
> +    MultiFDPacket_t *packet = p->packet;
> +    uint64_t packet_num;
> +
> +    memset(packet, 0, p->packet_len);
> +
> +    packet->magic = cpu_to_be32(MULTIFD_MAGIC);
> +    packet->version = cpu_to_be32(MULTIFD_VERSION);
> +
> +    packet->flags = cpu_to_be32(p->flags);
> +    packet->next_packet_size = cpu_to_be32(p->next_packet_size);
> +
> +    packet_num = qatomic_fetch_inc(&multifd_send_state->packet_num);
> +    packet->packet_num = cpu_to_be64(packet_num);
> +
> +    p->packets_sent++;
> +
> +    if (p->data) {

This needs to be !(p->flags & MULTIFD_SYNC). In v2 I'll add it in a
separate patch to make it clear:

-->8--
From e0dd1e0f10b6adb5d419ff68c1ef3b76d2fcf1d4 Mon Sep 17 00:00:00 2001
From: Fabiano Rosas <farosas@suse.de>
Date: Fri, 19 Jul 2024 11:28:33 -0300
Subject: [PATCH] migration/multifd: Don't send ram data during SYNC

Skip saving and loading any ram data in the packet in the case of a
SYNC. This fixes a shortcoming of the current code which requires a
reset of the MultiFDPages_t fields right after the previous
pending_job finishes, otherwise the very next job might be a SYNC and
multifd_send_fill_packet() will put the stale values in the packet.

By not calling multifd_ram_fill_packet(), we can stop resetting
MultiFDPages_t in the multifd core and leave that to the client code.

Actually moving the reset function is not yet done because
pages->num==0 is used by the client code to determine whether the
MultiFDPages_t needs to be flushed. The subsequent patches will
replace that with a generic flag that is not dependent on
MultiFDPages_t.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 migration/multifd.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/migration/multifd.c b/migration/multifd.c
index 3809890082..6e6e62d352 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -431,6 +431,7 @@ void multifd_send_fill_packet(MultiFDSendParams *p)
 {
     MultiFDPacket_t *packet = p->packet;
     uint64_t packet_num;
+    bool sync_packet = p->flags & MULTIFD_FLAG_SYNC;
 
     memset(packet, 0, p->packet_len);
 
@@ -445,7 +446,9 @@ void multifd_send_fill_packet(MultiFDSendParams *p)
 
     p->packets_sent++;
 
-    multifd_ram_fill_packet(p);
+    if (!sync_packet) {
+        multifd_ram_fill_packet(p);
+    }
 
     trace_multifd_send(p->id, packet_num,
                        be32_to_cpu(packet->normal_pages),
@@ -556,7 +559,12 @@ static int multifd_recv_unfill_packet(MultiFDRecvParams *p, Error **errp)
     p->packet_num = be64_to_cpu(packet->packet_num);
     p->packets_recved++;
 
-    ret = multifd_ram_unfill_packet(p, errp);
+    if (p->flags & MULTIFD_FLAG_SYNC) {
+        p->normal_num = 0;
+        p->zero_num = 0;
+    } else {
+        ret = multifd_ram_unfill_packet(p, errp);
+    }
 
     trace_multifd_recv(p->id, p->packet_num, p->normal_num, p->zero_num,
                        p->flags, p->next_packet_size);
-- 
2.35.3