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.
--
Peter Xu
>From c5dc2052794efd6da6a1e6f4b49f25d5b32879f7 Mon Sep 17 00:00:00 2001
From: Peter Xu <pet...@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 <pet...@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 <pet...@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 <pet...@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 <pet...@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....@suse.de
Signed-off-by: Peter Xu <pet...@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;
+}
+
+static inline bool multifd_queue_full(MultiFDPages_t *pages)
+{
+ return pages->num == pages->allocated;
+}
+
+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;
}
--
2.43.0
