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migration/multifd: Separate SYNC request with normal jobs

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Multifd provide a threaded model for processing jobs.  On sender side,
there can be two kinds of job: (1) a list of pages to send, or (2) a sync
request.

The sync request is a very special kind of job.  It never contains a page
array, but only a multifd packet telling the dest side to synchronize with
sent pages.

Before this patch, both requests use the pending_job field, no matter what
the request is, it will boost pending_job, while multifd sender thread will
decrement it after it finishes one job.

However this should be racy, because SYNC is special in that it needs to
set p->flags with MULTIFD_FLAG_SYNC, showing that this is a sync request.
Consider a sequence of operations where:

  - migration thread enqueue a job to send some pages, pending_job++ (0->1)

  - [...before the selected multifd sender thread wakes up...]

  - migration thread enqueue another job to sync, pending_job++ (1->2),
    setup p->flags=MULTIFD_FLAG_SYNC

  - multifd sender thread wakes up, found pending_job==2
    - send the 1st packet with MULTIFD_FLAG_SYNC and list of pages
    - send the 2nd packet with flags==0 and no pages

This is not expected, because MULTIFD_FLAG_SYNC should hopefully be done
after all the pages are received.  Meanwhile, the 2nd packet will be
completely useless, which contains zero information.

I didn't verify above, but I think this issue is still benign in that at
least on the recv side we always receive pages before handling
MULTIFD_FLAG_SYNC.  However that's not always guaranteed and just tricky.

One other reason I want to separate it is using p->flags to communicate
between the two threads is also not clearly defined, it's very hard to read
and understand why accessing p->flags is always safe; see the current impl
of multifd_send_thread() where we tried to cache only p->flags.  It doesn't
need to be that complicated.

This patch introduces pending_sync, a separate flag just to show that the
requester needs a sync.  Alongside, we remove the tricky caching of
p->flags now because after this patch p->flags should only be used by
multifd sender thread now, which will be crystal clear.  So it is always
thread safe to access p->flags.

With that, we can also safely convert the pending_job into a boolean,
because we don't support >1 pending jobs anyway.

Always use atomic ops to access both flags to make sure no cache effect.
When at it, drop the initial setting of "pending_job = 0" because it's
always allocated using g_new0().

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240202102857.110210-7-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
This commit is contained in:
Peter Xu 2024-02-02 18:28:40 +08:00
parent efd8c5439d
commit f5f48a7891
2 changed files with 36 additions and 16 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

39
migration/multifd.c
View file

@ -442,8 +442,8 @@ static int multifd_send_pages(void)
}
p = &multifd_send_state->params[i];
qemu_mutex_lock(&p->mutex);
if (!p->pending_job) {
p->pending_job++;
if (qatomic_read(&p->pending_job) == false) {
qatomic_set(&p->pending_job, true);
next_channel = (i + 1) % migrate_multifd_channels();
break;
}
@ -631,8 +631,12 @@ int multifd_send_sync_main(void)
qemu_mutex_lock(&p->mutex);
p->packet_num = multifd_send_state->packet_num++;
p->flags |= MULTIFD_FLAG_SYNC;
p->pending_job++;
/*
* We should be the only user so far, so not possible to be set by
* others concurrently.
*/
assert(qatomic_read(&p->pending_sync) == false);
qatomic_set(&p->pending_sync, true);
qemu_mutex_unlock(&p->mutex);
qemu_sem_post(&p->sem);
}
@ -685,10 +689,9 @@ static void *multifd_send_thread(void *opaque)
}
qemu_mutex_lock(&p->mutex);
if (p->pending_job) {
if (qatomic_read(&p->pending_job)) {
uint64_t packet_num = p->packet_num;
MultiFDPages_t *pages = p->pages;
uint32_t flags;
if (use_zero_copy_send) {
p->iovs_num = 0;
@ -704,13 +707,11 @@ static void *multifd_send_thread(void *opaque)
}
}
multifd_send_fill_packet(p);
flags = p->flags;
p->flags = 0;
p->num_packets++;
p->total_normal_pages += pages->num;
qemu_mutex_unlock(&p->mutex);
trace_multifd_send(p->id, packet_num, pages->num, flags,
trace_multifd_send(p->id, packet_num, pages->num, p->flags,
p->next_packet_size);
if (use_zero_copy_send) {
@ -738,12 +739,23 @@ static void *multifd_send_thread(void *opaque)
multifd_pages_reset(p->pages);
p->next_packet_size = 0;
qemu_mutex_lock(&p->mutex);
p->pending_job--;
qatomic_set(&p->pending_job, false);
qemu_mutex_unlock(&p->mutex);
if (flags & MULTIFD_FLAG_SYNC) {
qemu_sem_post(&p->sem_sync);
} else if (qatomic_read(&p->pending_sync)) {
p->flags = MULTIFD_FLAG_SYNC;
multifd_send_fill_packet(p);
ret = qio_channel_write_all(p->c, (void *)p->packet,
p->packet_len, &local_err);
if (ret != 0) {
qemu_mutex_unlock(&p->mutex);
break;
}
/* p->next_packet_size will always be zero for a SYNC packet */
stat64_add(&mig_stats.multifd_bytes, p->packet_len);
p->flags = 0;
qatomic_set(&p->pending_sync, false);
qemu_mutex_unlock(&p->mutex);
qemu_sem_post(&p->sem_sync);
} else {
qemu_mutex_unlock(&p->mutex);
/* sometimes there are spurious wakeups */
@ -907,7 +919,6 @@ int multifd_save_setup(Error **errp)
qemu_mutex_init(&p->mutex);
qemu_sem_init(&p->sem, 0);
qemu_sem_init(&p->sem_sync, 0);
p->pending_job = 0;
p->id = i;
p->pages = multifd_pages_init(page_count);
p->packet_len = sizeof(MultiFDPacket_t)

13
migration/multifd.h
View file

@ -99,8 +99,17 @@ typedef struct {
uint32_t flags;
/* global number of generated multifd packets */
uint64_t packet_num;
/* thread has work to do */
int pending_job;
/*
* The sender thread has work to do if either of below boolean is set.
*
* @pending_job: a job is pending
* @pending_sync: a sync request is pending
*
* For both of these fields, they're only set by the requesters, and
* cleared by the multifd sender threads.
*/
bool pending_job;
bool pending_sync;
/* array of pages to sent.
* The owner of 'pages' depends of 'pending_job' value:
* pending_job == 0 -> migration_thread can use it.