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author | Peter Xu <peterx@redhat.com> | 2024-02-02 18:28:41 +0800 |
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committer | Peter Xu <peterx@redhat.com> | 2024-02-05 14:42:10 +0800 |
commit | e3cce9af10b06c51434ced4e1a6686f1ce43e124 (patch) | |
tree | eabbd21ad2f4bde133c1b5fc26a58ee9cf0f3fe4 /migration | |
parent | f5f48a7891cf6664a920ba52f6f4dea1646049a4 (diff) | |
download | qemu-e3cce9af10b06c51434ced4e1a6686f1ce43e124.zip qemu-e3cce9af10b06c51434ced4e1a6686f1ce43e124.tar.gz qemu-e3cce9af10b06c51434ced4e1a6686f1ce43e124.tar.bz2 |
migration/multifd: Simplify locking in sender thread
The sender thread will yield the p->mutex before IO starts, trying to not
block the requester thread. This may be unnecessary lock optimizations,
because the requester can already read pending_job safely even without the
lock, because the requester is currently the only one who can assign a
task.
Drop that lock complication on both sides:
(1) in the sender thread, always take the mutex until job done
(2) in the requester thread, check pending_job clear lockless
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240202102857.110210-8-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Diffstat (limited to 'migration')
-rw-r--r-- | migration/multifd.c | 23 |
1 files changed, 16 insertions, 7 deletions
diff --git a/migration/multifd.c b/migration/multifd.c index ea25bbe..4d5a01e 100644 --- a/migration/multifd.c +++ b/migration/multifd.c @@ -429,7 +429,9 @@ static int multifd_send_pages(void) return -1; } + /* We wait here, until at least one channel is ready */ qemu_sem_wait(&multifd_send_state->channels_ready); + /* * next_channel can remain from a previous migration that was * using more channels, so ensure it doesn't overflow if the @@ -441,17 +443,26 @@ static int multifd_send_pages(void) return -1; } p = &multifd_send_state->params[i]; - qemu_mutex_lock(&p->mutex); + /* + * Lockless read to p->pending_job is safe, because only multifd + * sender thread can clear it. + */ if (qatomic_read(&p->pending_job) == false) { - qatomic_set(&p->pending_job, true); next_channel = (i + 1) % migrate_multifd_channels(); break; } - qemu_mutex_unlock(&p->mutex); } + + qemu_mutex_lock(&p->mutex); assert(!p->pages->num); assert(!p->pages->block); - + /* + * Double check on pending_job==false with the lock. In the future if + * we can have >1 requester thread, we can replace this with a "goto + * retry", but that is for later. + */ + assert(qatomic_read(&p->pending_job) == false); + qatomic_set(&p->pending_job, true); p->packet_num = multifd_send_state->packet_num++; multifd_send_state->pages = p->pages; p->pages = pages; @@ -709,8 +720,6 @@ static void *multifd_send_thread(void *opaque) multifd_send_fill_packet(p); p->num_packets++; p->total_normal_pages += pages->num; - qemu_mutex_unlock(&p->mutex); - trace_multifd_send(p->id, packet_num, pages->num, p->flags, p->next_packet_size); @@ -730,6 +739,7 @@ static void *multifd_send_thread(void *opaque) ret = qio_channel_writev_full_all(p->c, p->iov, p->iovs_num, NULL, 0, p->write_flags, &local_err); if (ret != 0) { + qemu_mutex_unlock(&p->mutex); break; } @@ -738,7 +748,6 @@ static void *multifd_send_thread(void *opaque) multifd_pages_reset(p->pages); p->next_packet_size = 0; - qemu_mutex_lock(&p->mutex); qatomic_set(&p->pending_job, false); qemu_mutex_unlock(&p->mutex); } else if (qatomic_read(&p->pending_sync)) { |