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Re: [PATCH] multifd: Avoid busy-wait in multifd_send_pages()
From: |
Juan Quintela |
Subject: |
Re: [PATCH] multifd: Avoid busy-wait in multifd_send_pages() |
Date: |
Wed, 26 Apr 2023 12:28:56 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux) |
"manish.mishra" <manish.mishra@nutanix.com> wrote:
> multifd_send_sync_main() posts request on the multifd channel
> but does not call sem_wait() on channels_ready semaphore, making
> the channels_ready semaphore count keep increasing.
> As a result, sem_wait() on channels_ready in multifd_send_pages()
> is always non-blocking hence multifd_send_pages() keeps searching
> for a free channel in a busy loop until a channel is freed.
>
> Signed-off-by: manish.mishra <manish.mishra@nutanix.com>
> ---
> migration/multifd.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/migration/multifd.c b/migration/multifd.c
> index cce3ad6988..43d26e7012 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -615,6 +615,7 @@ int multifd_send_sync_main(QEMUFile *f)
>
> trace_multifd_send_sync_main_signal(p->id);
>
> + qemu_sem_wait(&multifd_send_state->channels_ready);
> qemu_mutex_lock(&p->mutex);
>
> if (p->quit) {
We need this, but I think it is better to put it on the second loop.
> @@ -919,7 +920,7 @@ int multifd_save_setup(Error **errp)
> multifd_send_state = g_malloc0(sizeof(*multifd_send_state));
> multifd_send_state->params = g_new0(MultiFDSendParams, thread_count);
> multifd_send_state->pages = multifd_pages_init(page_count);
> - qemu_sem_init(&multifd_send_state->channels_ready, 0);
> + qemu_sem_init(&multifd_send_state->channels_ready, thread_count);
> qatomic_set(&multifd_send_state->exiting, 0);
> multifd_send_state->ops = multifd_ops[migrate_multifd_compression()];
I think this bit is wrong.
We should not set the channels ready until the thread is ready and
channel is created.
What do you think about this patch:
>From bcb0ef9b97b858458c403d2e4dc9e0dbd96721b3 Mon Sep 17 00:00:00 2001
From: Juan Quintela <quintela@redhat.com>
Date: Wed, 26 Apr 2023 12:20:36 +0200
Subject: [PATCH] multifd: Fix the number of channels ready
We don't wait in the sem when we are doing a sync_main. Make it wait
there. To make things clearer, we mark the channel ready at the
begining of the thread loop.
This causes a busy loop in multifd_send_page().
Found-by: manish.mishra <manish.mishra@nutanix.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
---
migration/multifd.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/migration/multifd.c b/migration/multifd.c
index 903df2117b..e625e8725e 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -635,6 +635,7 @@ int multifd_send_sync_main(QEMUFile *f)
for (i = 0; i < migrate_multifd_channels(); i++) {
MultiFDSendParams *p = &multifd_send_state->params[i];
+ qemu_sem_wait(&multifd_send_state->channels_ready);
trace_multifd_send_sync_main_wait(p->id);
qemu_sem_wait(&p->sem_sync);
@@ -668,6 +669,7 @@ static void *multifd_send_thread(void *opaque)
p->num_packets = 1;
while (true) {
+ qemu_sem_post(&multifd_send_state->channels_ready);
qemu_sem_wait(&p->sem);
if (qatomic_read(&multifd_send_state->exiting)) {
@@ -736,7 +738,6 @@ static void *multifd_send_thread(void *opaque)
if (flags & MULTIFD_FLAG_SYNC) {
qemu_sem_post(&p->sem_sync);
}
- qemu_sem_post(&multifd_send_state->channels_ready);
} else if (p->quit) {
qemu_mutex_unlock(&p->mutex);
break;
--
2.40.0