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From 63255c13492f42a3236d96e706e5f8e70bb4e219 Mon Sep 17 00:00:00 2001
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From: Leonardo Bras <leobras@redhat.com>
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Date: Fri, 13 May 2022 03:28:36 -0300
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Subject: [PATCH 13/18] multifd: Send header packet without flags if
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zero-copy-send is enabled
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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RH-Author: Leonardo Brás <leobras@redhat.com>
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RH-MergeRequest: 95: MSG_ZEROCOPY + Multifd
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RH-Commit: [7/11] 137eea685e387d3d6aff187ec3fcac05bc16b6e3 (LeoBras/centos-qemu-kvm)
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RH-Bugzilla: 1968509
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RH-Acked-by: Miroslav Rezanina <mrezanin@redhat.com>
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RH-Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
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RH-Acked-by: Peter Xu <peterx@redhat.com>
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Since d48c3a0445 ("multifd: Use a single writev on the send side"),
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sending the header packet and the memory pages happens in the same
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writev, which can potentially make the migration faster.
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Using channel-socket as example, this works well with the default copying
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mechanism of sendmsg(), but with zero-copy-send=true, it will cause
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the migration to often break.
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This happens because the header packet buffer gets reused quite often,
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and there is a high chance that by the time the MSG_ZEROCOPY mechanism get
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to send the buffer, it has already changed, sending the wrong data and
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causing the migration to abort.
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It means that, as it is, the buffer for the header packet is not suitable
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for sending with MSG_ZEROCOPY.
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In order to enable zero copy for multifd, send the header packet on an
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individual write(), without any flags, and the remanining pages with a
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writev(), as it was happening before. This only changes how a migration
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with zero-copy-send=true works, not changing any current behavior for
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migrations with zero-copy-send=false.
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Signed-off-by: Leonardo Bras <leobras@redhat.com>
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Reviewed-by: Peter Xu <peterx@redhat.com>
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Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
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Message-Id: <20220513062836.965425-8-leobras@redhat.com>
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Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
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(cherry picked from commit b7dbdd8e76cd03453c234dbb9578d20969859d74)
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Signed-off-by: Leonardo Bras <leobras@redhat.com>
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---
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migration/multifd.c | 22 +++++++++++++++++++---
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1 file changed, 19 insertions(+), 3 deletions(-)
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diff --git a/migration/multifd.c b/migration/multifd.c
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index cdb57439a7..8fca6c970e 100644
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--- a/migration/multifd.c
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+++ b/migration/multifd.c
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@@ -619,6 +619,7 @@ static void *multifd_send_thread(void *opaque)
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MultiFDSendParams *p = opaque;
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Error *local_err = NULL;
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int ret = 0;
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+ bool use_zero_copy_send = migrate_use_zero_copy_send();
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trace_multifd_send_thread_start(p->id);
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rcu_register_thread();
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@@ -641,9 +642,14 @@ static void *multifd_send_thread(void *opaque)
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if (p->pending_job) {
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uint64_t packet_num = p->packet_num;
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uint32_t flags = p->flags;
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- p->iovs_num = 1;
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p->normal_num = 0;
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+ if (use_zero_copy_send) {
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+ p->iovs_num = 0;
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+ } else {
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+ p->iovs_num = 1;
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+ }
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+
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for (int i = 0; i < p->pages->num; i++) {
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p->normal[p->normal_num] = p->pages->offset[i];
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p->normal_num++;
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@@ -667,8 +673,18 @@ static void *multifd_send_thread(void *opaque)
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trace_multifd_send(p->id, packet_num, p->normal_num, flags,
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p->next_packet_size);
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- p->iov[0].iov_len = p->packet_len;
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- p->iov[0].iov_base = p->packet;
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+ if (use_zero_copy_send) {
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+ /* Send header first, without zerocopy */
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+ ret = qio_channel_write_all(p->c, (void *)p->packet,
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+ p->packet_len, &local_err);
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+ if (ret != 0) {
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+ break;
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+ }
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+ } else {
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+ /* Send header using the same writev call */
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+ p->iov[0].iov_len = p->packet_len;
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+ p->iov[0].iov_base = p->packet;
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+ }
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ret = qio_channel_writev_all(p->c, p->iov, p->iovs_num,
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&local_err);
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--
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2.35.3
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