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migration/multifd: cache migrate_multifd_channels() in send/recv hot paths

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multifd_send() and multifd_recv() are on the per-page-batch hot path
of live migration.  Both functions call migrate_multifd_channels()
multiple times (3-4 calls each) for modulo arithmetic in the
round-robin channel selection loop.

Each call goes through migrate_get_current() -> dereference
MigrationState -> read parameters.multifd_channels.  While each
individual call is cheap, these functions execute for every page
batch during the entire migration, easily millions of times.

Cache the return value in a local variable at function entry.  The
channel count is fixed for the duration of a migration and cannot
change mid-flight.

For multifd_send(): 3 calls reduced to 1.
For multifd_recv(): 4 calls reduced to 1.

Signed-off-by: Bin Guo <guobin@linux.alibaba.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20260518110112.21395-8-guobin@linux.alibaba.com
Signed-off-by: Peter Xu <peterx@redhat.com>
This commit is contained in:
Bin Guo
2026-05-18 19:01:11 +08:00
committed by Peter Xu
parent 810bf31a1b
commit 75d782a1e9
1 changed files with 9 additions and 6 deletions
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15
migration/multifd.c
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@@ -362,13 +362,15 @@ bool multifd_send(MultiFDSendData **send_data)
/* We wait here, until at least one channel is ready */
qemu_sem_wait(&multifd_send_state->channels_ready);
int thread_count = migrate_multifd_channels();
/*
* next_channel can remain from a previous migration that was
* using more channels, so ensure it doesn't overflow if the
* limit is lower now.
*/
next_channel %= migrate_multifd_channels();
for (i = next_channel;; i = (i + 1) % migrate_multifd_channels()) {
next_channel %= thread_count;
for (i = next_channel;; i = (i + 1) % thread_count) {
if (multifd_send_should_exit()) {
return false;
}
@@ -378,7 +380,7 @@ bool multifd_send(MultiFDSendData **send_data)
* sender thread can clear it.
*/
if (qatomic_read(&p->pending_job) == false) {
next_channel = (i + 1) % migrate_multifd_channels();
next_channel = (i + 1) % thread_count;
break;
}
}
@@ -998,6 +1000,7 @@ bool multifd_recv(void)
int i;
static int next_recv_channel;
MultiFDRecvParams *p = NULL;
int thread_count = migrate_multifd_channels();
MultiFDRecvData *data = multifd_recv_state->data;
/*
@@ -1005,8 +1008,8 @@ bool multifd_recv(void)
* using more channels, so ensure it doesn't overflow if the
* limit is lower now.
*/
next_recv_channel %= migrate_multifd_channels();
for (i = next_recv_channel;; i = (i + 1) % migrate_multifd_channels()) {
next_recv_channel %= thread_count;
for (i = next_recv_channel;; i = (i + 1) % thread_count) {
if (multifd_recv_should_exit()) {
return false;
}
@@ -1014,7 +1017,7 @@ bool multifd_recv(void)
p = &multifd_recv_state->params[i];
if (qatomic_read(&p->pending_job) == false) {
next_recv_channel = (i + 1) % migrate_multifd_channels();
next_recv_channel = (i + 1) % thread_count;
break;
}
}