[Qemu-devel] [PATCH 1/2] migrate.c: migrate_fd_put_buffer(): Do not busyloop: stop writing if EWOULDBLOCK

[Qemu-devel] [PATCH 1/2] migrate.c: migrate_fd_put_buffer(): Do not busyloop: stop writing if EWOULDBLOCK

[Uri Lublin]

Theoretically the migration code is non-blocking, designed for live migration.

Practically migrate_fd_put_buffer busy-loops trying to write. That's since
on many machines EWOULDBLOCK==EAGAIN (e.g. look in
<linux>/include/asm-generic/errno.h).
Busy looping is much worse then blocking (in case a write actually blocks).

Fixed by removing "(s->get_error(s)) == EWOULDBLOCK)" from the while condition.

Signed-off-by: Uri Lublin <uril@redhat.com>
---
 migration.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/migration.c b/migration.c
index 0ef777a..e8bafa6 100644
--- a/migration.c
+++ b/migration.c
@@ -168,7 +168,7 @@ ssize_t migrate_fd_put_buffer(void *opaque, const void *data, size_t size)
 
     do {
         ret = s->write(s, data, size);
-    } while (ret == -1 && ((s->get_error(s)) == EINTR || (s->get_error(s)) == EWOULDBLOCK));
+    } while (ret == -1 && s->get_error(s) == EINTR);
 
     if (ret == -1)
         ret = -(s->get_error(s));
-- 
1.6.0.6

[Qemu-devel] [DEBUGGING DO-NOT-COMMIT 2/2]: debugging migration busy-looping

[Uri Lublin]

Try it with/without fix for migration busy-loop, and migrate
a VM. (without the fix, patch fails just manually fix it).

To see the problem clearly try to suspend the destination while a migration
process is active (e.g. with CTRL-z and fg).

Signed-off-by: Uri Lublin <uril@redhat.com>
---
 migration.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/migration.c b/migration.c
index e8bafa6..5db9c47 100644
--- a/migration.c
+++ b/migration.c
@@ -165,11 +165,14 @@ ssize_t migrate_fd_put_buffer(void *opaque, const void *data, size_t size)
 {
     FdMigrationState *s = opaque;
     ssize_t ret;
+    int n = 0;
 
     do {
         ret = s->write(s, data, size);
+	if (++n % 1000 == 0) printf("LOOP size=%lu n=%d\n", size, n);
     } while (ret == -1 && s->get_error(s) == EINTR);
 
+    if (n > 4) printf("OUT size=%lu n=%d\n", size, n);
     if (ret == -1)
         ret = -(s->get_error(s));
 
-- 
1.6.0.6