[v4,3/3] migration/multifd: Report to user when zerocopy not working

Commit Message

Leonardo Bras July 11, 2022, 9:11 p.m. UTC

Some errors, like the lack of Scatter-Gather support by the network
interface(NETIF_F_SG) may cause sendmsg(...,MSG_ZEROCOPY) to fail on using
zero-copy, which causes it to fall back to the default copying mechanism.

After each full dirty-bitmap scan there should be a zero-copy flush
happening, which checks for errors each of the previous calls to
sendmsg(...,MSG_ZEROCOPY). If all of them failed to use zero-copy, then
increment dirty_sync_missed_zero_copy migration stat to let the user know
about it.

Signed-off-by: Leonardo Bras <[e-mail]>
Reviewed-by: Daniel P. Berrangé <[e-mail]>
---
 migration/ram.h     | 2 ++
 migration/multifd.c | 2 ++
 migration/ram.c     | 5 +++++
 3 files changed, 9 insertions(+)

Comments

Peter Xu July 12, 2022, 10:42 p.m. UTC | #1

On Mon, Jul 11, 2022 at 06:11:13PM -0300, Leonardo Bras wrote:
> Some errors, like the lack of Scatter-Gather support by the network
> interface(NETIF_F_SG) may cause sendmsg(...,MSG_ZEROCOPY) to fail on using
> zero-copy, which causes it to fall back to the default copying mechanism.
> 
> After each full dirty-bitmap scan there should be a zero-copy flush
> happening, which checks for errors each of the previous calls to
> sendmsg(...,MSG_ZEROCOPY). If all of them failed to use zero-copy, then
> increment dirty_sync_missed_zero_copy migration stat to let the user know
> about it.
> 
> Signed-off-by: Leonardo Bras <[e-mail]>
> Reviewed-by: Daniel P. Berrangé <[e-mail]>

Acked-by: Peter Xu <[e-mail]>

Leonardo Bras July 13, 2022, 12:33 a.m. UTC | #2

On Tue, Jul 12, 2022 at 7:42 PM Peter Xu <[e-mail]> wrote:
>
> On Mon, Jul 11, 2022 at 06:11:13PM -0300, Leonardo Bras wrote:
> > Some errors, like the lack of Scatter-Gather support by the network
> > interface(NETIF_F_SG) may cause sendmsg(...,MSG_ZEROCOPY) to fail on using
> > zero-copy, which causes it to fall back to the default copying mechanism.
> >
> > After each full dirty-bitmap scan there should be a zero-copy flush
> > happening, which checks for errors each of the previous calls to
> > sendmsg(...,MSG_ZEROCOPY). If all of them failed to use zero-copy, then
> > increment dirty_sync_missed_zero_copy migration stat to let the user know
> > about it.
> >
> > Signed-off-by: Leonardo Bras <[e-mail]>
> > Reviewed-by: Daniel P. Berrangé <[e-mail]>
>
> Acked-by: Peter Xu <[e-mail]>

Thanks Peter!

> --
> Peter Xu
>

Patch

diff --git a/migration/ram.h b/migration/ram.h
index ded0a3a086..d3c7eb96f5 100644
--- a/migration/ram.h
+++ b/migration/ram.h
@@ -87,4 +87,6 @@  void ram_write_tracking_prepare(void);
 int ram_write_tracking_start(void);
 void ram_write_tracking_stop(void);
 
+void dirty_sync_missed_zero_copy(void);
+
 #endif
diff --git a/migration/multifd.c b/migration/multifd.c
index 684c014c86..3909b34967 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -624,6 +624,8 @@  int multifd_send_sync_main(QEMUFile *f)
             if (ret < 0) {
                 error_report_err(err);
                 return -1;
+            } else if (ret == 1) {
+                dirty_sync_missed_zero_copy();
             }
         }
     }
diff --git a/migration/ram.c b/migration/ram.c
index 01f9cc1d72..db948c4787 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -407,6 +407,11 @@  static void ram_transferred_add(uint64_t bytes)
     ram_counters.transferred += bytes;
 }
 
+void dirty_sync_missed_zero_copy(void)
+{
+    ram_counters.dirty_sync_missed_zero_copy++;
+}
+
 /* used by the search for pages to send */
 struct PageSearchStatus {
     /* Current block being searched */