Peter Xu <[email protected]> writes:
> On Tue, Feb 20, 2024 at 07:41:26PM -0300, Fabiano Rosas wrote:
>> The fixed-ram migration can be performed live or non-live, but it is
>> always asynchronous, i.e. the source machine and the destination
>> machine are not migrating at the same time. We only need some pieces
>> of the multifd sync operations.
>>
>> multifd_send_sync_main()
>> ------------------------
>> Issued by the ram migration code on the migration thread, causes the
>> multifd send channels to synchronize with the migration thread and
>> makes the sending side emit a packet with the MULTIFD_FLUSH flag.
>>
>> With fixed-ram we want to maintain the sync on the sending side
>> because that provides ordering between the rounds of dirty pages when
>> migrating live.
>>
>> MULTIFD_FLUSH
>> -------------
>> On the receiving side, the presence of the MULTIFD_FLUSH flag on a
>> packet causes the receiving channels to start synchronizing with the
>> main thread.
>>
>> We're not using packets with fixed-ram, so there's no MULTIFD_FLUSH
>> flag and therefore no channel sync on the receiving side.
>>
>> multifd_recv_sync_main()
>> ------------------------
>> Issued by the migration thread when the ram migration flag
>> RAM_SAVE_FLAG_MULTIFD_FLUSH is received, causes the migration thread
>> on the receiving side to start synchronizing with the recv
>> channels. Due to compatibility, this is also issued when
>> RAM_SAVE_FLAG_EOS is received.
>>
>> For fixed-ram we only need to synchronize the channels at the end of
>> migration to avoid doing cleanup before the channels have finished
>> their IO.
>>
>> Make sure the multifd syncs are only issued at the appropriate
>> times. Note that due to pre-existing backward compatibility issues, we
>> have the multifd_flush_after_each_section property that enables an
>> older behavior of synchronizing channels more frequently (and
>> inefficiently). Fixed-ram should always run with that property
>> disabled (default).
>
> What if the user enables multifd_flush_after_each_section=true?
>
> IMHO we don't necessarily need to attach the fixed-ram loading flush to any
> flag in the stream. For fixed-ram IIUC all the loads will happen in one
> shot of ram_load() anyway when parsing the ramblock list, so.. how about we
> decouple the fixed-ram load flush from the stream by always do a sync in
> ram_load() unconditionally?
I would like to. But it's not possible because ram_load() is called once
per section. So once for each EOS flag on the stream. We'll have at
least two calls to ram_load(), once due to qemu_savevm_state_iterate()
and another due to qemu_savevm_state_complete_precopy().
The fact that fixed-ram can use just one load doesn't change the fact
that we perform more than one "save". So we'll need to use the FLUSH
flag in this case unfortunately.
>
> @@ -4368,6 +4367,15 @@ static int ram_load(QEMUFile *f, void *opaque, int
> version_id)
> ret = ram_load_precopy(f);
> }
> }
> +
> + /*
> + * Fixed-ram migration may queue load tasks to multifd threads; make
> + * sure they're all done.
> + */
> + if (migrate_fixed_ram() && migrate_multifd()) {
> + multifd_recv_sync_main();
> + }
> +
> trace_ram_load_complete(ret, seq_iter);
>
> return ret;
>
> Then ram_load() always guarantees synchronous loading of pages, and
> fixed-ram will completely ignore multifd flushes (then we also skip it for
> the ram_save_complete() like what this patch does for the rest).
>
>>
>> Signed-off-by: Fabiano Rosas <[email protected]>
>> ---
>> migration/ram.c | 19 ++++++++++++++++---
>> 1 file changed, 16 insertions(+), 3 deletions(-)
>>
>> diff --git a/migration/ram.c b/migration/ram.c
>> index 5932e1b8e1..c7050f6f68 100644
>> --- a/migration/ram.c
>> +++ b/migration/ram.c
>> @@ -1369,8 +1369,11 @@ static int find_dirty_block(RAMState *rs,
>> PageSearchStatus *pss)
>> if (ret < 0) {
>> return ret;
>> }
>> - qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
>> - qemu_fflush(f);
>> +
>> + if (!migrate_fixed_ram()) {
>> + qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
>> + qemu_fflush(f);
>> + }
>> }
>> /*
>> * If memory migration starts over, we will meet a dirtied page
>> @@ -3112,7 +3115,8 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>> return ret;
>> }
>>
>> - if (migrate_multifd() && !migrate_multifd_flush_after_each_section()) {
>> + if (migrate_multifd() && !migrate_multifd_flush_after_each_section()
>> + && !migrate_fixed_ram()) {
>> qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
>> }
>>
>> @@ -4253,6 +4257,15 @@ static int ram_load_precopy(QEMUFile *f)
>> break;
>> case RAM_SAVE_FLAG_EOS:
>> /* normal exit */
>> + if (migrate_fixed_ram()) {
>> + /*
>> + * The EOS flag appears multiple times on the
>> + * stream. Fixed-ram needs only one sync at the
>> + * end. It will be done on the flush flag above.
>> + */
>> + break;
>> + }
>> +
>> if (migrate_multifd() &&
>> migrate_multifd_flush_after_each_section()) {
>> multifd_recv_sync_main();
>> --
>> 2.35.3
>>
