Peter Xu <pet...@redhat.com> writes:
> On Wed, Feb 28, 2024 at 12:21:23PM -0300, Fabiano Rosas wrote:
>> The mapped-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 mapped-ram we want to maintain the sync on the sending side
>> because that provides ordering between the rounds of dirty pages when
>> migrating live.
>
> IIUC as I used to comment, we should probably only need that sync after
> each full iteration, which is find_dirty_block().
>
> I think keeping the setup/complete sync is fine, and that can be discussed
> separately. However IMHO we should still avoid the sync in
> ram_save_iterate() always, or on new qemu + old machine types (where
> flush_after_each_section=true) fixed-ram could suffer perf issues, IIUC.
>
> So I assume at a minimum below would still be preferred?
>
> @@ -3257,7 +3257,8 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
> out:
> if (ret >= 0
> && migration_is_setup_or_active(migrate_get_current()->state)) {
> - if (migrate_multifd() && migrate_multifd_flush_after_each_section())
> {
> + if (migrate_multifd() && migrate_multifd_flush_after_each_section()
> &&
> + !migrate_mapped_ram()) {
> ret = multifd_send_sync_main();
> if (ret < 0) {
> return ret;
>
I think I forgot this. I'll amend it.
>>
>> 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 mapped-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 mapped-ram we only need to synchronize the channels at the end of
>> migration to avoid doing cleanup before the channels have finished
>> their IO.
>
> Did you forget to add the sync at parse_ramblocks() for mapped-ram?
>
Ugh, I messed it up. I'll fix it.
>>
>> 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 can cause a sync to
>> happen at EOS. Since the EOS flag is needed on the stream, allow
>> mapped-ram to just ignore it.
>
> Skipping EOS makes sense, but I suggest do that without invalid_flags. See
> below.
>
>>
>> Also emit an error if any other unexpected flags are found on the
>> stream.
>>
>> Signed-off-by: Fabiano Rosas <faro...@suse.de>
>> ---
>> - skipped all FLUSH flags
>> - added invalid flags
>> - skipped EOS
>> ---
>> migration/ram.c | 26 ++++++++++++++++++++++----
>> 1 file changed, 22 insertions(+), 4 deletions(-)
>>
>> diff --git a/migration/ram.c b/migration/ram.c
>> index 18620784c6..250dcd110c 100644
>> --- a/migration/ram.c
>> +++ b/migration/ram.c
>> @@ -1368,8 +1368,11 @@ 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_mapped_ram()) {
>> + qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
>> + qemu_fflush(f);
>> + }
>> }
>> /*
>> * If memory migration starts over, we will meet a dirtied page
>> @@ -3111,7 +3114,8 @@ 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_mapped_ram()) {
>> qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
>> }
>>
>> @@ -3334,7 +3338,8 @@ static int ram_save_complete(QEMUFile *f, void *opaque)
>> }
>> }
>>
>> - if (migrate_multifd() && !migrate_multifd_flush_after_each_section()) {
>> + if (migrate_multifd() && !migrate_multifd_flush_after_each_section() &&
>> + !migrate_mapped_ram()) {
>> qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
>> }
>> qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
>> @@ -4137,6 +4142,12 @@ static int ram_load_precopy(QEMUFile *f)
>> invalid_flags |= RAM_SAVE_FLAG_COMPRESS_PAGE;
>> }
>>
>> + if (migrate_mapped_ram()) {
>> + invalid_flags |= (RAM_SAVE_FLAG_EOS | RAM_SAVE_FLAG_HOOK |
>> + RAM_SAVE_FLAG_MULTIFD_FLUSH | RAM_SAVE_FLAG_PAGE |
>> + RAM_SAVE_FLAG_XBZRLE | RAM_SAVE_FLAG_ZERO);
>
> IMHO EOS cannot be accounted as "invalid" here because it always exists.
> Rather than this trick (then explicitly ignore it below... which is even
> hackier, IMHO), we can avoid setting EOS in invalid_flags, but explicitly
> ignore EOS in below code to bypass it for mapped-ram:
>
> @@ -4301,7 +4302,12 @@ static int ram_load_precopy(QEMUFile *f)
> case RAM_SAVE_FLAG_EOS:
> /* normal exit */
> if (migrate_multifd() &&
> - migrate_multifd_flush_after_each_section()) {
> + migrate_multifd_flush_after_each_section() &&
> + /*
> + * Mapped-ram migration flushes once and for all after
> + * parsing ramblocks. Always ignore EOS for it.
> + */
> + !migrate_mapped_ram()) {
> multifd_recv_sync_main();
> }
> break;
I thought we were already spraying too many migrate_mapped_ram() checks
all over the code. But wat you said makes sense, I'll change it.
>> + }
>> +
>> while (!ret && !(flags & RAM_SAVE_FLAG_EOS)) {
>> ram_addr_t addr;
>> void *host = NULL, *host_bak = NULL;
>> @@ -4158,6 +4169,13 @@ static int ram_load_precopy(QEMUFile *f)
>> addr &= TARGET_PAGE_MASK;
>>
>> if (flags & invalid_flags) {
>> + if (invalid_flags & RAM_SAVE_FLAG_EOS) {
>> + /* EOS is always present, just ignore it */
>> + continue;
>> + }
>> +
>> + error_report("Unexpected RAM flags: %d", flags & invalid_flags);
>> +
>> if (flags & invalid_flags & RAM_SAVE_FLAG_COMPRESS_PAGE) {
>> error_report("Received an unexpected compressed page");
>> }
>> --
>> 2.35.3
>>
