Re: [PATCH v3 08/10] migration: Allow network to fail even during recovery

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> Normally the postcopy recover phase should only exist for a super short
> period, that's the duration when QEMU is trying to recover from an
> interrupted postcopy migration, during which handshake will be carried out
> for continuing the procedure with state changes from PAUSED -> RECOVER ->
> POSTCOPY_ACTIVE again.
>
> Here RECOVER phase should be super small, that happens right after the
> admin specified a new but working network link for QEMU to reconnect to
> dest QEMU.
>
> However there can still be case where the channel is broken in this small
> RECOVER window.
>
> If it happens, with current code there's no way the src QEMU can got kicked
> out of RECOVER stage. No way either to retry the recover in another channel
> when established.
>
> This patch allows the RECOVER phase to fail itself too - we're mostly
> ready, just some small things missing, e.g. properly kick the main
> migration thread out when sleeping on rp_sem when we found that we're at
> RECOVER stage.  When this happens, it fails the RECOVER itself, and
> rollback to PAUSED stage.  Then the user can retry another round of
> recovery.
>
> To make it even stronger, teach QMP command migrate-pause to explicitly
> kick src/dst QEMU out when needed, so even if for some reason the migration
> thread didn't got kicked out already by a failing rethrn-path thread, the
> admin can also kick it out.
>
> This will be an super, super corner case, but still try to cover that.
>
> One can try to test this with two proxy channels for migration:
>
>   (a) socat unix-listen:/tmp/src.sock,reuseaddr,fork tcp:localhost:10000
>   (b) socat tcp-listen:10000,reuseaddr,fork unix:/tmp/dst.sock
>
> So the migration channel will be:
>
>                       (a)          (b)
>   src -> /tmp/src.sock -> tcp:10000 -> /tmp/dst.sock -> dst
>
> Then to make QEMU hang at RECOVER stage, one can do below:
>
>   (1) stop the postcopy using QMP command postcopy-pause
>   (2) kill the 2nd proxy (b)
>   (3) try to recover the postcopy using /tmp/src.sock on src
>   (4) src QEMU will go into RECOVER stage but won't be able to continue
>       from there, because the channel is actually broken at (b)
>
> Before this patch, step (4) will make src QEMU stuck in RECOVER stage,
> without a way to kick the QEMU out or continue the postcopy again.  After
> this patch, (4) will quickly fail qemu and bounce back to PAUSED stage.
>
> Admin can also kick QEMU from (4) into PAUSED when needed using
> migrate-pause when needed.
>
> After bouncing back to PAUSED stage, one can recover again.
>
> Reported-by: Xiaohui Li <xiaohli@redhat.com>
> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2111332
> Signed-off-by: Peter Xu <peterx@redhat.com>

Reviewed-by: Fabiano Rosas <farosas@suse.de>