Re: QEMU migration-test CI intermittent failure

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> On Thu, Sep 14, 2023 at 07:54:17PM -0300, Fabiano Rosas wrote:
>> Fabiano Rosas <farosas@suse.de> writes:
>> 
>> > Peter Xu <peterx@redhat.com> writes:
>> >
>> >> On Thu, Sep 14, 2023 at 12:57:08PM -0300, Fabiano Rosas wrote:
>> >>> I managed to reproduce it. It's not the return path error. In hindsight
>> >>> that's obvious because that error happens in the 'recovery' test and this
>> >>> one in the 'plain' one. Sorry about the noise.
>> >>
>> >> No worry.  It's good to finally identify that.
>> >>
>> >>> 
>> >>> This one reproduced with just 4 iterations of preempt/plain. I'll
>> >>> investigate.
>> >
>> > It seems that we're getting a tcp disconnect (ECONNRESET) on when doing
>> > that shutdown() on postcopy_qemufile_src. The one from commit 6621883f93
>> > ("migration: Fix potential race on postcopy_qemufile_src").
>> >
>> > I'm trying to determine why that happens when other times it just
>> > returns 0 as expected.
>> >
>> > Could this mean that we're kicking the dest too soon while it is still
>> > receiving valid data?
>> 
>> Looking a bit more into this, what's happening is that
>> postcopy_ram_incoming_cleanup() is shutting the postcopy_qemufile_dst
>> while ram_load_postcopy() is still running.
>> 
>> The postcopy_ram_listen_thread() function waits for the
>> main_thread_load_event, but that only works when not using preempt. With
>> the preempt thread, the event is set right away and we proceed to do the
>> cleanup without waiting.
>> 
>> So the assumption of commit 6621883f93 that the incoming side knows when
>> it has finished migrating is wrong IMO. Without the EOS we're relying on
>> the chance that the shutdown() happens after the last recvmsg has
>> returned and not during it.
>> 
>> Peter, what do you think?
>
> That's a good point.
>
> One thing to verify that (sorry, I still cannot reproduce it myself, which
> is so weirdly... it seems loads won't really help reproducing this) is to
> let the main thread wait for all requested pages to arrive:
>
> diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> index 29aea9456d..df055c51ea 100644
> --- a/migration/postcopy-ram.c
> +++ b/migration/postcopy-ram.c
> @@ -597,6 +597,12 @@ int postcopy_ram_incoming_cleanup(MigrationIncomingState *mis)
>      trace_postcopy_ram_incoming_cleanup_entry();
>  
>      if (mis->preempt_thread_status == PREEMPT_THREAD_CREATED) {
> +        /*
> +         * NOTE!  it's possible that the preempt thread is still handling
> +         * the last pages to arrive which were requested by faults.  Making
> +         * sure nothing is left behind.
> +         */
> +        while (qatomic_read(&mis->page_requested_count));
>          /* Notify the fast load thread to quit */
>          mis->preempt_thread_status = PREEMPT_THREAD_QUIT;
>          if (mis->postcopy_qemufile_dst) {
>
> If that can work solidly, we can figure out a better way than a dead loop
> here.

Yep, 2000+ iterations so far and no error.

Should we add something that makes ram_load_postcopy return once it's
finished? Then this code could just set PREEMPT_THREAD_QUIT and join the
preempt thread.