Re: [PATCH v6 10/13] migration: Respect postcopy request order in preemption mode

[Peter Xu <peterx@redhat.com>]

On Mon, May 23, 2022 at 11:56:14AM +0100, Dr. David Alan Gilbert wrote:
> * Peter Xu (peterx@redhat.com) wrote:
> > With preemption mode on, when we see a postcopy request that was requesting
> > for exactly the page that we have preempted before (so we've partially sent
> > the page already via PRECOPY channel and it got preempted by another
> > postcopy request), currently we drop the request so that after all the
> > other postcopy requests are serviced then we'll go back to precopy stream
> > and start to handle that.
> > 
> > We dropped the request because we can't send it via postcopy channel since
> > the precopy channel already contains partial of the data, and we can only
> > send a huge page via one channel as a whole.  We can't split a huge page
> > into two channels.
> > 
> > That's a very corner case and that works, but there's a change on the order
> > of postcopy requests that we handle since we're postponing this (unlucky)
> > postcopy request to be later than the other queued postcopy requests.  The
> > problem is there's a possibility that when the guest was very busy, the
> > postcopy queue can be always non-empty, it means this dropped request will
> > never be handled until the end of postcopy migration. So, there's a chance
> > that there's one dest QEMU vcpu thread waiting for a page fault for an
> > extremely long time just because it's unluckily accessing the specific page
> > that was preempted before.
> > 
> > The worst case time it needs can be as long as the whole postcopy migration
> > procedure.  It's extremely unlikely to happen, but when it happens it's not
> > good.
> > 
> > The root cause of this problem is because we treat pss->postcopy_requested
> > variable as with two meanings bound together, as the variable shows:
> > 
> >   1. Whether this page request is urgent, and,
> >   2. Which channel we should use for this page request.
> > 
> > With the old code, when we set postcopy_requested it means either both (1)
> > and (2) are true, or both (1) and (2) are false.  We can never have (1)
> > and (2) to have different values.
> > 
> > However it doesn't necessarily need to be like that.  It's very legal that
> > there's one request that has (1) very high urgency, but (2) we'd like to
> > use the precopy channel.  Just like the corner case we were discussing
> > above.
> > 
> > To differenciate the two meanings better, introduce a new field called
> > postcopy_target_channel, showing which channel we should use for this page
> > request, so as to cover the old meaning (2) only.  Then we leave the
> > postcopy_requested variable to stand only for meaning (1), which is the
> > urgency of this page request.
> > 
> > With this change, we can easily boost priority of a preempted precopy page
> > as long as we know that page is also requested as a postcopy page.  So with
> > the new approach in get_queued_page() instead of dropping that request, we
> > send it right away with the precopy channel so we get back the ordering of
> > the page faults just like how they're requested on dest.
> > 
> > Alongside, I touched up find_dirty_block() to only set the postcopy fields
> > in the pss section if we're going through a postcopy migration.  That's a
> > very light optimization and shouldn't affect much.
> > 
> > Reported-by: manish.mishra@nutanix.com
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> 
> So I think this is OK; getting a bit complicated!

Yes it is.  I added some more comment, hopefully it'll help a little bit.

> 
> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

Thanks!

> >  static bool find_dirty_block(RAMState *rs, PageSearchStatus *pss, bool *again)
> >  {
> > -    /* This is not a postcopy requested page */
> > -    pss->postcopy_requested = false;
> > +    if (migration_in_postcopy()) {
> > +        /*
> > +         * This is not a postcopy requested page, mark it "not urgent", and
> > +         * use precopy channel to send it.
> > +         */
> > +        pss->postcopy_requested = false;
> > +        pss->postcopy_target_channel = RAM_CHANNEL_PRECOPY;
> > +    }
> 
> Do you need the 'if' here?

Hmm good question..  precopy should always have these two fields cleared
anyway so I wanted to avoid setting them every time, but I just noticed
that pss is not initialized at all when used..

static int ram_find_and_save_block(RAMState *rs)
{
    PageSearchStatus pss;
    ...
}

So either we'd reset pss explicitly on these fields, or simpler - let me
drop the if..

Thanks,

-- 
Peter Xu