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Re: [PATCH v6 10/13] migration: Respect postcopy request order in preemp
From: |
Peter Xu |
Subject: |
Re: [PATCH v6 10/13] migration: Respect postcopy request order in preemption mode |
Date: |
Tue, 24 May 2022 14:42:41 -0400 |
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
- Re: [PATCH v6 11/13] tests: Add postcopy tls migration test, (continued)
- [PATCH v6 12/13] tests: Add postcopy tls recovery migration test, Peter Xu, 2022/05/17
- [PATCH v6 13/13] tests: Add postcopy preempt tests, Peter Xu, 2022/05/17
- [PATCH v6 06/13] migration: Add property x-postcopy-preempt-break-huge, Peter Xu, 2022/05/17
- [PATCH v6 04/13] migration: Postcopy recover with preempt enabled, Peter Xu, 2022/05/17
- [PATCH v6 10/13] migration: Respect postcopy request order in preemption mode, Peter Xu, 2022/05/17