Re: [RFC][Patch v9 0/6] KVM: Guest Free Page Hinting

From: Alexander Duyck
Date: Thu Mar 07 2019 - 17:20:05 EST

On Thu, Mar 7, 2019 at 1:28 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> On 07.03.19 22:14, Alexander Duyck wrote:
> > On Thu, Mar 7, 2019 at 10:53 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> >>
> >> On Thu, Mar 07, 2019 at 10:45:58AM -0800, Alexander Duyck wrote:
> >>> To that end what I think w may want to do is instead just walk the LRU
> >>> list for a given zone/order in reverse order so that we can try to
> >>> identify the pages that are most likely to be cold and unused and
> >>> those are the first ones we want to be hinting on rather than the ones
> >>> that were just freed. If we can look at doing something like adding a
> >>> jiffies value to the page indicating when it was last freed we could
> >>> even have a good point for determining when we should stop processing
> >>> pages in a given zone/order list.
> >>>
> >>> In reality the approach wouldn't be too different from what you are
> >>> doing now, the only real difference would be that we would just want
> >>> to walk the LRU list for the given zone/order rather then pulling
> >>> hints on what to free from the calls to free_one_page. In addition we
> >>> would need to add a couple bits to indicate if the page has been
> >>> hinted on, is in the middle of getting hinted on, and something such
> >>> as the jiffies value I mentioned which we could use to determine how
> >>> old the page is.
> >>
> >> Do we really need bits in the page?
> >> Would it be bad to just have a separate hint list?
> >
> > The issue is lists are expensive to search. If we have a single bit in
> > the page we can check it as soon as we have the page.
> >
> >> If you run out of free memory you can check the hint
> >> list, if you find stuff there you can spin
> >> or kick the hypervisor to hurry up.
> >
> > This implies you are keeping a separate list of pages for what has
> > been hinted on. If we are pulling pages out of the LRU list for that
> > it will require the zone lock to move the pages back and forth and for
> > higher core counts that isn't going to scale very well, and if you are
> > trying to pull out a page that is currently being hinted on you will
> > run into the same issue of having to wait for the hint to be completed
> > before proceeding.
> >
> >> Core mm/ changes, so nothing's easy, I know.
> >
> > We might be able to reuse some existing page flags. For example, there
> > is the PG_young and PG_idle flags that would actually be a pretty good
> > fit in terms of what we are looking for in behavior. We could set
> > PG_young when the page is initially freed, then clear it when we start
> > to perform the hint, and set PG_idle once the hint has been completed.
> Just noting that when hinting, we have to set all affected sub-page bits
> as far as I see.

You may be correct there. One thing I hadn't thought about is what
happens if the page is split or merged up to a higher order. I guess I
could be talked into being okay with a side list that we maintain a
few pages in that are isolated from the rest.

> >
> > The check for if we could use a page would be pretty fast as a result
> > as well since if PG_young or PG_idle are set it means the page is free
> > to use so the check in arch_alloc_page would be pretty cheap since we
> > could probably test for both bits in one read.
> >
> I still dislike spinning on ordinary allocation paths. If we want to go
> that way, core mm has to consider these bits and try other pages first.

Agreed. I was just thinking that would be follow-on work since in my
mind the collision rate for these should be low.