Re: [Xen-devel] [PATCH for-4.12 v2 15/17] xen/arm: p2m: Add support for preemption in p2m_cache_flush_range

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 7 Dec 2018, Julien Grall wrote:
> > > @@ -1547,6 +1551,25 @@ int p2m_cache_flush_range(struct domain *d, gfn_t
> > > start, gfn_t end)
> > >         while ( gfn_x(start) < gfn_x(end) )
> > >       {
> > > +       /*
> > > +         * Cleaning the cache for the P2M may take a long time. So we
> > > +         * need to be able to preempt. We will arbitrarily preempt every
> > > +         * time count reach 512 or above.
> > > +
> > > +         *
> > > +         * The count will be incremented by:
> > > +         *  - 1 on region skipped
> > > +         *  - 10 for each page requiring a flush
> > 
> > Why this choice? A page flush should cost much more than 10x a region
> > skipped, more like 100x or 1000x. In fact, doing the full loop without
> > calling flush_page_to_ram should be cheap and fast, right?.
> 
> It is cheaper than a flush of the page but it still has a cost. You have to
> walk the stage-2 in software that will require to map the tables. As all the
> memory is not mapped in the hypervisor on arm32 this will require a map/unmap
> operation. On arm64, so far the full memory is mapped, so the map/unmap is
> pretty much a NOP.

Good point, actually the cost of an "empty" iteration is significantly
different on arm32 and arm64.


> > I would:
> > 
> > - not increase count on region skipped at all
> > - increase it by 1 on each page requiring a flush
> > - set the limit lower, if we go with your proposal it would be about 50,
> >    I am not sure what the limit should be though
> I don't think you can avoid incrementing count on region skipped. While one
> lookup is pretty cheap, all the lookups for hole added together may result to
> a pretty long time.

Thinking of the arm32 case where map/unmap need to be done, you are
right.


> Even if stage-2 mappings are handled by the hypervisor, the guest is still
> somewhat in control of it because it can balloon in/out pages. The operation
> may result to shatter stage-2 mappings.
> 
> It would be feasible for a guest to shatter 1GB of memory in 4KB mappings in
> stage-2 entries and then remove all the entries. This means the stage-2 would
> contains 262144 holes. This would result to 262144 iterations, so no matter
> how cheap it is the resulting time spent without preemption is going to be
> quite important.

OK


> The choice in the numbers 1 vs 10 is pretty much random. The question is how
> often we want to check for pending softirq. The check is pretty much trivial,
> yet it has a cost to preempt. With the current solution, we check preemption
> every 512 holes or 51 pages flushed (~204KB flushed).
> 
> This sounds ok to me. Feel free to suggest better number.

One suggestion is that we might want to treat the arm32 case differently
from the arm64 case, given the different cost of mapping/unmapping pages
during the walk. Would it be fair to say that if an arm64 empty
iteration is worth "1" unit of work, then an arm32 empty iteration is
worth at least "2" unit of work? Or more? My gut feeling is that it is
more like:

empty arm64:       1
empty arm32:       5
flush arm32/arm64: 10

Or even:

empty arm64:       1
empty arm32:       10
flush arm32/arm64: 20

and the overall limits for checks could be 512 or 1024 like you
suggested.

But I don't really know, we would need precise benchmarks to have an
idea about what are the best numbers for this. I am not suggesting you
have to do any more benchmarks now, we'll just hand-wave it for now.



> > > +         */
> > > +        if ( count >= 512 )
> > > +        {
> > > +            if ( softirq_pending(smp_processor_id()) )
> > > +            {
> > > +                rc = -ERESTART;
> > > +                break;
> > > +            }
> > > +            count = 0;
> > 
> > No need to set count to 0 here
> 
> Well, the code would not do the same here. If you don't reset to 0, you would
> check softirq_pending() all the iteration when count reached 512.
> 
> If you reset 0, you will avoid to check softirq_pending() until the next time
> count reached 512.
> 
> The both are actually valid. It just a matter on whether we are assuming that
> a softirq will happen soon after reaching 512?

My comment was wrong, the code is right as is, I think we want to check
softirq_pending every 512 iterations.

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