Re: [PATCH] memory: avoid unnecessary iteration when updating ioeventfds

[Peter Xu]

On Wed, Mar 01, 2023 at 04:36:20PM +0800, Jason Wang wrote:
> On Tue, Feb 28, 2023 at 10:25 PM Longpeng(Mike) <longpeng2@huawei.com> wrote:
> >
> > From: Longpeng <longpeng2@huawei.com>
> >
> > When updating ioeventfds, we need to iterate all address spaces and
> > iterate all flat ranges of each address space. There is so much
> > redundant process that a FlatView would be iterated for so many times
> > during one commit (memory_region_transaction_commit).
> >
> > We can mark a FlatView as UPDATED and then skip it in the next iteration
> > and clear the UPDATED flag at the end of the commit. The overhead can
> > be significantly reduced.
> >
> > For example, a VM with 16 vdpa net devices and each one has 65 vectors,
> > can reduce the time spent on memory_region_transaction_commit by 95%.
> >
> > Signed-off-by: Longpeng <longpeng2@huawei.com>
> > ---
> >  include/exec/memory.h |  2 ++
> >  softmmu/memory.c      | 28 +++++++++++++++++++++++++++-
> >  2 files changed, 29 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/exec/memory.h b/include/exec/memory.h
> > index 2e602a2fad..974eabf765 100644
> > --- a/include/exec/memory.h
> > +++ b/include/exec/memory.h
> > @@ -1093,6 +1093,8 @@ struct FlatView {
> >      unsigned nr_allocated;
> >      struct AddressSpaceDispatch *dispatch;
> >      MemoryRegion *root;
> > +#define FLATVIEW_FLAG_IOEVENTFD_UPDATED (1 << 0)
> > +    unsigned flags;
> >  };
> >
> >  static inline FlatView *address_space_to_flatview(AddressSpace *as)
> > diff --git a/softmmu/memory.c b/softmmu/memory.c
> > index 9d64efca26..71ff996712 100644
> > --- a/softmmu/memory.c
> > +++ b/softmmu/memory.c
> > @@ -815,6 +815,15 @@ FlatView *address_space_get_flatview(AddressSpace *as)
> >      return view;
> >  }
> >
> > +static void address_space_reset_view_flags(AddressSpace *as, unsigned mask)
> > +{
> > +    FlatView *view = address_space_get_flatview(as);
> > +
> > +    if (view->flags & mask) {
> > +        view->flags &= ~mask;
> > +    }
> > +}
> > +
> >  static void address_space_update_ioeventfds(AddressSpace *as)
> >  {
> >      FlatView *view;
> > @@ -825,6 +834,12 @@ static void 
> > address_space_update_ioeventfds(AddressSpace *as)
> >      AddrRange tmp;
> >      unsigned i;
> >
> > +    view = address_space_get_flatview(as);
> > +    if (view->flags & FLATVIEW_FLAG_IOEVENTFD_UPDATED) {
> > +        return;
> > +    }
> > +    view->flags |= FLATVIEW_FLAG_IOEVENTFD_UPDATED;
> > +
> 
> Won't we lose the listener calls if multiple address spaces have the
> same flatview?

I have the same concern with Jason.  I don't think it matters in reality,
since only address_space_io uses it so far. but it doesn't really look
reasonable and clean.

One other idea of optimizing ioeventfd update is we can add a per-AS
counter (ioeventfd_notifiers), increase if any eventfd_add|del is
registered in memory_listener_register(), and decrease when unregister.
Then address_space_update_ioeventfds() can be skipped completely if
ioeventfd_notifiers==0.

Side note: Jason, do you think we should drop vhost_eventfd_add|del?
They're all no-ops right now.

Thanks,

-- 
Peter Xu