Re: [RFC][PATCH] srcu: Implement call_srcu()

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Feb 01, 2012 at 12:49:57PM +0200, Avi Kivity wrote:
> On 02/01/2012 12:44 PM, Avi Kivity wrote:
> > On 02/01/2012 12:22 PM, Peter Zijlstra wrote:
> > > One of the things I was thinking of is adding a sequence counter in the
> > > per-cpu data. Using that we could do something like:
> > >
> > >   unsigned int seq1 = 0, seq2 = 0, count = 0;
> > >   int cpu, idx;
> > >
> > >   idx = ACCESS_ONCE(sp->completions) & 1;
> > >
> > >   for_each_possible_cpu(cpu)
> > > 	seq1 += per_cpu(sp->per_cpu_ref, cpu)->seq;
> > >
> > >   for_each_possible_cpu(cpu)
> > > 	count += per_cpu(sp->per_cpu_ref, cpu)->c[idx];
> > >
> > >   for_each_possible_cpu(cpu)
> > > 	seq2 += per_cpu(sp->per_cpu_ref, cpu)->seq;
> > >
> > >   /*
> > >    * there's no active references and no activity, we pass
> > >    */
> > >   if (seq1 == seq2 && count == 0)
> > > 	return;
> > >
> > >   synchronize_srcu_slow();
> > >
> > >
> > > This would add a fast-path which should catch the case Avi outlined
> > > where we call sync_srcu() when there's no other SRCU activity.
> >
> > Sorry, I was inaccurate.  In two of the cases indeed we don't expect
> > guest activity, and we're okay with waiting a bit if there is guest
> > activity - when we're altering the guest physical memory map.  But the
> > third case does have concurrent guest activity with
> > synchronize_srcu_expedited() and we still need it fast - that's when
> > userspace reads the dirty bitmap log of a running guest and replaces it
> > with a new bitmap.
> >
> > There may be a way to convert it to call_srcu() though.  Without
> > synchronize_srcu_expedited(), kvm sees both the old and the new bitmaps,
> > but that's fine, since the dirty bits will go *somewhere*, and we can
> > pick them up later in call_srcu().  The only problem is if this is the
> > very last call to kvm_vm_ioctl_get_dirty_log(), and the callback
> > triggers after it returns - we end up with a bag of bits with not one to
> > return them to.  Maybe we can detect this conditions (all vcpus ought to
> > be stopped), and do something like:
> >
> >
> >    if (all vcpus stopped) {
> >         /* no activity, this should be fast */
> >         synchronize_srcu()
> >         /* collect and return bits */
> >    } else {
> >         call_srcu(collect bits)
> >    }
> >
> > still a snag - we can't reliably detect that all vcpus are stopped, they
> > may be just resting in userspace, and restart while synchronize_srcu()
> > is running.
> >
> > Marcelo?
> >

VGABIOS mode constantly destroys and creates 0xa0000 slot, so
performance is required for KVM_SET_MEM too (it can probably be fixed in
qemu, but older qemu's must be supported).

For KVM it would be better for synchronize_srcu_expedited() to remain
relatively fast (think heavy users of GET_DIRTY_LOG such as Kemari),
even if slower than it is today.

synchronize_srcu() takes 10ms or so.

> Or something completely different - we can remove srcu from the equation
> completely in this case.  Use just one bitmap (so no
> rcu_assign_pointer), and use atomic operations to copy and clear:
> 
>   word = bitmap[i]
>   put_user(word)
>   atomic_and(&bitmap[i], ~word)
> 
> 
> -- 
> error compiling committee.c: too many arguments to function