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

[Avi Kivity <avi@redhat.com>]

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?
>

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