Re: Commit 282d8998e997 (srcu: Prevent expedited GPs and blocking readers from consuming CPU) cause qemu boot slow

[Paolo Bonzini]

On 6/12/22 19:29, Paolo Bonzini wrote:
> On 6/12/22 18:40, Paul E. McKenney wrote:
> > > Do these reserved memory regions really need to be allocated separately?
> > > (For example, are they really all non-contiguous?  If not, that is, if
> > > there are a lot of contiguous memory regions, could you sort the IORT
> > > by address and do one ioctl() for each set of contiguous memory 
> > > regions?)
> > >
> > > Are all of these reserved memory regions set up before init is spawned?
> > >
> > > Are all of these reserved memory regions set up while there is only a
> > > single vCPU up and running?
> > >
> > > Is the SRCU grace period really needed in this case?  (I freely confess
> > > to not being all that familiar with KVM.)
> >
> > Oh, and there was a similar many-requests problem with networking many
> > years ago.  This was solved by adding a new syscall/ioctl()/whatever
> > that permitted many requests to be presented to the kernel with a single
> > system call.
> >
> > Could a new ioctl() be introduced that requested a large number
> > of these memory regions in one go so as to make each call to
> > synchronize_rcu_expedited() cover a useful fraction of your 9000+
> > requests?  Adding a few of the KVM guys on CC for their thoughts.

Another question: how much can call_srcu() callbacks pile up these days? 
 I've always been a bit wary of letting userspace do an arbitrary 
number of allocations that can only be freed after a grace period, but 
perhaps there's a way to query SRCU and apply some backpressure?

Paolo