Re: qemu-kvm.git build problem

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jan 12, 2010 at 01:23:15AM +0100, Jan Kiszka wrote:
> Jan Kiszka wrote:
> > Avi Kivity wrote:
> >> On 01/11/2010 12:13 PM, Jan Kiszka wrote:
> >>> BTW, does anybody know how to back-port synchronize_srcu_expedited best?
> >>> It looked like a simple mapping to synchronize_srcu was not sufficient
> >>> to achieve the same performance as with the pre-srcu locking (e.g.
> >>> guest&host stalled during guest's framebuffer setup).
> >>>    
> >> Isn't it sufficient to backport kernel/srcu.c?  I thought no sched.c 
> >> changes were necessary.
> > 
> > Haven't looked yet, but if that's the case, it would indeed be
> > straightforward.
> 
> It's far away from being straightforward: synchronize_rcu_expedited is
> based on synchronize_sched_expedited, introduced to 2.6.32. But that
> services is hooked deep into the scheduler, fiddling directly with
> runqueues (which are completely private to sched.c). This path looks
> like a dead end, specifically when its about supporting ~8 major Linux
> releases backwards.
> 
> Paul, we have a problem here on the KVM-for-older-kernels front: We need
> synchronize_rcu_expedited for acceptable write-side performance (there
> are certain phases with lots of changes, plain synchronize_rcu just
> stalls both guest and host for several seconds). Our target kernels
> (down to 2.6.27, unofficially even 2.6.24) do not have the expedited
> service. Can you think of a poor man's solution for those kernels?
> 
> Unfortunately, I don't think there is mechanical patching possible to
> role-back our srcu use to a rw-sem. But I will check this once again
> tomorrow.

Would it help if I backported the RCU expedited stuff?  Yes, it does
involve kernel/sched.c, but it is reasonably hands-off.

If this would help, please let me know to which kernel version you need
the backport.

FYI -- will be AFK Friday and most of the weekend, flying over the
Pacific.

							Thanx, Paul