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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
