Re: qemu-kvm.git build problem

[Jan Kiszka <jan.kiszka@web.de>]

[-- Attachment #1: Type: text/plain, Size: 1534 bytes --]

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.

Jan


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 257 bytes --]