From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kiszka Subject: Re: qemu-kvm.git build problem Date: Tue, 12 Jan 2010 01:23:15 +0100 Message-ID: <4B4BC0F3.2010507@web.de> References: <1262025646.2774.16.camel@localhost.localdomain> <1263014581.2439.207.camel@localhost.localdomain> <4B4AF9D2.3080100@siemens.com> <4B4AFAD9.3010604@redhat.com> <4B4AFBC7.1020801@siemens.com> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enigD7425DD6B9DCCF0858EC670F" Cc: Avi Kivity , Lucas Meneghel Rodrigues , KVM mailing list , Michael Goldish , Eduardo Habkost , Dor Laor To: "Paul E. McKenney" Return-path: Received: from fmmailgate02.web.de ([217.72.192.227]:49771 "EHLO fmmailgate02.web.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753393Ab0ALAXZ (ORCPT ); Mon, 11 Jan 2010 19:23:25 -0500 In-Reply-To: <4B4AFBC7.1020801@siemens.com> Sender: kvm-owner@vger.kernel.org List-ID: This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enigD7425DD6B9DCCF0858EC670F Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable 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 be= st? >>> It looked like a simple mapping to synchronize_srcu was not sufficien= t >>> to achieve the same performance as with the pre-srcu locking (e.g. >>> guest&host stalled during guest's framebuffer setup). >>> =20 >> Isn't it sufficient to backport kernel/srcu.c? I thought no sched.c=20 >> changes were necessary. >=20 > 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 --------------enigD7425DD6B9DCCF0858EC670F Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.9 (GNU/Linux) Comment: Using GnuPG with SUSE - http://enigmail.mozdev.org iEYEARECAAYFAktLwPwACgkQitSsb3rl5xSRwgCff90edpTCsxqhs0LRj3Fo9Rz0 O6cAoNGNY0nN9p4rLperRySSrjalmzEr =qir+ -----END PGP SIGNATURE----- --------------enigD7425DD6B9DCCF0858EC670F--