From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH 4/6] kvm tools: Add rwlock wrapper Date: Sun, 29 May 2011 08:38:54 -0700 Message-ID: <20110529153854.GH2668@linux.vnet.ibm.com> References: <20110527110729.GA26920@elte.hu> <4DE13AF0.2080001@redhat.com> <20110528183259.GA15019@elte.hu> <4DE1EA93.6040401@redhat.com> <20110529073550.GA21254@elte.hu> <4DE1FBA5.6080905@redhat.com> <20110529123755.GC26627@elte.hu> <4DE2409D.1050701@redhat.com> <20110529142747.GA15441@elte.hu> <4DE25F70.5080700@redhat.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Ingo Molnar , Mathieu Desnoyers , Pekka Enberg , Sasha Levin , john@jfloren.net, kvm@vger.kernel.org, asias.hejun@gmail.com, gorcunov@gmail.com, prasadjoshi124@gmail.com To: Avi Kivity Return-path: Received: from e6.ny.us.ibm.com ([32.97.182.146]:52716 "EHLO e6.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754688Ab1E2PjD (ORCPT ); Sun, 29 May 2011 11:39:03 -0400 Received: from d01relay06.pok.ibm.com (d01relay06.pok.ibm.com [9.56.227.116]) by e6.ny.us.ibm.com (8.14.4/8.13.1) with ESMTP id p4TFEmPg004020 for ; Sun, 29 May 2011 11:14:48 -0400 Received: from d01av01.pok.ibm.com (d01av01.pok.ibm.com [9.56.224.215]) by d01relay06.pok.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id p4TFcwa31425460 for ; Sun, 29 May 2011 11:38:58 -0400 Received: from d01av01.pok.ibm.com (loopback [127.0.0.1]) by d01av01.pok.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id p4TFcv9h011847 for ; Sun, 29 May 2011 11:38:58 -0400 Content-Disposition: inline In-Reply-To: <4DE25F70.5080700@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Sun, May 29, 2011 at 06:00:00PM +0300, Avi Kivity wrote: > On 05/29/2011 05:27 PM, Ingo Molnar wrote: > >* Avi Kivity wrote: > > > >> I don't understand how you expect per_cpu to work in userspace. As > >> soon as you calculate the per-cpu address, it can be invalidated. > >> It doesn't help that you get a signal; you've already calculated > >> the address. > > > >I was thinking of some sort of transactional mechanism, a tightly > >controlled set of assembly instructions updating the percpu fields, > >where the migration event would be able to 'unroll' incomplete > >modifications done to the 'wrong' percpu data structure. (It would be > >rather complex and every percpu op would have to be an atomic because > >there's always the chance that it's executed on the wrong CPU.) > > > >But note that we do not even need any notification if there's a > >(local) lock on the percpu fields: > > > >It will work because it's statistically percpu the lock will not > >SMP-bounce between CPUs generally so it will be very fast to > >acquire/release it, and we get the full cache benefits of percpu > >variables. > > > >The migration notification would still be useful to detect grace > >periods at natural points - but as Paul pointed out polling it via > >SIGALRM works as well. The two (migration and SIGALRM) could be > >combined as well. > > I think it's way simpler to map cpu == thread. And in fact, when > you run a Linux kernel in a kvm guest, that's what happens, since > each vcpu _is_ a host thread. I have to agree with Avi here. If a stop_machine()-like approach is going to work, the updates have to be very rare, so any additional cache-nonlocality from having lots of threads should not be a problem. Especially given that in this particular case, there are exactly as many CPUs as threads anyway. The readers should only need to touch a constant number of cache lines either way. Or am I missing something here? Thanx, Paul