Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754068AbbBJUTy (ORCPT ); Tue, 10 Feb 2015 15:19:54 -0500 Received: from mail-lb0-f169.google.com ([209.85.217.169]:50812 "EHLO mail-lb0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751562AbbBJUTv (ORCPT ); Tue, 10 Feb 2015 15:19:51 -0500 MIME-Version: 1.0 In-Reply-To: <20150210201402.GU4166@linux.vnet.ibm.com> References: <1423579310-24555-1-git-send-email-riel@redhat.com> <1423579310-24555-7-git-send-email-riel@redhat.com> <54DA630D.6020601@amacapital.net> <20150210201402.GU4166@linux.vnet.ibm.com> From: Andy Lutomirski Date: Tue, 10 Feb 2015 12:19:28 -0800 Message-ID: Subject: Re: [PATCH 6/6] kvm,rcu,nohz: use RCU extended quiescent state when running KVM guest To: Paul McKenney Cc: Rik van Riel , Will Deacon , "linux-kernel@vger.kernel.org" , Catalin Marinas , =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= , kvm list , Marcelo Tosatti , Christian Borntraeger , Ingo Molnar , Oleg Nesterov , Luiz Capitulino , Paolo Bonzini Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2752 Lines: 65 On Tue, Feb 10, 2015 at 12:14 PM, Paul E. McKenney wrote: > On Tue, Feb 10, 2015 at 11:59:09AM -0800, Andy Lutomirski wrote: >> On 02/10/2015 06:41 AM, riel@redhat.com wrote: >> >From: Rik van Riel >> > >> >The host kernel is not doing anything while the CPU is executing >> >a KVM guest VCPU, so it can be marked as being in an extended >> >quiescent state, identical to that used when running user space >> >code. >> > >> >The only exception to that rule is when the host handles an >> >interrupt, which is already handled by the irq code, which >> >calls rcu_irq_enter and rcu_irq_exit. >> > >> >The guest_enter and guest_exit functions already switch vtime >> >accounting independent of context tracking. Leave those calls >> >where they are, instead of moving them into the context tracking >> >code. >> > >> >Signed-off-by: Rik van Riel >> >--- >> > include/linux/context_tracking.h | 6 ++++++ >> > include/linux/context_tracking_state.h | 1 + >> > include/linux/kvm_host.h | 3 ++- >> > 3 files changed, 9 insertions(+), 1 deletion(-) >> > >> >diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h >> >index 954253283709..b65fd1420e53 100644 >> >--- a/include/linux/context_tracking.h >> >+++ b/include/linux/context_tracking.h >> >@@ -80,10 +80,16 @@ static inline void guest_enter(void) >> > vtime_guest_enter(current); >> > else >> > current->flags |= PF_VCPU; >> >+ >> >+ if (context_tracking_is_enabled()) >> >+ context_tracking_enter(IN_GUEST); >> >> Why the if statement? >> >> Also, have you checked how much this hurts guest lightweight >> entry/exit latency? Context tracking is shockingly expensive for >> reasons I don't fully understand, but hopefully most of it is the >> vtime stuff. (Context tracking is *so* expensive that I almost >> think we should set the performance taint flag if we enable it, >> assuming that flag ended up getting merged. Also, we should make >> context tracking faster.) > > It turns out that context_tracking_is_enabled() is a static inline > that uses a static_key, so the overhead should be minimal on platforms > having a full implementation of static keys. Shouldn't we just fold that into context_tracking_xyz_enter? Also, why does the vtime stuff depend on RCU extended quiescent states? To me, they seem mostly orthogonal other than the fact that they hook into the same places. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/