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McKenney" To: Christian Borntraeger Cc: David Woodhouse , peterz@infradead.org, mhillenb@amazon.de, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering guest mode Reply-To: paulmck@linux.vnet.ibm.com References: <20180711174843.GX3593@linux.vnet.ibm.com> <20180711180101.3711464-1-dwmw2@infradead.org> <20180711182053.GA3593@linux.vnet.ibm.com> <20180711183645.GA23820@linux.vnet.ibm.com> <62e945ee-a58e-f9b3-279c-74cd0f5809da@de.ibm.com> <20180711202752.GC3593@linux.vnet.ibm.com> <2f7ba67e-0442-13cc-628c-2dca56520c21@de.ibm.com> <20180711213259.GF3593@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180711213259.GF3593@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18071123-0060-0000-0000-0000028ACD0A X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009353; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01059997; UDB=6.00544066; IPR=6.00837917; MB=3.00022108; MTD=3.00000008; XFM=3.00000015; UTC=2018-07-11 23:35:10 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18071123-0061-0000-0000-000045C3CCF6 Message-Id: <20180711233727.GA9888@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-07-11_06:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1807110247 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 11, 2018 at 02:32:59PM -0700, Paul E. McKenney wrote: > On Wed, Jul 11, 2018 at 11:11:19PM +0200, Christian Borntraeger wrote: > > > > > > On 07/11/2018 10:27 PM, Paul E. McKenney wrote: > > > On Wed, Jul 11, 2018 at 08:39:36PM +0200, Christian Borntraeger wrote: > > >> > > >> > > >> On 07/11/2018 08:36 PM, Paul E. McKenney wrote: > > >>> On Wed, Jul 11, 2018 at 11:20:53AM -0700, Paul E. McKenney wrote: > > >>>> On Wed, Jul 11, 2018 at 07:01:01PM +0100, David Woodhouse wrote: > > >>>>> From: David Woodhouse > > >>>>> > > >>>>> RCU can spend long periods of time waiting for a CPU which is actually in > > >>>>> KVM guest mode, entirely pointlessly. Treat it like the idle and userspace > > >>>>> modes, and don't wait for it. > > >>>>> > > >>>>> Signed-off-by: David Woodhouse > > >>>> > > >>>> And idiot here forgot about some of the debugging code in RCU's dyntick-idle > > >>>> code. I will reply with a fixed patch. > > >>>> > > >>>> The code below works just fine as long as you don't enable CONFIG_RCU_EQS_DEBUG, > > >>>> so should be OK for testing, just not for mainline. > > >>> > > >>> And here is the updated code that allegedly avoids splatting when run with > > >>> CONFIG_RCU_EQS_DEBUG. > > >>> > > >>> Thoughts? > > >>> > > >>> Thanx, Paul > > >>> > > >>> ------------------------------------------------------------------------ > > >>> > > >>> commit 12cd59e49cf734f907f44b696e2c6e4b46a291c3 > > >>> Author: David Woodhouse > > >>> Date: Wed Jul 11 19:01:01 2018 +0100 > > >>> > > >>> kvm/x86: Inform RCU of quiescent state when entering guest mode > > >>> > > >>> RCU can spend long periods of time waiting for a CPU which is actually in > > >>> KVM guest mode, entirely pointlessly. Treat it like the idle and userspace > > >>> modes, and don't wait for it. > > >>> > > >>> Signed-off-by: David Woodhouse > > >>> Signed-off-by: Paul E. McKenney > > >>> [ paulmck: Adjust to avoid bad advice I gave to dwmw, avoid WARN_ON()s. ] > > >>> > > >>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > > >>> index 0046aa70205a..b0c82f70afa7 100644 > > >>> --- a/arch/x86/kvm/x86.c > > >>> +++ b/arch/x86/kvm/x86.c > > >>> @@ -7458,7 +7458,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) > > >>> vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD; > > >>> } > > >>> > > >>> + rcu_kvm_enter(); > > >>> kvm_x86_ops->run(vcpu); > > >>> + rcu_kvm_exit(); > > >> > > >> As indicated in my other mail. This is supposed to be handled in the guest_enter|exit_ calls around > > >> the run function. This would also handle other architectures. So if the guest_enter_irqoff code is > > >> not good enough, we should rather fix that instead of adding another rcu hint. > > > > > > Something like this, on top of the earlier patch? I am not at all > > > confident of this patch because there might be other entry/exit > > > paths I am missing. Plus there might be RCU uses on the arch-specific > > > patch to and from the guest OS. > > > > > > Thoughts? > > > > > > > If you instrment guest_enter/exit, you should cover all cases and all architectures as far > > as I can tell. FWIW, we did this rcu_note thing back then actually handling this particular > > case of long running guests blocking rcu for many seconds. And I am pretty sure that > > this did help back then. > > And my second patch on the email you replied to replaced the only call > to rcu_virt_note_context_switch(). So maybe it covers what it needs to, > but yes, there might well be things I missed. Let's see what David > comes up with. > > What changed was RCU's reactions to longish grace periods. It used to > be very aggressive about forcing the scheduler to do otherwise-unneeded > context switches, which became a problem somewhere between v4.9 and v4.15. > I therefore reduced the number of such context switches, which in turn > caused KVM to tell RCU about quiescent states way too infrequently. > > The advantage of the rcu_kvm_enter()/rcu_kvm_exit() approach is that > it tells RCU of an extended duration in the guest, which means that > RCU can ignore the corresponding CPU, which in turn allows the guest > to proceed without any RCU-induced interruptions. > > Does that make sense, or am I missing something? I freely admit to > much ignorance of both kvm and s390! ;-) But I am getting some rcutorture near misses on the commit that introduces rcu_kvm_enter() and rcu_kvm_exit() to the x86 arch-specific vcpu_enter_guest() function. These near misses occur when running rcutorture scenarios TREE01 and TREE03, and in my -rcu tree rather than the v4.15 version of this patch. Given that I am making pervasive changes to the way that RCU works, it might well be that this commit is an innocent bystander. I will run tests overnight and let you know what comes up. Thanx, Paul