On Wed, Jul 11, 2018 at 04:37:27PM -0700, Paul E. McKenney wrote: > 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 <dwmw@xxxxxxxxxxxx> > > > >>>>> > > > >>>>> 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 <dwmw@xxxxxxxxxxxx> > > > >>>> > > > >>>> 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 <dwmw@xxxxxxxxxxxx> > > > >>> 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 <dwmw@xxxxxxxxxxxx> > > > >>> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > > > >>> [ 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. And thus far, the verdict is "intermittent". I am starting a new series with CONFIG_RCU_EQS_DEBUG=y, which should detect any mismatched rcu_kvm_enter()/rcu_kvm_exit() pairs. Thanx, Paul
