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.
