On Tue, May 07, 2024, Sean Christopherson wrote: > On Tue, May 07, 2024, Paul E. McKenney wrote: > > On Tue, May 07, 2024 at 02:00:12PM -0700, Sean Christopherson wrote: > > > On Tue, May 07, 2024, Paul E. McKenney wrote: > > > > On Tue, May 07, 2024 at 10:55:54AM -0700, Sean Christopherson wrote: > > > > > On Fri, May 03, 2024, Paul E. McKenney wrote: > > > > > > On Fri, May 03, 2024 at 02:29:57PM -0700, Sean Christopherson wrote: > > > > > > > So if we're comfortable relying on the 1 second timeout to guard against a > > > > > > > misbehaving userspace, IMO we might as well fully rely on that guardrail. I.e. > > > > > > > add a generic PF_xxx flag (or whatever flag location is most appropriate) to let > > > > > > > userspace communicate to the kernel that it's a real-time task that spends the > > > > > > > overwhelming majority of its time in userspace or guest context, i.e. should be > > > > > > > given extra leniency with respect to rcuc if the task happens to be interrupted > > > > > > > while it's in kernel context. > > > > > > > > > > > > But if the task is executing in host kernel context for quite some time, > > > > > > then the host kernel's RCU really does need to take evasive action. > > > > > > > > > > Agreed, but what I'm saying is that RCU already has the mechanism to do so in the > > > > > form of the 1 second timeout. > > > > > > > > Plus RCU will force-enable that CPU's scheduler-clock tick after about > > > > ten milliseconds of that CPU not being in a quiescent state, with > > > > the time varying depending on the value of HZ and the number of CPUs. > > > > After about ten seconds (halfway to the RCU CPU stall warning), it will > > > > resched_cpu() that CPU every few milliseconds. > > > > > > > > > And while KVM does not guarantee that it will immediately resume the guest after > > > > > servicing the IRQ, neither does the existing userspace logic. E.g. I don't see > > > > > anything that would prevent the kernel from preempting the interrupt task. > > > > > > > > Similarly, the hypervisor could preempt a guest OS's RCU read-side > > > > critical section or its preempt_disable() code. > > > > > > > > Or am I missing your point? > > > > > > I think you're missing my point? I'm talking specifically about host RCU, what > > > is or isn't happening in the guest is completely out of scope. > > > > Ah, I was thinking of nested virtualization. > > > > > My overarching point is that the existing @user check in rcu_pending() is optimistic, > > > in the sense that the CPU is _likely_ to quickly enter a quiescent state if @user > > > is true, but it's not 100% guaranteed. And because it's not guaranteed, RCU has > > > the aforementioned guardrails. > > > > You lost me on this one. > > > > The "user" argument to rcu_pending() comes from the context saved at > > the time of the scheduling-clock interrupt. In other words, the CPU > > really was executing in user mode (which is an RCU quiescent state) > > when the interrupt arrived. > > > > And that suffices, 100% guaranteed. > > Ooh, that's where I'm off in the weeds. I was viewing @user as "this CPU will be > quiescent", but it really means "this CPU _was_ quiescent". Hrm, I'm still confused though. That's rock solid for this check: /* Is the RCU core waiting for a quiescent state from this CPU? */ But I don't understand how it plays into the next three checks that can result in rcuc being awakened. I suspect it's these checks that Leo and Marcelo are trying squash, and these _do_ seem like they are NOT 100% guaranteed by the @user check. /* Does this CPU have callbacks ready to invoke? */ /* Has RCU gone idle with this CPU needing another grace period? */ /* Have RCU grace period completed or started? */ > > The reason that it suffices is that other RCU code such as rcu_qs() and > > rcu_note_context_switch() ensure that this CPU does not pay attention to > > the user-argument-induced quiescent state unless this CPU had previously > > acknowledged the current grace period. > > > > And if the CPU has previously acknowledged the current grace period, that > > acknowledgement must have preceded the interrupt from user-mode execution. > > Thus the prior quiescent state represented by that user-mode execution > > applies to that previously acknowledged grace period. > > To confirm my own understanding: > > 1. Acknowledging the current grace period means any future rcu_read_lock() on > the CPU will be accounted to the next grace period. > > 2. A CPU can acknowledge a grace period without being quiescent. > > 3. Userspace can't acknowledge a grace period, because it doesn't run kernel > code (stating the obvious). > > 4. All RCU read-side critical sections must complete before exiting to usersepace. > > And so if an IRQ interrupts userspace, and the CPU previously acknowledged grace > period N, RCU can infer that grace period N elapsed on the CPU, because all > "locks" held on grace period N are guaranteed to have been dropped. > > > This is admittedly a bit indirect, but then again this is Linux-kernel > > RCU that we are talking about. > > > > > And I'm arguing that, since the @user check isn't bombproof, there's no reason to > > > try to harden against every possible edge case in an equivalent @guest check, > > > because it's unnecessary for kernel safety, thanks to the guardrails. > > > > And the same argument above would also apply to an equivalent check for > > execution in guest mode at the time of the interrupt. > > This is partly why I was off in the weeds. KVM cannot guarantee that the > interrupt that leads to rcu_pending() actually interrupted the guest. And the > original patch didn't help at all, because a time-based check doesn't come > remotely close to the guarantees that the @user check provides. > > > Please understand that I am not saying that we absolutely need an > > additional check (you tell me!). > > Heh, I don't think I'm qualified to answer that question, at least not yet. > > > But if we do need RCU to be more aggressive about treating guest execution as > > an RCU quiescent state within the host, that additional check would be an > > excellent way of making that happen. > > It's not clear to me that being more agressive is warranted. If my understanding > of the existing @user check is correct, we _could_ achieve similar functionality > for vCPU tasks by defining a rule that KVM must never enter an RCU critical section > with PF_VCPU set and IRQs enabled, and then rcu_pending() could check PF_VCPU. > On x86, this would be relatively straightforward (hack-a-patch below), but I've > no idea what it would look like on other architectures. > > But the value added isn't entirely clear to me, probably because I'm still missing > something. KVM will have *very* recently called __ct_user_exit(CONTEXT_GUEST) to > note the transition from guest to host kernel. Why isn't that a sufficient hook > for RCU to infer grace period completion? > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 1a9e1e0c9f49..259b60adaad7 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -11301,6 +11301,11 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) > if (vcpu->arch.guest_fpu.xfd_err) > wrmsrl(MSR_IA32_XFD_ERR, 0); > > + RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) || > + lock_is_held(&rcu_lock_map) || > + lock_is_held(&rcu_sched_lock_map), > + "KVM in RCU read-side critical section with PF_VCPU set and IRQs enabled"); > + > /* > * Consume any pending interrupts, including the possible source of > * VM-Exit on SVM and any ticks that occur between VM-Exit and now. > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index b2bccfd37c38..cdb815105de4 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -3929,7 +3929,8 @@ static int rcu_pending(int user) > return 1; > > /* Is this a nohz_full CPU in userspace or idle? (Ignore RCU if so.) */ > - if ((user || rcu_is_cpu_rrupt_from_idle()) && rcu_nohz_full_cpu()) > + if ((user || rcu_is_cpu_rrupt_from_idle() || (current->flags & PF_VCPU)) && > + rcu_nohz_full_cpu()) > return 0; > > /* Is the RCU core waiting for a quiescent state from this CPU? */ > >
