On Thu, Aug 01, 2019 at 02:35:50PM -0700, Sean Christopherson wrote: > On Thu, Aug 01, 2019 at 08:34:53PM +0200, Thomas Gleixner wrote: > > On Thu, 1 Aug 2019, Oleg Nesterov wrote: > > > On 08/01, Thomas Gleixner wrote: > > > > > > > > @@ -8172,6 +8174,10 @@ static int vcpu_run(struct kvm_vcpu *vcp > > > > ++vcpu->stat.signal_exits; > > > > break; > > > > } > > > > + > > > > + if (notify_resume_pending()) > > > > + tracehook_handle_notify_resume(); > > > > > > shouldn't you drop kvm->srcu before tracehook_handle_notify_resume() ? > > > > > > I don't understand this code at all, but vcpu_run() does this even before > > > cond_resched(). > > > > Yeah, I noticed that it's dropped around cond_resched(). > > > > My understanding is that for voluntary giving up the CPU via cond_resched() > > it needs to be dropped. > > > > For involuntary preemption (CONFIG_PREEMPT=y) it's not required as the > > whole code section after preempt_enable() is fully preemptible. > > > > Now the 1Mio$ question is whether any of the notify functions invokes > > cond_resched() and whether that really matters. Paolo? > > cond_resched() is called via tracehook_notify_resume()->task_work_run(), > and "kernel code can only call cond_resched() in places where it ... > cannot hold references to any RCU-protected data structures" according to > https://lwn.net/Articles/603252/. This is SRCU, you can reschedule while holding that just fine. It will just delay some kvm operations, like the memslot stuff. I don't think it is terrible to keep it, but people more versed in KVM might know of a good reason to drop it anyway.
