Il 16/01/2014 10:23, Christian Borntraeger ha scritto: > When starting lots of dataplane devices the bootup takes very long on my > s390 system(prototype irqfd code). With larger setups we are even able to > trigger some timeouts in some components. > Turns out that the KVM_SET_GSI_ROUTING ioctl takes very > long (strace claims up to 0.1 sec) when having multiple CPUs. > This is caused by the synchronize_rcu and the HZ=100 of s390. > We can defer the freeing outside of the ioctl path by using kfree_rcu. > > Please note that we now have to check for a NULL pointer, since > the underlying call_rcu in kfree_rcu cannot handle broken rcu > callbacks. > > This patch reduces the boot time till mounting root from 8 to 2 seconds > on my s390 guest with 100 disks. > > Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx> Hi Christian, please take a look at the thread at http://permalink.gmane.org/gmane.comp.emulators.kvm.devel/116933 - uses of call_rcu need to be rate limited, otherwise the guest can allocate arbitrarily large amounts of memory. Can you check if using a dedicated SRCU (possibly with synchronize_srcu_expedited in kvm_set_irq_routing) speeds up KVM_SET_GSI_ROUTING enough? Back-of-the-envelope, the latency should be on the order of tens of microseconds. Thanks, Paolo > --- > include/linux/kvm_host.h | 1 + > virt/kvm/irqchip.c | 5 ++--- > 2 files changed, 3 insertions(+), 3 deletions(-) > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index 3d1b0e6..2f19079 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -327,6 +327,7 @@ struct kvm_kernel_irq_routing_entry { > #ifdef CONFIG_HAVE_KVM_IRQ_ROUTING > > struct kvm_irq_routing_table { > + struct rcu_head rcu; > int chip[KVM_NR_IRQCHIPS][KVM_IRQCHIP_NUM_PINS]; > struct kvm_kernel_irq_routing_entry *rt_entries; > u32 nr_rt_entries; > diff --git a/virt/kvm/irqchip.c b/virt/kvm/irqchip.c > index 20dc9e4..3e2ebed 100644 > --- a/virt/kvm/irqchip.c > +++ b/virt/kvm/irqchip.c > @@ -226,12 +226,11 @@ int kvm_set_irq_routing(struct kvm *kvm, > kvm_irq_routing_update(kvm, new); > mutex_unlock(&kvm->irq_lock); > > - synchronize_rcu(); > - > new = old; > r = 0; > > out: > - kfree(new); > + if (new) > + kfree_rcu(new, rcu); > return r; > } > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
