| Message ID | 20241202090628.67919-1-bk@alpico.io (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] KVM: make uevents configurable | expand |
On Mon, 02 Dec 2024 09:06:28 +0000, Bernhard Kauer <bk@alpico.io> wrote: > > Handling of uevents in userlevel is a bottleneck for tiny VMs. > > Running 10_000 VMs keeps one and a half cores busy for 5.4 seconds to let > systemd-udevd handle all messages. That is roughly 27x longer than > the 0.2 seconds needed for running the VMs without them. > > We choose a module parameter here due to its simplicity and ease of > maintenance. > > v1->v2: make the parameter read-write to avoid reboots on ARM That's also to avoid killing all VMs on *any* architecture, modular or not, just to switch a parameter. Also, please don't include this sort of changelogs in the commit message... > > Signed-off-by: Bernhard Kauer <bk@alpico.io> > --- ... but instead place them *here*. > virt/kvm/kvm_main.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > index 38620c16739b..9e714cf45617 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -97,6 +97,9 @@ EXPORT_SYMBOL_GPL(halt_poll_ns_shrink); > bool debugfs_per_vm = true; > module_param(debugfs_per_vm, bool, 0644); > > +bool disable_uevent_notify; static? > +module_param(disable_uevent_notify, bool, 0644); > + > /* > * Allow direct access (from KVM or the CPU) without MMU notifier protection > * to unpinned pages. > @@ -6141,7 +6144,7 @@ static void kvm_uevent_notify_change(unsigned int type, struct kvm *kvm) > struct kobj_uevent_env *env; > unsigned long long created, active; > > - if (!kvm_dev.this_device || !kvm) > + if (!kvm_dev.this_device || !kvm || disable_uevent_notify) > return; > > mutex_lock(&kvm_lock); <bikeshed-time> I'd rather have a positive logic. Something like: +static bool uevent_notify = true; +module_param(uevent_notify, bool, 0644); + /* * Allow direct access (from KVM or the CPU) without MMU notifier protection * to unpinned pages. @@ -6141,7 +6144,7 @@ static void kvm_uevent_notify_change(unsigned int type, struct kvm *kvm) struct kobj_uevent_env *env; unsigned long long created, active; - if (!kvm_dev.this_device || !kvm) + if (!kvm_dev.this_device || !kvm || !uevent_notify) return; mutex_lock(&kvm_lock); which is overall more readable. </bikeshed-time> I would also expect some form of documentation in Documentation/admin-guide/kernel-parameters.txt. Thanks, M.
On Mon, Dec 02, 2024 at 10:40:00AM +0000, Marc Zyngier wrote: > > +bool disable_uevent_notify; > > static? Yes. > <bikeshed-time> > I'd rather have a positive logic. Something like: > > +static bool uevent_notify = true; > +module_param(uevent_notify, bool, 0644); Choosing readability vs one byte in the data-section seems to be a good tradeoff. > </bikeshed-time> > > I would also expect some form of documentation in > Documentation/admin-guide/kernel-parameters.txt. Good point. I will add this in v3. Thanks, Bernhard
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 38620c16739b..9e714cf45617 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -97,6 +97,9 @@ EXPORT_SYMBOL_GPL(halt_poll_ns_shrink); bool debugfs_per_vm = true; module_param(debugfs_per_vm, bool, 0644); +bool disable_uevent_notify; +module_param(disable_uevent_notify, bool, 0644); + /* * Allow direct access (from KVM or the CPU) without MMU notifier protection * to unpinned pages. @@ -6141,7 +6144,7 @@ static void kvm_uevent_notify_change(unsigned int type, struct kvm *kvm) struct kobj_uevent_env *env; unsigned long long created, active; - if (!kvm_dev.this_device || !kvm) + if (!kvm_dev.this_device || !kvm || disable_uevent_notify) return; mutex_lock(&kvm_lock);
Handling of uevents in userlevel is a bottleneck for tiny VMs. Running 10_000 VMs keeps one and a half cores busy for 5.4 seconds to let systemd-udevd handle all messages. That is roughly 27x longer than the 0.2 seconds needed for running the VMs without them. We choose a module parameter here due to its simplicity and ease of maintenance. v1->v2: make the parameter read-write to avoid reboots on ARM Signed-off-by: Bernhard Kauer <bk@alpico.io> --- virt/kvm/kvm_main.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)