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Re: [Qemu-devel] [PATCH 1/9] qom/user-creatable: add a few helper macros
From: |
Eduardo Habkost |
Subject: |
Re: [Qemu-devel] [PATCH 1/9] qom/user-creatable: add a few helper macros |
Date: |
Tue, 30 Oct 2018 20:07:17 -0300 |
User-agent: |
Mutt/1.9.2 (2017-12-15) |
On Tue, Oct 30, 2018 at 03:22:43PM +0100, Igor Mammedov wrote:
> On Tue, 30 Oct 2018 13:26:40 +0400
> Marc-André Lureau <address@hidden> wrote:
>
> > Hi
> >
> > On Tue, Oct 30, 2018 at 5:37 AM Eduardo Habkost <address@hidden> wrote:
> > >
> > > On Mon, Oct 29, 2018 at 10:56:57AM +0100, Igor Mammedov wrote:
> > > > On Fri, 26 Oct 2018 12:13:21 -0300
> > > > Eduardo Habkost <address@hidden> wrote:
> > > >
> > > > > On Mon, Oct 22, 2018 at 03:33:30PM +0100, Igor Mammedov wrote:
> > > > > > On Wed, 12 Sep 2018 16:55:23 +0400
> > > > > > Marc-André Lureau <address@hidden> wrote:
> > > > > >
> > > > > > > Improve a bit code readability.
> > > > > > >
> > > > > > > Signed-off-by: Marc-André Lureau <address@hidden>
> > > > > > > ---
> > > > > > > include/qom/object_interfaces.h | 4 ++++
> > > > > > > qom/object.c | 4 ++--
> > > > > > > qom/object_interfaces.c | 9 +++------
> > > > > > > 3 files changed, 9 insertions(+), 8 deletions(-)
> > > > > > >
> > > > > > > diff --git a/include/qom/object_interfaces.h
> > > > > > > b/include/qom/object_interfaces.h
> > > > > > > index 4d513fb329..46b0861457 100644
> > > > > > > --- a/include/qom/object_interfaces.h
> > > > > > > +++ b/include/qom/object_interfaces.h
> > > > > > > @@ -9,9 +9,13 @@
> > > > > > > #define USER_CREATABLE_CLASS(klass) \
> > > > > > > OBJECT_CLASS_CHECK(UserCreatableClass, (klass), \
> > > > > > > TYPE_USER_CREATABLE)
> > > > > > > +#define IS_USER_CREATABLE_CLASS(klass) \
> > > > > > > + object_class_dynamic_cast(OBJECT_CLASS(oc),
> > > > > > > TYPE_USER_CREATABLE)
> > > > > > > #define USER_CREATABLE_GET_CLASS(obj) \
> > > > > > > OBJECT_GET_CLASS(UserCreatableClass, (obj), \
> > > > > > > TYPE_USER_CREATABLE)
> > > > > > > +#define IS_USER_CREATABLE(obj) \
> > > > > > > + object_dynamic_cast(OBJECT(obj), TYPE_USER_CREATABLE)
> > > > > > > #define USER_CREATABLE(obj) \
> > > > > > > INTERFACE_CHECK(UserCreatable, (obj), \
> > > > > > > TYPE_USER_CREATABLE)
> > > > > > > diff --git a/qom/object.c b/qom/object.c
> > > > > > > index 75d1d48944..0703e8e4ff 100644
> > > > > > > --- a/qom/object.c
> > > > > > > +++ b/qom/object.c
> > > > > > > @@ -424,7 +424,7 @@ void object_initialize_childv(Object
> > > > > > > *parentobj, const char *propname,
> > > > > > > goto out;
> > > > > > > }
> > > > > > >
> > > > > > > - if (object_dynamic_cast(obj, TYPE_USER_CREATABLE)) {
> > > > > > > + if (IS_USER_CREATABLE(obj)) {
> > > > > > > user_creatable_complete(obj, &local_err);
> > > > > > > if (local_err) {
> > > > > > > object_unparent(obj);
> > > > > > > @@ -605,7 +605,7 @@ Object *object_new_with_propv(const char
> > > > > > > *typename,
> > > > > > > goto error;
> > > > > > > }
> > > > > > >
> > > > > > > - if (object_dynamic_cast(obj, TYPE_USER_CREATABLE)) {
> > > > > > > + if (IS_USER_CREATABLE(obj)) {
> > > > > > > user_creatable_complete(obj, &local_err);
> > > > > > > if (local_err) {
> > > > > > > object_unparent(obj);
> > > > > > > diff --git a/qom/object_interfaces.c b/qom/object_interfaces.c
> > > > > > > index 72b97a8bed..e3084bc04a 100644
> > > > > > > --- a/qom/object_interfaces.c
> > > > > > > +++ b/qom/object_interfaces.c
> > > > > > > @@ -10,18 +10,15 @@
> > > > > > >
> > > > > > > void user_creatable_complete(Object *obj, Error **errp)
> > > > > > > {
> > > > > > > -
> > > > > > > UserCreatableClass *ucc;
> > > > > > > - UserCreatable *uc =
> > > > > > > - (UserCreatable *)object_dynamic_cast(obj,
> > > > > > > TYPE_USER_CREATABLE);
> > > > > > >
> > > > > > > - if (!uc) {
> > > > > > > + if (!IS_USER_CREATABLE(obj)) {
> > > > > > > return;
> > > > > > > }
> > > > > > >
> > > > > > > - ucc = USER_CREATABLE_GET_CLASS(uc);
> > > > > > > + ucc = USER_CREATABLE_GET_CLASS(obj);
> > > > > > > if (ucc->complete) {
> > > > > > > - ucc->complete(uc, errp);
> > > > > > > + ucc->complete(USER_CREATABLE(obj), errp);
> > > > > > ^^^
> > > > > > even though function becomes more concise,
> > > > > > this will call expensive dynamic cast 2nd time (IS_USER_CREATABLE
> > > > > > was the 1st and discarded)
> > > > > > so I'm not sure is a good idea to regress startup time for
> > > > > > readability.
> >
> > I hope it's not measurable, unless we create billions of objects. Do
> > you want some figures?
> I recall penalty was big enough for QEMU, that we added qom debug option,
> in case of -object, the cost could be multiplied by hundreds
This sounds like premature optimization. Did you measure how
many nanoseconds we're saving per -object option?
> > > > > (INTERFACE_CHECK is a nop if CONFIG_QOM_CAST_DEBUG is not
> > > > > enabled, so I don't understand how it would regress startup time.
> > > > Isn't it enabled by default though?
> > > > Maybe we should flip default to disabled then cast should be ok and
> > > > enable it
> > > > when generic '--debug' is enabled.
> > >
> > > This is called only once for each -object option. Even if QOM
> > > debugging is enabled by default I don't see why it wouldn't be OK
> > > to call object_dynamic_cast_assert() here.
> >
> > Yes, imho it's very marginal. Thus I would value more readability.
> Probably not marginal for folks who are trying to reduce every
> tiny bit of start up time.
These people should be disabling QOM debugging, then.
>
> Considering readablity change here doesn't really make a difference,
> I'd pick performance. i.e. do cast and then reuse result later.
>
> > Alternatively, we can consider replacing
> > ucc->complete(USER_CREATABLE(obj), errp); by
> > ucc->complete((UserCreatable*)obj, errp); to get back to the same
> > number of dynamic_cast calls.
> Unsafe casts (foo*) typically frowned upon but it should be safe
> since one gets here after IS_USER_CREATABLE check, so I don't really
> care in this case. But I'd drop user_creatable_complete() hunk,
> so that original cast result is being reused.
Please don't do that. Readability, safety, and consistency are
more important. For the few users where QOM debugging has a
measurable performance impact, we already have a mechanism to
disable it.
--
Eduardo