When debugging QEMU it is often useful to put a breakpoint on the
error_setg_internal method impl.
Unfortunately the object_property_add / object_class_property_add
methods call object_property_find / object_class_property_find methods
to check if a property exists already before adding the new property.
As a result there are a huge number of calls to error_setg_internal
on startup of most QEMU commands, making it very painful to set a
breakpoint on this method.
This puts a minor optimization on the code so that we avoid calling
error_setg() when errp is NULL. Functionally there's no difference
since error_setg() is a no-op when errp is NULL, but this lets us
use breakpoints in GDB in a practical way.
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
qom/object.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)
diff --git a/qom/object.c b/qom/object.c
index 547dcf97c3..ddd5e7a30e 100644
--- a/qom/object.c
+++ b/qom/object.c
@@ -1087,7 +1087,12 @@ ObjectProperty *object_property_find(Object *obj, const char *name,
return prop;
}
- error_setg(errp, "Property '.%s' not found", name);
+ /* Optimized to avoid calling error_setg if errp == NULL
+ * otherwise every property add call hits error_setg
+ * making it impratical to set breakpoints in GDB */
+ if (errp) {
+ error_setg(errp, "Property '.%s' not found", name);
+ }
return NULL;
}
@@ -1133,7 +1138,10 @@ ObjectProperty *object_class_property_find(ObjectClass *klass, const char *name,
}
prop = g_hash_table_lookup(klass->properties, name);
- if (!prop) {
+ /* Optimized to avoid calling error_setg if errp == NULL
+ * otherwise every property add call hits error_setg
+ * making it impratical to set breakpoints in GDB */
+ if (!prop && errp) {
error_setg(errp, "Property '.%s' not found", name);
}
return prop;
--
2.19.1
Hi On Mon, Nov 19, 2018 at 5:59 PM Daniel P. Berrangé <berrange@redhat.com> wrote: > > When debugging QEMU it is often useful to put a breakpoint on the > error_setg_internal method impl. > > Unfortunately the object_property_add / object_class_property_add > methods call object_property_find / object_class_property_find methods > to check if a property exists already before adding the new property. > > As a result there are a huge number of calls to error_setg_internal > on startup of most QEMU commands, making it very painful to set a > breakpoint on this method. > > This puts a minor optimization on the code so that we avoid calling > error_setg() when errp is NULL. Functionally there's no difference > since error_setg() is a no-op when errp is NULL, but this lets us > use breakpoints in GDB in a practical way. > > Signed-off-by: Daniel P. Berrangé <berrange@redhat.com> Or maybe change the error_setg* macros? After all, if errp is NULL, it's an error qemu can handle. > --- > qom/object.c | 12 ++++++++++-- > 1 file changed, 10 insertions(+), 2 deletions(-) > > diff --git a/qom/object.c b/qom/object.c > index 547dcf97c3..ddd5e7a30e 100644 > --- a/qom/object.c > +++ b/qom/object.c > @@ -1087,7 +1087,12 @@ ObjectProperty *object_property_find(Object *obj, const char *name, > return prop; > } > > - error_setg(errp, "Property '.%s' not found", name); > + /* Optimized to avoid calling error_setg if errp == NULL > + * otherwise every property add call hits error_setg > + * making it impratical to set breakpoints in GDB */ > + if (errp) { > + error_setg(errp, "Property '.%s' not found", name); > + } > return NULL; > } > > @@ -1133,7 +1138,10 @@ ObjectProperty *object_class_property_find(ObjectClass *klass, const char *name, > } > > prop = g_hash_table_lookup(klass->properties, name); > - if (!prop) { > + /* Optimized to avoid calling error_setg if errp == NULL > + * otherwise every property add call hits error_setg > + * making it impratical to set breakpoints in GDB */ > + if (!prop && errp) { > error_setg(errp, "Property '.%s' not found", name); > } > return prop; > -- > 2.19.1 > > -- Marc-André Lureau
On Tue, Nov 20, 2018 at 09:45:23AM +0400, Marc-André Lureau wrote: > Hi > > On Mon, Nov 19, 2018 at 5:59 PM Daniel P. Berrangé <berrange@redhat.com> wrote: > > > > When debugging QEMU it is often useful to put a breakpoint on the > > error_setg_internal method impl. > > > > Unfortunately the object_property_add / object_class_property_add > > methods call object_property_find / object_class_property_find methods > > to check if a property exists already before adding the new property. > > > > As a result there are a huge number of calls to error_setg_internal > > on startup of most QEMU commands, making it very painful to set a > > breakpoint on this method. > > > > This puts a minor optimization on the code so that we avoid calling > > error_setg() when errp is NULL. Functionally there's no difference > > since error_setg() is a no-op when errp is NULL, but this lets us > > use breakpoints in GDB in a practical way. > > > > Signed-off-by: Daniel P. Berrangé <berrange@redhat.com> > > Or maybe change the error_setg* macros? After all, if errp is NULL, > it's an error qemu can handle. That would mean that all callers would get short circuited which I don't think it is desirable in general. This QOM usage is special in that the calling code is using the error scenario as a means to trigger different code flow. IOW it doesn't consider it an error scenario at all. > > --- > > qom/object.c | 12 ++++++++++-- > > 1 file changed, 10 insertions(+), 2 deletions(-) > > > > diff --git a/qom/object.c b/qom/object.c > > index 547dcf97c3..ddd5e7a30e 100644 > > --- a/qom/object.c > > +++ b/qom/object.c > > @@ -1087,7 +1087,12 @@ ObjectProperty *object_property_find(Object *obj, const char *name, > > return prop; > > } > > > > - error_setg(errp, "Property '.%s' not found", name); > > + /* Optimized to avoid calling error_setg if errp == NULL > > + * otherwise every property add call hits error_setg > > + * making it impratical to set breakpoints in GDB */ > > + if (errp) { > > + error_setg(errp, "Property '.%s' not found", name); > > + } > > return NULL; > > } > > > > @@ -1133,7 +1138,10 @@ ObjectProperty *object_class_property_find(ObjectClass *klass, const char *name, > > } > > > > prop = g_hash_table_lookup(klass->properties, name); > > - if (!prop) { > > + /* Optimized to avoid calling error_setg if errp == NULL > > + * otherwise every property add call hits error_setg > > + * making it impratical to set breakpoints in GDB */ > > + if (!prop && errp) { > > error_setg(errp, "Property '.%s' not found", name); > > } > > return prop; > > -- > > 2.19.1 > > > > > > > -- > Marc-André Lureau Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
Daniel P. Berrangé <berrange@redhat.com> writes: > When debugging QEMU it is often useful to put a breakpoint on the > error_setg_internal method impl. > > Unfortunately the object_property_add / object_class_property_add > methods call object_property_find / object_class_property_find methods > to check if a property exists already before adding the new property. > > As a result there are a huge number of calls to error_setg_internal > on startup of most QEMU commands, making it very painful to set a > breakpoint on this method. > > This puts a minor optimization on the code so that we avoid calling > error_setg() when errp is NULL. Functionally there's no difference > since error_setg() is a no-op when errp is NULL, but this lets us > use breakpoints in GDB in a practical way. > > Signed-off-by: Daniel P. Berrangé <berrange@redhat.com> > --- > qom/object.c | 12 ++++++++++-- > 1 file changed, 10 insertions(+), 2 deletions(-) > > diff --git a/qom/object.c b/qom/object.c > index 547dcf97c3..ddd5e7a30e 100644 > --- a/qom/object.c > +++ b/qom/object.c > @@ -1087,7 +1087,12 @@ ObjectProperty *object_property_find(Object *obj, const char *name, > return prop; > } > > - error_setg(errp, "Property '.%s' not found", name); > + /* Optimized to avoid calling error_setg if errp == NULL > + * otherwise every property add call hits error_setg > + * making it impratical to set breakpoints in GDB */ > + if (errp) { > + error_setg(errp, "Property '.%s' not found", name); > + } > return NULL; > } > In my opinion, this function's design is awkward. Stress on *opinion*. On success, it returns a (non-null) pointer. On failure, it sets an error and returns null. Note that it has just one failure mode: "Property '.%s' not found". Setting an error is just a convenience for those callers that want to propagate exactly this error to their callers. I count 30 callers. Only six pass a non-NULL argument to @errp. I'd rather have a pair of functions similar to how Python has both .get() and .__getitem__(): the former doesn't fail, but returns None instead, and the latter does fail, throwing KeyError. In QEMU, we can't throw, so we set an error. Here's the obvious code: ObjectProperty *object_property_find(Object *obj, const char *name) { ObjectProperty *prop; ObjectClass *klass = object_get_class(obj); prop = object_class_property_find(klass, name, NULL); if (prop) { return prop; } prop = g_hash_table_lookup(obj->properties, name); if (prop) { return prop; } return NULL; } ObjectProperty *object_property_find_err(Object *obj, const char *name, Error **errp) { ObjectProperty *prop = object_property_find(obj, name); if (!prop) { error_setg(errp, "Property '.%s' not found", name); } return prop; } > @@ -1133,7 +1138,10 @@ ObjectProperty *object_class_property_find(ObjectClass *klass, const char *name, > } > > prop = g_hash_table_lookup(klass->properties, name); > - if (!prop) { > + /* Optimized to avoid calling error_setg if errp == NULL > + * otherwise every property add call hits error_setg > + * making it impratical to set breakpoints in GDB */ > + if (!prop && errp) { > error_setg(errp, "Property '.%s' not found", name); > } > return prop; Likewise, just more so: callers passing non-NULL do not exist.
On Tue, Nov 20, 2018 at 07:16:31PM +0100, Markus Armbruster wrote: > Daniel P. Berrangé <berrange@redhat.com> writes: > > > When debugging QEMU it is often useful to put a breakpoint on the > > error_setg_internal method impl. > > > > Unfortunately the object_property_add / object_class_property_add > > methods call object_property_find / object_class_property_find methods > > to check if a property exists already before adding the new property. > > > > As a result there are a huge number of calls to error_setg_internal > > on startup of most QEMU commands, making it very painful to set a > > breakpoint on this method. > > > > This puts a minor optimization on the code so that we avoid calling > > error_setg() when errp is NULL. Functionally there's no difference > > since error_setg() is a no-op when errp is NULL, but this lets us > > use breakpoints in GDB in a practical way. > > > > Signed-off-by: Daniel P. Berrangé <berrange@redhat.com> > > --- > > qom/object.c | 12 ++++++++++-- > > 1 file changed, 10 insertions(+), 2 deletions(-) > > > > diff --git a/qom/object.c b/qom/object.c > > index 547dcf97c3..ddd5e7a30e 100644 > > --- a/qom/object.c > > +++ b/qom/object.c > > @@ -1087,7 +1087,12 @@ ObjectProperty *object_property_find(Object *obj, const char *name, > > return prop; > > } > > > > - error_setg(errp, "Property '.%s' not found", name); > > + /* Optimized to avoid calling error_setg if errp == NULL > > + * otherwise every property add call hits error_setg > > + * making it impratical to set breakpoints in GDB */ > > + if (errp) { > > + error_setg(errp, "Property '.%s' not found", name); > > + } > > return NULL; > > } > > > > In my opinion, this function's design is awkward. Stress on *opinion*. > > On success, it returns a (non-null) pointer. > > On failure, it sets an error and returns null. Note that it has just > one failure mode: "Property '.%s' not found". Setting an error is just > a convenience for those callers that want to propagate exactly this > error to their callers. > > I count 30 callers. Only six pass a non-NULL argument to @errp. > > I'd rather have a pair of functions similar to how Python has both > .get() and .__getitem__(): the former doesn't fail, but returns None > instead, and the latter does fail, throwing KeyError. In QEMU, we can't > throw, so we set an error. Here's the obvious code: > > ObjectProperty *object_property_find(Object *obj, const char *name) > { > ObjectProperty *prop; > ObjectClass *klass = object_get_class(obj); > > prop = object_class_property_find(klass, name, NULL); > if (prop) { > return prop; > } > > prop = g_hash_table_lookup(obj->properties, name); > if (prop) { > return prop; > } > > return NULL; > } > > ObjectProperty *object_property_find_err(Object *obj, const char *name, > Error **errp) > { > ObjectProperty *prop = object_property_find(obj, name); > > if (!prop) { > error_setg(errp, "Property '.%s' not found", name); > } > return prop; > } Yes, I'd be happy with this approach as I think it is more useful in general Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
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