* [Qemu-devel] [PATCH] qom: avoid reporting errors for NULL error object
@ 2018-11-19 13:59 Daniel P. Berrangé
2018-11-20 5:45 ` Marc-André Lureau
2018-11-20 18:16 ` Markus Armbruster
0 siblings, 2 replies; 5+ messages in thread
From: Daniel P. Berrangé @ 2018-11-19 13:59 UTC (permalink / raw)
To: qemu-devel; +Cc: Andreas Färber, Daniel P. Berrangé
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
^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: [Qemu-devel] [PATCH] qom: avoid reporting errors for NULL error object
2018-11-19 13:59 [Qemu-devel] [PATCH] qom: avoid reporting errors for NULL error object Daniel P. Berrangé
@ 2018-11-20 5:45 ` Marc-André Lureau
2018-11-20 9:45 ` Daniel P. Berrangé
2018-11-20 18:16 ` Markus Armbruster
1 sibling, 1 reply; 5+ messages in thread
From: Marc-André Lureau @ 2018-11-20 5:45 UTC (permalink / raw)
To: Daniel P. Berrange; +Cc: QEMU, Andreas Färber
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
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [Qemu-devel] [PATCH] qom: avoid reporting errors for NULL error object
2018-11-20 5:45 ` Marc-André Lureau
@ 2018-11-20 9:45 ` Daniel P. Berrangé
0 siblings, 0 replies; 5+ messages in thread
From: Daniel P. Berrangé @ 2018-11-20 9:45 UTC (permalink / raw)
To: Marc-André Lureau; +Cc: QEMU, Andreas Färber
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 :|
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [Qemu-devel] [PATCH] qom: avoid reporting errors for NULL error object
2018-11-19 13:59 [Qemu-devel] [PATCH] qom: avoid reporting errors for NULL error object Daniel P. Berrangé
2018-11-20 5:45 ` Marc-André Lureau
@ 2018-11-20 18:16 ` Markus Armbruster
2018-11-21 9:31 ` Daniel P. Berrangé
1 sibling, 1 reply; 5+ messages in thread
From: Markus Armbruster @ 2018-11-20 18:16 UTC (permalink / raw)
To: Daniel P. Berrangé; +Cc: qemu-devel, Andreas Färber
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.
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [Qemu-devel] [PATCH] qom: avoid reporting errors for NULL error object
2018-11-20 18:16 ` Markus Armbruster
@ 2018-11-21 9:31 ` Daniel P. Berrangé
0 siblings, 0 replies; 5+ messages in thread
From: Daniel P. Berrangé @ 2018-11-21 9:31 UTC (permalink / raw)
To: Markus Armbruster; +Cc: qemu-devel, Andreas Färber
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 :|
^ permalink raw reply [flat|nested] 5+ messages in thread
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2018-11-19 13:59 [Qemu-devel] [PATCH] qom: avoid reporting errors for NULL error object Daniel P. Berrangé
2018-11-20 5:45 ` Marc-André Lureau
2018-11-20 9:45 ` Daniel P. Berrangé
2018-11-20 18:16 ` Markus Armbruster
2018-11-21 9:31 ` Daniel P. Berrangé
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