| Message ID | 1416482702-10820-1-git-send-email-thunder.leizhen@huawei.com |
|---|---|
| State | New |
| Headers | show |
On Thu, 2014-11-20 at 19:25 +0800, Zhen Lei wrote: > The next code fragment "list_for_each_entry" is not depend on "minor". With this > patch, the free of "minor" in "list_for_each_entry" can be reduced, and there is > no functional change. A reasonable micro-optimization, but I'm curious if you're actually seeing some measurable overhead from this. It seems like we'd need to have multiple devices, all within the same IOMMU group, all probed by vfio-pci at the same time to exercise the race condition. Thanks, Alex > Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com> > --- > drivers/vfio/vfio.c | 13 ++++++------- > 1 file changed, 6 insertions(+), 7 deletions(-) > > diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c > index f018d8d..737eb468 100644 > --- a/drivers/vfio/vfio.c > +++ b/drivers/vfio/vfio.c > @@ -225,22 +225,21 @@ static struct vfio_group *vfio_create_group(struct iommu_group *iommu_group) > > mutex_lock(&vfio.group_lock); > > - minor = vfio_alloc_group_minor(group); > - if (minor < 0) { > - vfio_group_unlock_and_free(group); > - return ERR_PTR(minor); > - } > - > /* Did we race creating this group? */ > list_for_each_entry(tmp, &vfio.group_list, vfio_next) { > if (tmp->iommu_group == iommu_group) { > vfio_group_get(tmp); > - vfio_free_group_minor(minor); > vfio_group_unlock_and_free(group); > return tmp; > } > } > > + minor = vfio_alloc_group_minor(group); > + if (minor < 0) { > + vfio_group_unlock_and_free(group); > + return ERR_PTR(minor); > + } > + > dev = device_create(vfio.class, NULL, > MKDEV(MAJOR(vfio.group_devt), minor), > group, "%d", iommu_group_id(iommu_group)); > -- > 1.8.0 > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On 2014/11/20 23:37, Alex Williamson wrote: > On Thu, 2014-11-20 at 19:25 +0800, Zhen Lei wrote: >> The next code fragment "list_for_each_entry" is not depend on "minor". With this >> patch, the free of "minor" in "list_for_each_entry" can be reduced, and there is >> no functional change. > > A reasonable micro-optimization, but I'm curious if you're actually > seeing some measurable overhead from this. It seems like we'd need to Oh, I just found this by code review. It's good to put pure check first and resource allocation later, if they are order independent. Isn't it? I think it will not improve performance but only save a little code space and looks feel better. > have multiple devices, all within the same IOMMU group, all probed by > vfio-pci at the same time to exercise the race condition. Thanks, I think vfio_create_group can not be invoked frequently. So, the performance of this function is not a major consideration, we don't care slightly performance change. > > Alex > >> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com> >> --- >> drivers/vfio/vfio.c | 13 ++++++------- >> 1 file changed, 6 insertions(+), 7 deletions(-) >> >> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c >> index f018d8d..737eb468 100644 >> --- a/drivers/vfio/vfio.c >> +++ b/drivers/vfio/vfio.c >> @@ -225,22 +225,21 @@ static struct vfio_group *vfio_create_group(struct iommu_group *iommu_group) >> >> mutex_lock(&vfio.group_lock); >> >> - minor = vfio_alloc_group_minor(group); >> - if (minor < 0) { >> - vfio_group_unlock_and_free(group); >> - return ERR_PTR(minor); >> - } >> - >> /* Did we race creating this group? */ >> list_for_each_entry(tmp, &vfio.group_list, vfio_next) { >> if (tmp->iommu_group == iommu_group) { >> vfio_group_get(tmp); >> - vfio_free_group_minor(minor); >> vfio_group_unlock_and_free(group); >> return tmp; >> } >> } >> >> + minor = vfio_alloc_group_minor(group); >> + if (minor < 0) { >> + vfio_group_unlock_and_free(group); >> + return ERR_PTR(minor); >> + } >> + >> dev = device_create(vfio.class, NULL, >> MKDEV(MAJOR(vfio.group_devt), minor), >> group, "%d", iommu_group_id(iommu_group)); >> -- >> 1.8.0 >> >> >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in >> the body of a message to majordomo@vger.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> Please read the FAQ at http://www.tux.org/lkml/ > > > > > . > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c index f018d8d..737eb468 100644 --- a/drivers/vfio/vfio.c +++ b/drivers/vfio/vfio.c @@ -225,22 +225,21 @@ static struct vfio_group *vfio_create_group(struct iommu_group *iommu_group) mutex_lock(&vfio.group_lock); - minor = vfio_alloc_group_minor(group); - if (minor < 0) { - vfio_group_unlock_and_free(group); - return ERR_PTR(minor); - } - /* Did we race creating this group? */ list_for_each_entry(tmp, &vfio.group_list, vfio_next) { if (tmp->iommu_group == iommu_group) { vfio_group_get(tmp); - vfio_free_group_minor(minor); vfio_group_unlock_and_free(group); return tmp; } } + minor = vfio_alloc_group_minor(group); + if (minor < 0) { + vfio_group_unlock_and_free(group); + return ERR_PTR(minor); + } + dev = device_create(vfio.class, NULL, MKDEV(MAJOR(vfio.group_devt), minor), group, "%d", iommu_group_id(iommu_group));
The next code fragment "list_for_each_entry" is not depend on "minor". With this patch, the free of "minor" in "list_for_each_entry" can be reduced, and there is no functional change. Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com> --- drivers/vfio/vfio.c | 13 ++++++------- 1 file changed, 6 insertions(+), 7 deletions(-) -- 1.8.0 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/