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@xxxxxxxxxx>
>> ---
>> 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
>>
>>
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>
>
>
>
> .
>
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