Re: [Patch v2 2/3] iommu: optimize iova_magazine_free_pfns()

John Garry
2019-12-02T10:02:48Z

On 30/11/2019 06:02, Cong Wang wrote:
> On Fri, Nov 29, 2019 at 5:24 AM John Garry <john.garry@huawei.com> wrote:
>>
>> On 29/11/2019 00:48, Cong Wang wrote:
>>> If the maganize is empty, iova_magazine_free_pfns() should
>>
>> magazine
>
> Good catch!
>
>>
>>> be a nop, however it misses the case of mag->size==0. So we
>>> should just call iova_magazine_empty().
>>>
>>> This should reduce the contention on iovad->iova_rbtree_lock
>>> a little bit.
>>>
>>> Cc: Joerg Roedel <joro@8bytes.org>
>>> Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
>>> ---
>>> drivers/iommu/iova.c | 22 +++++++++++-----------
>>> 1 file changed, 11 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
>>> index cb473ddce4cf..184d4c0e20b5 100644
>>> --- a/drivers/iommu/iova.c
>>> +++ b/drivers/iommu/iova.c
>>> @@ -797,13 +797,23 @@ static void iova_magazine_free(struct iova_magazine *mag)
>>> kfree(mag);
>>> }
>>>
>>> +static bool iova_magazine_full(struct iova_magazine *mag)
>>> +{
>>> + return (mag && mag->size == IOVA_MAG_SIZE);
>>> +}
>>> +
>>> +static bool iova_magazine_empty(struct iova_magazine *mag)
>>> +{
>>> + return (!mag || mag->size == 0);
>>> +}
>>> +
>>> static void
>>> iova_magazine_free_pfns(struct iova_magazine *mag, struct iova_domain *iovad)
>>> {
>>> unsigned long flags;
>>> int i;
>>>
>>> - if (!mag)
>>> + if (iova_magazine_empty(mag))
>>
>> The only hot path we this call is
>> __iova_rcache_insert()->iova_magazine_free_pfns(mag_to_free) and
>> mag_to_free is full in this case, so I am sure how the additional check
>> helps, right?
>
> This is what I mean by "a little bit" in changelog, did you miss it or
> misunderstand it? :)

I was concerned that in the fastpath we actually make things very
marginally slower by adding a check which will fail.

Thanks,
John

>
> Thanks.
> .