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

[John Garry]

On 30/11/2019 06:02, Cong Wang wrote:
> On Fri, Nov 29, 2019 at 5:24 AM John Garry <john.garry@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > Signed-off-by: Cong Wang <xiyou.wangcong@xxxxxxxxx>
> > > ---
> > >    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.
> .