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

[Cong Wang]

On Mon, Dec 2, 2019 at 2:02 AM John Garry <john.garry@xxxxxxxxxx> wrote:
>
> 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.

The check is done without any locking, so it is cheap. And it is a
common pattern that we do a check without lock and do a second same
check with lock:

https://en.wikipedia.org/wiki/Double-checked_locking

Thanks.