linux-kernel - Re: [Patch v2 2/3] iommu: optimize iova_magazine_free

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Date:   Fri, 29 Nov 2019 22:02:08 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     John Garry <john.garry@...wei.com>
Cc:     iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Patch v2 2/3] iommu: optimize iova_magazine_free_pfns()

On Fri, Nov 29, 2019 at 5:24 AM John Garry <john.garry@...wei.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@...tes.org>
> > Signed-off-by: Cong Wang <xiyou.wangcong@...il.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? :)

Thanks.