linux-kernel - Re: [Patch v3 2/3] iommu: optimize iova_magazine_free

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Date:   Tue, 21 Jan 2020 09:29:12 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        John Garry <john.garry@...wei.com>
Subject: Re: [Patch v3 2/3] iommu: optimize iova_magazine_free_pfns()

On Tue, Jan 21, 2020 at 1:52 AM Robin Murphy <robin.murphy@....com> wrote:
>
> On 18/12/2019 4:39 am, Cong Wang wrote:
> > If the magazine is empty, iova_magazine_free_pfns() should
> > 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, not much at all.
>
> Have you measured that in any way? AFAICS the only time this can get
> called with a non-full magazine is in the CPU hotplug callback, where
> the impact of taking the rbtree lock and immediately releasing it seems
> unlikely to be significant on top of everything else involved in that
> operation.

This patchset is only tested as a whole, it is not easy to deploy
each to production and test it separately.

Is there anything wrong to optimize a CPU hotplug path? :) And,
it is called in alloc_iova_fast() too when, for example, over-cached.

Thanks.