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Date: Wed, 27 Sep 2017 16:00:51 +0200 From: Joerg Roedel <joro@...tes.org> To: Robin Murphy <robin.murphy@....com> Cc: iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH 3/3] iommu/iova: Try harder to allocate from rcache magazine On Tue, Sep 19, 2017 at 02:48:41PM +0100, Robin Murphy wrote: > When devices with different DMA masks are using the same domain, or for > PCI devices where we usually try a speculative 32-bit allocation first, > there is a fair possibility that the top PFN of the rcache stack at any > given time may be unsuitable for the lower limit, prompting a fallback > to allocating anew from the rbtree. Consequently, we may end up > artifically increasing pressure on the 32-bit IOVA space as unused IOVAs > accumulate lower down in the rcache stacks, while callers with 32-bit > masks also impose unnecessary rbtree overhead. > > In such cases, let's try a bit harder to satisfy the allocation locally > first - scanning the whole stack should still be relatively inexpensive, > and even rotating an entry up from the very bottom probably has less > overall impact than going to the rbtree. > > Signed-off-by: Robin Murphy <robin.murphy@....com> > --- > drivers/iommu/iova.c | 19 ++++++++++++++++--- > 1 file changed, 16 insertions(+), 3 deletions(-) > > diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c > index 8f8b436afd81..a7af8273fa98 100644 > --- a/drivers/iommu/iova.c > +++ b/drivers/iommu/iova.c > @@ -826,12 +826,25 @@ static bool iova_magazine_empty(struct iova_magazine *mag) > static unsigned long iova_magazine_pop(struct iova_magazine *mag, > unsigned long limit_pfn) > { > + int i; > + unsigned long pfn; > + > BUG_ON(iova_magazine_empty(mag)); > > - if (mag->pfns[mag->size - 1] > limit_pfn) > - return 0; > + /* > + * If we can pull a suitable pfn from anywhere in the stack, that's > + * still probably preferable to falling back to the rbtree. > + */ > + for (i = mag->size - 1; mag->pfns[i] > limit_pfn; i--) > + if (i == 0) > + return 0; > > - return mag->pfns[--mag->size]; > + pfn = mag->pfns[i]; > + mag->size--; > + for (; i < mag->size; i++) > + mag->pfns[i] = mag->pfns[i + 1]; Do we need to preserve the order of the elements on the stack or would it also suffice to just copy the top-element to the position we are removing? Joerg
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