linux-kernel - Re: [PATCH 3/3] iommu/iova: Try harder to allocate from rcache magazine

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Date:   Wed, 27 Sep 2017 17:50:31 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        nd@....com
Subject: Re: [PATCH 3/3] iommu/iova: Try harder to allocate from rcache
 magazine

On Wed, 27 Sep 2017 16:00:51 +0200
Joerg Roedel <joro@...tes.org> wrote:

> 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?

Ooh, good point - the order is more or less meaningless, and if it *did*
matter then that would imply we couldn't do this anyway. Getting rid of
the second loop makes it even more compelling.

Robin.