From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joerg Roedel Subject: Re: [PATCH 3/3] iommu/iova: Try harder to allocate from rcache magazine Date: Wed, 27 Sep 2017 16:00:51 +0200 Message-ID: <20170927140051.GO8398@8bytes.org> References: <8127fabc219811d8169189e9d7177d42bc74bcbf.1505827369.git.robin.murphy@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <8127fabc219811d8169189e9d7177d42bc74bcbf.1505827369.git.robin.murphy-5wv7dgnIgG8@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: Robin Murphy Cc: iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: iommu@lists.linux-foundation.org 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 > --- > 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