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Date: Fri, 22 Sep 2017 18:21:07 +0200
From: Tomasz Nowicki <tn@...ihalf.com>
To: Robin Murphy <robin.murphy@....com>, joro@...tes.org
Cc: dwoods@...lanox.com, tomasz.nowicki@...iumnetworks.com,
linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH v5 3/6] iommu/iova: Extend rbtree node caching
Hi Robin,
On 21.09.2017 17:52, Robin Murphy wrote:
> The cached node mechanism provides a significant performance benefit for
> allocations using a 32-bit DMA mask, but in the case of non-PCI devices
> or where the 32-bit space is full, the loss of this benefit can be
> significant - on large systems there can be many thousands of entries in
> the tree, such that walking all the way down to find free space every
> time becomes increasingly awful.
>
> Maintain a similar cached node for the whole IOVA space as a superset of
> the 32-bit space so that performance can remain much more consistent.
>
> Inspired by work by Zhen Lei <thunder.leizhen@...wei.com>.
>
> Tested-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Tested-by: Zhen Lei <thunder.leizhen@...wei.com>
> Tested-by: Nate Watterson <nwatters@...eaurora.org>
> Signed-off-by: Robin Murphy <robin.murphy@....com>
> ---
>
> v5: Fixed __cached_rbnode_delete_update() logic to update both nodes
> when necessary
>
> drivers/iommu/iova.c | 60 ++++++++++++++++++++++++----------------------------
> include/linux/iova.h | 3 ++-
> 2 files changed, 30 insertions(+), 33 deletions(-)
>
> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
> index 20be9a8b3188..c6f5a22f8d20 100644
> --- a/drivers/iommu/iova.c
> +++ b/drivers/iommu/iova.c
> @@ -48,6 +48,7 @@ init_iova_domain(struct iova_domain *iovad, unsigned long granule,
>
> spin_lock_init(&iovad->iova_rbtree_lock);
> iovad->rbroot = RB_ROOT;
> + iovad->cached_node = NULL;
> iovad->cached32_node = NULL;
> iovad->granule = granule;
> iovad->start_pfn = start_pfn;
> @@ -110,48 +111,44 @@ EXPORT_SYMBOL_GPL(init_iova_flush_queue);
> static struct rb_node *
> __get_cached_rbnode(struct iova_domain *iovad, unsigned long *limit_pfn)
> {
> - if ((*limit_pfn > iovad->dma_32bit_pfn) ||
> - (iovad->cached32_node == NULL))
> + struct rb_node *cached_node = NULL;
> + struct iova *curr_iova;
> +
> + if (*limit_pfn <= iovad->dma_32bit_pfn)
> + cached_node = iovad->cached32_node;
> + if (!cached_node)
> + cached_node = iovad->cached_node;
> + if (!cached_node)
> return rb_last(&iovad->rbroot);
> - else {
> - struct rb_node *prev_node = rb_prev(iovad->cached32_node);
> - struct iova *curr_iova =
> - rb_entry(iovad->cached32_node, struct iova, node);
> - *limit_pfn = curr_iova->pfn_lo;
> - return prev_node;
> - }
> +
> + curr_iova = rb_entry(cached_node, struct iova, node);
> + *limit_pfn = min(*limit_pfn, curr_iova->pfn_lo);
I guess this it the fix for stale pointer in iovad->cached32_node from
previous series but I think this is something more.
With this min() here we have real control over the limit_pfn we would
like to allocate at most. In other works, without your series two
subsequent calls can give us:
iova (ffff) = alloc_iova_fast(iovad, 1, DMA_BIT_MASK(32) >> shift);
iova (fffe)= alloc_iova_fast(iovad, 1, DMA_BIT_MASK(16) >> shift);
We do not see this since nobody uses limit_pfn below DMA_BIT_MASK(32)
now. That might be done intentionally so I ask for your opinion.
Also, with your patch and two the same alloc_iova_fast() calls in
iteration may get 32-bit space full much faster. Please correct me if I
messed things up.
Thanks,
Tomasz
> +
> + return rb_prev(cached_node);
> }
>
> static void
> -__cached_rbnode_insert_update(struct iova_domain *iovad,
> - unsigned long limit_pfn, struct iova *new)
> +__cached_rbnode_insert_update(struct iova_domain *iovad, struct iova *new)
> {
> - if (limit_pfn != iovad->dma_32bit_pfn)
> - return;
> - iovad->cached32_node = &new->node;
> + if (new->pfn_hi < iovad->dma_32bit_pfn)
> + iovad->cached32_node = &new->node;
> + else
> + iovad->cached_node = &new->node;
> }
>
> static void
> __cached_rbnode_delete_update(struct iova_domain *iovad, struct iova *free)
> {
> struct iova *cached_iova;
> - struct rb_node *curr;
>
> - if (!iovad->cached32_node)
> - return;
> - curr = iovad->cached32_node;
> - cached_iova = rb_entry(curr, struct iova, node);
> + cached_iova = rb_entry(iovad->cached32_node, struct iova, node);
> + if (free->pfn_hi < iovad->dma_32bit_pfn &&
> + iovad->cached32_node && free->pfn_lo >= cached_iova->pfn_lo)
> + iovad->cached32_node = rb_next(&free->node);
>
> - if (free->pfn_lo >= cached_iova->pfn_lo) {
> - struct rb_node *node = rb_next(&free->node);
> - struct iova *iova = rb_entry(node, struct iova, node);
> -
> - /* only cache if it's below 32bit pfn */
> - if (node && iova->pfn_lo < iovad->dma_32bit_pfn)
> - iovad->cached32_node = node;
> - else
> - iovad->cached32_node = NULL;
> - }
> + cached_iova = rb_entry(iovad->cached_node, struct iova, node);
> + if (iovad->cached_node && free->pfn_lo >= cached_iova->pfn_lo)
> + iovad->cached_node = rb_next(&free->node);
> }
>
> /* Insert the iova into domain rbtree by holding writer lock */
> @@ -188,7 +185,7 @@ static int __alloc_and_insert_iova_range(struct iova_domain *iovad,
> {
> struct rb_node *prev, *curr = NULL;
> unsigned long flags;
> - unsigned long saved_pfn, new_pfn;
> + unsigned long new_pfn;
> unsigned long align_mask = ~0UL;
>
> if (size_aligned)
> @@ -196,7 +193,6 @@ static int __alloc_and_insert_iova_range(struct iova_domain *iovad,
>
> /* Walk the tree backwards */
> spin_lock_irqsave(&iovad->iova_rbtree_lock, flags);
> - saved_pfn = limit_pfn;
> curr = __get_cached_rbnode(iovad, &limit_pfn);
> prev = curr;
> while (curr) {
> @@ -226,7 +222,7 @@ static int __alloc_and_insert_iova_range(struct iova_domain *iovad,
>
> /* If we have 'prev', it's a valid place to start the insertion. */
> iova_insert_rbtree(&iovad->rbroot, new, prev);
> - __cached_rbnode_insert_update(iovad, saved_pfn, new);
> + __cached_rbnode_insert_update(iovad, new);
>
> spin_unlock_irqrestore(&iovad->iova_rbtree_lock, flags);
>
> diff --git a/include/linux/iova.h b/include/linux/iova.h
> index d179b9bf7814..69ea3e258ff2 100644
> --- a/include/linux/iova.h
> +++ b/include/linux/iova.h
> @@ -70,7 +70,8 @@ struct iova_fq {
> struct iova_domain {
> spinlock_t iova_rbtree_lock; /* Lock to protect update of rbtree */
> struct rb_root rbroot; /* iova domain rbtree root */
> - struct rb_node *cached32_node; /* Save last alloced node */
> + struct rb_node *cached_node; /* Save last alloced node */
> + struct rb_node *cached32_node; /* Save last 32-bit alloced node */
> unsigned long granule; /* pfn granularity for this domain */
> unsigned long start_pfn; /* Lower limit for this domain */
> unsigned long dma_32bit_pfn;
>
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