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SubjectRe: [PATCH] iommu/iova: Optimise attempts to allocate iova from 32bit address range
Hi Robin,

On Fri, Aug 10, 2018 at 2:13 AM, Robin Murphy <robin.murphy@arm.com> wrote:
> On 2018-08-09 6:49 PM, Ganapatrao Kulkarni wrote:
>>
>> Hi Robin,
>>
>> On Thu, Aug 9, 2018 at 9:54 PM, Robin Murphy <robin.murphy@arm.com> wrote:
>>>
>>> On 07/08/18 09:54, Ganapatrao Kulkarni wrote:
>>>>
>>>>
>>>> As an optimisation for PCI devices, there is always first attempt
>>>> been made to allocate iova from SAC address range. This will lead
>>>> to unnecessary attempts/function calls, when there are no free ranges
>>>> available.
>>>>
>>>> This patch optimises by adding flag to track previous failed attempts
>>>> and avoids further attempts until replenish happens.
>>>
>>>
>>>
>>> Agh, what I overlooked is that this still suffers from the original
>>> problem,
>>> wherein a large allocation which fails due to fragmentation then blocks
>>> all
>>> subsequent smaller allocations, even if they may have succeeded.
>>>
>>> For a minimal change, though, what I think we could do is instead of just
>>> having a flag, track the size of the last 32-bit allocation which failed.
>>> If
>>> we're happy to assume that nobody's likely to mix aligned and unaligned
>>> allocations within the same domain, then that should be sufficiently
>>> robust
>>> whilst being no more complicated than this version, i.e. (modulo thinking
>>> up
>>> a better name for it):
>>
>>
>> I agree, it would be better to track size and attempt to allocate for
>> smaller chunks, if not for bigger one.
>>
>>>
>>>>
>>>> Signed-off-by: Ganapatrao Kulkarni <ganapatrao.kulkarni@cavium.com>
>>>> ---
>>>> This patch is based on comments from Robin Murphy <robin.murphy@arm.com>
>>>> for patch [1]
>>>>
>>>> [1] https://lkml.org/lkml/2018/4/19/780
>>>>
>>>> drivers/iommu/iova.c | 11 ++++++++++-
>>>> include/linux/iova.h | 1 +
>>>> 2 files changed, 11 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
>>>> index 83fe262..d97bb5a 100644
>>>> --- a/drivers/iommu/iova.c
>>>> +++ b/drivers/iommu/iova.c
>>>> @@ -56,6 +56,7 @@ init_iova_domain(struct iova_domain *iovad, unsigned
>>>> long granule,
>>>> iovad->granule = granule;
>>>> iovad->start_pfn = start_pfn;
>>>> iovad->dma_32bit_pfn = 1UL << (32 - iova_shift(iovad));
>>>> + iovad->free_32bit_pfns = true;
>>>
>>>
>>>
>>> iovad->max_32bit_free = iovad->dma_32bit_pfn;
>>>
>>>> iovad->flush_cb = NULL;
>>>> iovad->fq = NULL;
>>>> iovad->anchor.pfn_lo = iovad->anchor.pfn_hi = IOVA_ANCHOR;
>>>> @@ -139,8 +140,10 @@ __cached_rbnode_delete_update(struct iova_domain
>>>> *iovad, struct iova *free)
>>>> cached_iova = rb_entry(iovad->cached32_node, struct iova,
>>>> node);
>>>> if (free->pfn_hi < iovad->dma_32bit_pfn &&
>>>> - free->pfn_lo >= cached_iova->pfn_lo)
>>>> + free->pfn_lo >= cached_iova->pfn_lo) {
>>>> iovad->cached32_node = rb_next(&free->node);
>>>> + iovad->free_32bit_pfns = true;
>>>
>>>
>>>
>>> iovad->max_32bit_free = iovad->dma_32bit_pfn;
>>
>>
>> i think, you intended to say,
>> iovad->max_32bit_free += (free->pfn_hi - free->pfn_lo);
>
>
> Nope, that's why I said it needed a better name ;)
>
> (I nearly called it last_failed_32bit_alloc_size, but that's a bit much)

may be we can name it "max32_alloc_size"?
>
> The point of this value (whetever it's called) is that at any given time it
> holds an upper bound on the size of the largest contiguous free area. It
> doesn't have to be the *smallest* upper bound, which is why we can keep
> things simple and avoid arithmetic - in realistic use-cases like yours when
> the allocations are a pretty constant size, this should work out directly
> equivalent to the boolean, only with values of "size" and "dma_32bit_pfn"
> instead of 0 and 1, so we don't do any more work than necessary. In the edge
> cases where allocations are all different sizes, it does mean that we will
> probably end up performing more failing allocations than if we actually
> tried to track all of the free space exactly, but I think that's reasonable
> - just because I want to make sure we handle such cases fairly gracefully,
> doesn't mean that we need to do extra work on the typical fast path to try
> and actually optimise for them (which is why I didn't really like the
> accounting implementation I came up with).
>

ok got it, thanks for the explanation.
>>>
>>>> + }
>>>> cached_iova = rb_entry(iovad->cached_node, struct iova, node);
>>>> if (free->pfn_lo >= cached_iova->pfn_lo)
>>>> @@ -290,6 +293,10 @@ alloc_iova(struct iova_domain *iovad, unsigned long
>>>> size,
>>>> struct iova *new_iova;
>>>> int ret;
>>>> + if (limit_pfn <= iovad->dma_32bit_pfn &&
>>>> + !iovad->free_32bit_pfns)
>>>
>>>
>>>
>>> size >= iovad->max_32bit_free)
>>>
>>>> + return NULL;
>>>> +
>>>> new_iova = alloc_iova_mem();
>>>> if (!new_iova)
>>>> return NULL;
>>>> @@ -299,6 +306,8 @@ alloc_iova(struct iova_domain *iovad, unsigned long
>>>> size,
>>>> if (ret) {
>>>> free_iova_mem(new_iova);
>>>> + if (limit_pfn <= iovad->dma_32bit_pfn)
>>>> + iovad->free_32bit_pfns = false;
>>>
>>>
>>>
>>> iovad->max_32bit_free = size;
>>
>>
>> same here, we should decrease available free range after successful
>> allocation.
>> iovad->max_32bit_free -= size;
>
>
> Equivalently, the simple assignment is strictly decreasing the upper bound
> already, since we can only get here if size < max_32bit_free in the first
> place. One more thing I've realised is that this is all potentially a bit
> racy as we're outside the lock here, so it might need to be pulled into
> __alloc_and_insert_iova_range(), something like the rough diff below (name
> changed again for the sake of it; it also occurs to me that we don't really
> need to re-check limit_pfn in the failure path either, because even a 64-bit
> allocation still has to walk down through the 32-bit space in order to fail
> completely)
>
>>>
>>> What do you think?
>>
>>
>> most likely this should work, i will try this and confirm at the earliest,
>
>
> Thanks for sticking with this.
>
> Robin.
>
> ----->8-----
>
> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
> index 83fe2621effe..7cbc58885877 100644
> --- a/drivers/iommu/iova.c
> +++ b/drivers/iommu/iova.c
> @@ -190,6 +190,10 @@ static int __alloc_and_insert_iova_range(struct
> iova_domain *iovad,
>
> /* Walk the tree backwards */
> spin_lock_irqsave(&iovad->iova_rbtree_lock, flags);
> + if (limit_pfn <= iovad->dma_32bit_pfn &&
> + size >= iovad->failed_alloc_size)
> + goto out_err;
> +
> curr = __get_cached_rbnode(iovad, limit_pfn);
> curr_iova = rb_entry(curr, struct iova, node);
> do {
> @@ -200,10 +204,8 @@ static int __alloc_and_insert_iova_range(struct
> iova_domain *iovad,
> curr_iova = rb_entry(curr, struct iova, node);
> } while (curr && new_pfn <= curr_iova->pfn_hi);
>
> - if (limit_pfn < size || new_pfn < iovad->start_pfn) {
> - spin_unlock_irqrestore(&iovad->iova_rbtree_lock, flags);
> - return -ENOMEM;
> - }
> + if (limit_pfn < size || new_pfn < iovad->start_pfn)
> + goto out_err;
>
> /* pfn_lo will point to size aligned address if size_aligned is set
> */
> new->pfn_lo = new_pfn;
> @@ -214,9 +216,12 @@ static int __alloc_and_insert_iova_range(struct
> iova_domain *iovad,
> __cached_rbnode_insert_update(iovad, new);
>
> spin_unlock_irqrestore(&iovad->iova_rbtree_lock, flags);
> -
> -
> return 0;
> +
> +out_err:
> + iovad->failed_alloc_size = size;
> + spin_unlock_irqrestore(&iovad->iova_rbtree_lock, flags);
> + return -ENOMEM;
> }
>
> static struct kmem_cache *iova_cache;


cant we bump up the size when ranges are freed? otherwise we never be
able to attempt in 32bit range, even there is enough replenish.


@@ -139,8 +139,10 @@ __cached_rbnode_delete_update(struct iova_domain
*iovad, struct iova *free)

cached_iova = rb_entry(iovad->cached32_node, struct iova, node);
if (free->pfn_hi < iovad->dma_32bit_pfn &&
- free->pfn_lo >= cached_iova->pfn_lo)
+ free->pfn_lo >= cached_iova->pfn_lo) {
iovad->cached32_node = rb_next(&free->node);
+ iovad->failed_alloc_size += (free->pfn_hi - free->pfn_lo);
+ }

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