Re: [PATCH] iommu/iova: Optimise attempts to allocate iova from 32bit address range

[Robin Murphy <robin.murphy@arm.com>]

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)

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).

>>
>>> +       }
>>>          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;