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

[Ganapatrao Kulkarni]

On Fri, Aug 10, 2018 at 3:19 PM, Robin Murphy <robin.murphy@xxxxxxx> wrote:
> On 10/08/18 10:24, Ganapatrao Kulkarni wrote:
>>
>> Hi Robin,
>>
>> On Fri, Aug 10, 2018 at 2:13 AM, Robin Murphy <robin.murphy@xxxxxxx>
>> 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@xxxxxxx>
>>>> 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@xxxxxxxxxx>
>>>>>> ---
>>>>>> This patch is based on comments from Robin Murphy
>>>>>> <robin.murphy@xxxxxxx>
>>>>>> 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.
>
>
> Oh, I just left that part out of the example for clarity, since it's already
> under the lock - I didn't mean to suggest that that we should remove it!

ok, thanks
>
> (I was just too lazy to actually apply your patch to generate a real diff on
> top of it)
>
no problem, i will post next version at the earliest.

> Robin.
>
>
>>
>>
>> @@ -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);
>> +       }
>>
>

thanks
Ganapat