[PATCH] drm/gpuvm: merge adjacent gpuva range during a map operation
Danilo Krummrich
dakr at kernel.org
Mon Sep 23 08:22:25 UTC 2024
On 9/19/24 5:09 PM, Zeng, Oak wrote:
>
>
>> -----Original Message-----
>> From: Brost, Matthew <matthew.brost at intel.com>
>> Sent: Wednesday, September 18, 2024 2:38 PM
>> To: Zeng, Oak <oak.zeng at intel.com>
>> Cc: intel-xe at lists.freedesktop.org; dakr at redhat.com
>> Subject: Re: [PATCH] drm/gpuvm: merge adjacent gpuva range during
>> a map operation
>>
>> On Wed, Sep 18, 2024 at 12:47:40PM -0400, Oak Zeng wrote:
>>
>> Please sent patches which touch common code to dri-devel.
>>
>>> Considder this example. Before a map operation, the gpuva ranges
>>> in a vm looks like below:
>>>
>>> VAs | start | range | end | object |
>> object offset
>>> -----------------------------------------------------------------------------------
>> --------------------------
>>> | 0x0000000000000000 | 0x00007ffff5cd0000 | 0x00007ffff5cd0000
>> | 0x0000000000000000 | 0x0000000000000000
>>> | 0x00007ffff5cf0000 | 0x00000000000c7000 | 0x00007ffff5db7000
>> | 0x0000000000000000 | 0x0000000000000000
>>>
>>> Now user want to map range [0x00007ffff5cd0000 -
>> 0x00007ffff5cf0000).
>>> With existing codes, the range walking in __drm_gpuvm_sm_map
>> won't
>>> find any range, so we end up a single map operation for range
>>> [0x00007ffff5cd0000 - 0x00007ffff5cf0000). This result in:
>>>
>>> VAs | start | range | end | object |
>> object offset
>>> -----------------------------------------------------------------------------------
>> --------------------------
>>> | 0x0000000000000000 | 0x00007ffff5cd0000 | 0x00007ffff5cd0000
>> | 0x0000000000000000 | 0x0000000000000000
>>> | 0x00007ffff5cd0000 | 0x0000000000020000 | 0x00007ffff5cf0000
>> | 0x0000000000000000 | 0x0000000000000000
>>> | 0x00007ffff5cf0000 | 0x00000000000c7000 | 0x00007ffff5db7000
>> | 0x0000000000000000 | 0x0000000000000000
>>>
>>> The correct behavior is to merge those 3 ranges. So
>> __drm_gpuvm_sm_map
>>
>> Danilo - correct me if I'm wrong, but I believe early in gpuvm you had
>> similar code to this which could optionally be used. I was of the
>> thinking Xe didn't want this behavior and eventually this behavior was
>> ripped out prior to merging.
>>
>>> is slightly modified to handle this corner case. The walker is changed
>>> to find the range just before or after the mapping request, and
>> merge
>>> adjacent ranges using unmap and map operations. with this change,
>> the
>>
>> This would problematic in Xe for several reasons.
>>
>> 1. This would create a window in which previously valid mappings are
>> unmapped by our bind code implementation which could result in a
>> fault.
>> Remap operations can create a similar window but it is handled by
>> either
>> only unmapping the required range or using dma-resv slots to close
>> this
>> window ensuring nothing is running on the GPU while valid mappings
>> are
>> unmapped. A series of UNMAP, UNMAP, and MAP ops currently
>> doesn't detect
>> the problematic window. If we wanted to do something like this, we'd
>> probably need to a new op like MERGE or something to help detect
>> this
>> window.
>>
>> 2. Consider this case.
>>
>> 0x0000000000000000-0x00007ffff5cd0000 VMA[A]
>> 0x00007ffff5cf0000-0x00000000000c7000 VMA[B]
>> 0x00007ffff5cd0000-0x0000000000020000 VMA[C]
>>
>> What is VMA[A], VMA[B], and VMA[C] are all setup with different
>> driver
>> specific implmentation properties (e.g. pat_index). These VMAs
>> cannot be
>> merged. GPUVM has no visablity to this. If we wanted to do this I
>> think
>> we'd need a gpuvm vfunc that calls into the driver to determine if we
>> can merge VMAs.
>
> #1, #2 are all reasonable to me. Agree if we want this merge behavior, more work is needed.
>
>>
>> 3. What is the ROI of this? Slightly reducing the VMA count? Perhaps
>> allowing larger GPU is very specific corner cases? Give 1), 2) I'd say
>> just leave GPUVM as is rather than add this complexity and then
>> make all
>> driver use GPUVM absorb this behavior change.
>
> This patch is an old one in my back log. I roughly remember I ran into a situation where there were two duplicated VMAs covering
> Same virtual address range are kept in gpuvm's RB-tree. One VMA was actually already destroyed. This further caused issues as
> The destroyed VMA was found during a GPUVM RB-tree walk. This triggered me to look into the gpuvm merge split logic and end
> Up with this patch. This patch did fix that issue.
That would indeed be a big issue. As Matt suggests, is there a reproducer?
Either way, adding merge support can't be the fix for this, we need a separate
one, that's back-portable.
Also, can we move this on DRI-devel please?
- Danilo
>
> But I don't remember the details now. I need to go back to it to find more details.
>
> From design perspective, I think merging adjacent contiguous ranges is a cleaner design. Merging for some use cases (I am not sure
> We do merge for some cases, just guess from the function name _sm_) but not merging for other use cases creates a design hole and
> Eventually such behavior can potentially mess things up. Maybe xekmd today doesn't have such use cases, but people may run into
> Situation where they want a merge behavior.
>
> If we decide only merge for some case but not for other cases, we need a clear documentation of the behavior.
>
> Oak
>
>>
>> Matt
>>
>>> end result of above example is as below:
>>>
>>> VAs | start | range | end | object |
>> object offset
>>> -----------------------------------------------------------------------------------
>> --------------------------
>>> | 0x0000000000000000 | 0x00007ffff5db7000 |
>> 0x00007ffff5db7000 | 0x0000000000000000 | 0x0000000000000000
>>>
>>> Even though this fixes a real problem, the codes looks a little ugly.
>>> So I welcome any better fix or suggestion.
>>>
>>> Signed-off-by: Oak Zeng <oak.zeng at intel.com>
>>> ---
>>> drivers/gpu/drm/drm_gpuvm.c | 62
>> +++++++++++++++++++++++++------------
>>> 1 file changed, 43 insertions(+), 19 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/drm_gpuvm.c
>> b/drivers/gpu/drm/drm_gpuvm.c
>>> index 4b6fcaea635e..51825c794bdc 100644
>>> --- a/drivers/gpu/drm/drm_gpuvm.c
>>> +++ b/drivers/gpu/drm/drm_gpuvm.c
>>> @@ -2104,28 +2104,30 @@ __drm_gpuvm_sm_map(struct
>> drm_gpuvm *gpuvm,
>>> {
>>> struct drm_gpuva *va, *next;
>>> u64 req_end = req_addr + req_range;
>>> + u64 merged_req_addr = req_addr;
>>> + u64 merged_req_end = req_end;
>>> int ret;
>>>
>>> if (unlikely(!drm_gpuvm_range_valid(gpuvm, req_addr,
>> req_range)))
>>> return -EINVAL;
>>>
>>> - drm_gpuvm_for_each_va_range_safe(va, next, gpuvm,
>> req_addr, req_end) {
>>> + drm_gpuvm_for_each_va_range_safe(va, next, gpuvm,
>> req_addr - 1, req_end + 1) {
>>> struct drm_gem_object *obj = va->gem.obj;
>>> u64 offset = va->gem.offset;
>>> u64 addr = va->va.addr;
>>> u64 range = va->va.range;
>>> u64 end = addr + range;
>>> - bool merge = !!va->gem.obj;
>>> + bool merge;
>>>
>>> if (addr == req_addr) {
>>> - merge &= obj == req_obj &&
>>> + merge = obj == req_obj &&
>>> offset == req_offset;
>>>
>>> if (end == req_end) {
>>> ret = op_unmap_cb(ops, priv, va,
>> merge);
>>> if (ret)
>>> return ret;
>>> - break;
>>> + continue;
>>> }
>>>
>>> if (end < req_end) {
>>> @@ -2162,22 +2164,33 @@ __drm_gpuvm_sm_map(struct
>> drm_gpuvm *gpuvm,
>>> };
>>> struct drm_gpuva_op_unmap u = { .va = va };
>>>
>>> - merge &= obj == req_obj &&
>>> - offset + ls_range == req_offset;
>>> + merge = (obj && obj == req_obj &&
>>> + offset + ls_range == req_offset) ||
>>> + (!obj && !req_obj);
>>> u.keep = merge;
>>>
>>> if (end == req_end) {
>>> ret = op_remap_cb(ops, priv, &p,
>> NULL, &u);
>>> if (ret)
>>> return ret;
>>> - break;
>>> + continue;
>>> }
>>>
>>> if (end < req_end) {
>>> - ret = op_remap_cb(ops, priv, &p,
>> NULL, &u);
>>> - if (ret)
>>> - return ret;
>>> - continue;
>>> + if (end == req_addr) {
>>> + if (merge) {
>>> + ret =
>> op_unmap_cb(ops, priv, va, merge);
>>> + if (ret)
>>> + return ret;
>>> + merged_req_addr =
>> addr;
>>> + continue;
>>> + }
>>> + } else {
>>> + ret = op_remap_cb(ops, priv,
>> &p, NULL, &u);
>>> + if (ret)
>>> + return ret;
>>> + continue;
>>> + }
>>> }
>>>
>>> if (end > req_end) {
>>> @@ -2195,15 +2208,16 @@ __drm_gpuvm_sm_map(struct
>> drm_gpuvm *gpuvm,
>>> break;
>>> }
>>> } else if (addr > req_addr) {
>>> - merge &= obj == req_obj &&
>>> + merge = (obj && obj == req_obj &&
>>> offset == req_offset +
>>> - (addr - req_addr);
>>> + (addr - req_addr)) ||
>>> + (!obj && !req_obj);
>>>
>>> if (end == req_end) {
>>> ret = op_unmap_cb(ops, priv, va,
>> merge);
>>> if (ret)
>>> return ret;
>>> - break;
>>> + continue;
>>> }
>>>
>>> if (end < req_end) {
>>> @@ -2225,16 +2239,26 @@ __drm_gpuvm_sm_map(struct
>> drm_gpuvm *gpuvm,
>>> .keep = merge,
>>> };
>>>
>>> - ret = op_remap_cb(ops, priv, NULL,
>> &n, &u);
>>> - if (ret)
>>> - return ret;
>>> - break;
>>> + if (addr == req_end) {
>>> + if (merge) {
>>> + ret =
>> op_unmap_cb(ops, priv, va, merge);
>>> + if (ret)
>>> + return ret;
>>> + merged_req_end =
>> end;
>>> + break;
>>> + }
>>> + } else {
>>> + ret = op_remap_cb(ops, priv,
>> NULL, &n, &u);
>>> + if (ret)
>>> + return ret;
>>> + break;
>>> + }
>>> }
>>> }
>>> }
>>>
>>> return op_map_cb(ops, priv,
>>> - req_addr, req_range,
>>> + merged_req_addr, merged_req_end -
>> merged_req_addr,
>>> req_obj, req_offset);
>>> }
>>>
>>> --
>>> 2.26.3
>>>
>
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