[PATCH] drm/gpuvm: merge adjacent gpuva range during a map operation
Matthew Brost
matthew.brost at intel.com
Thu Sep 19 15:48:02 UTC 2024
On Thu, Sep 19, 2024 at 09:09:57AM -0600, 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.
>
If a destroyed VMA is in the RB tree that would be a big issue and
definitely would need to be fixed.
Adding a test case to show the issue you describe would be good. Also if
we end doing something with merging adding a test case for the
description in the commit message would also be good.
> But I don't remember the details now. I need to go back to it to find more details.
>
That would be good.
> 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.
>
I don't think Xe has a current use case, but the situation you describe
is very similar to a system allocator case where we would want merging.
Simple example below.
Initital State:
VMA[A] 0x0000-0x0fff - System allocator VMA
VMA[B] 0x1000-0x1fff - BO binding VMA
VMA[C] 0x2000-0x2fff - System allocator VMA
User op:
Bind 0x1000-0x1fff to sytem allocator
Ideally we really want this final state:
VMA[D] 0x0000-0x2fff - System allocator VMA
The without merging like above as BO bindings are bound / unbound the
system allocator space will get fragmented into lots of VMA which is not
ideal.
So here 1) from my list is a non-issue as UNMAP system allocator VMAs
don't interact with the hardware. 2) could still be an issue as VMA[A],
VMA[C] could have different caching or migration policies.
> If we decide only merge for some case but not for other cases, we need a clear documentation of the behavior.
>
If this was added merging it likely would a be optional user controled
thing. I suggested a vfunc or something to test for merge condition, we
could just use a user defined cookie attached to VMA that GPUVM could
match on for merging (also could be used as enable merging if cookie is
non-zero). That actually seems pretty clean.
Matt
> 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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