[Intel-gfx] [PATCH 3/3] drm/ttm: remove ttm_bo_wait_unreserved
Thomas Hellström (VMware)
thomas_os at shipmail.org
Wed Aug 21 14:27:39 UTC 2019
On 8/21/19 4:09 PM, Daniel Vetter wrote:
> On Wed, Aug 21, 2019 at 2:47 PM Thomas Hellström (VMware)
> <thomas_os at shipmail.org> wrote:
>> On 8/21/19 2:40 PM, Thomas Hellström (VMware) wrote:
>>> On 8/20/19 4:53 PM, Daniel Vetter wrote:
>>>> With nouveau fixed all ttm-using drives have the correct nesting of
>>>> mmap_sem vs dma_resv, and we can just lock the buffer.
>>>>
>>>> Assuming I didn't screw up anything with my audit of course.
>>>>
>>>> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
>>>> Cc: Christian Koenig <christian.koenig at amd.com>
>>>> Cc: Huang Rui <ray.huang at amd.com>
>>>> Cc: Gerd Hoffmann <kraxel at redhat.com>
>>>> Cc: "VMware Graphics" <linux-graphics-maintainer at vmware.com>
>>>> Cc: Thomas Hellstrom <thellstrom at vmware.com>
>>>> ---
>>>> drivers/gpu/drm/ttm/ttm_bo.c | 34 ---------------------------------
>>>> drivers/gpu/drm/ttm/ttm_bo_vm.c | 26 +------------------------
>>>> include/drm/ttm/ttm_bo_api.h | 1 -
>>>> 3 files changed, 1 insertion(+), 60 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
>>>> index 20ff56f27aa4..a952dd624b06 100644
>>>> --- a/drivers/gpu/drm/ttm/ttm_bo.c
>>>> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
>>>> @@ -1954,37 +1954,3 @@ void ttm_bo_swapout_all(struct ttm_bo_device
>>>> *bdev)
>>>> ;
>>>> }
>>>> EXPORT_SYMBOL(ttm_bo_swapout_all);
>>>> -
>>>> -/**
>>>> - * ttm_bo_wait_unreserved - interruptible wait for a buffer object
>>>> to become
>>>> - * unreserved
>>>> - *
>>>> - * @bo: Pointer to buffer
>>>> - */
>>>> -int ttm_bo_wait_unreserved(struct ttm_buffer_object *bo)
>>>> -{
>>>> - int ret;
>>>> -
>>>> - /*
>>>> - * In the absense of a wait_unlocked API,
>>>> - * Use the bo::wu_mutex to avoid triggering livelocks due to
>>>> - * concurrent use of this function. Note that this use of
>>>> - * bo::wu_mutex can go away if we change locking order to
>>>> - * mmap_sem -> bo::reserve.
>>>> - */
>>>> - ret = mutex_lock_interruptible(&bo->wu_mutex);
>>>> - if (unlikely(ret != 0))
>>>> - return -ERESTARTSYS;
>>>> - if (!dma_resv_is_locked(bo->base.resv))
>>>> - goto out_unlock;
>>>> - ret = dma_resv_lock_interruptible(bo->base.resv, NULL);
>>>> - if (ret == -EINTR)
>>>> - ret = -ERESTARTSYS;
>>>> - if (unlikely(ret != 0))
>>>> - goto out_unlock;
>>>> - dma_resv_unlock(bo->base.resv);
>>>> -
>>>> -out_unlock:
>>>> - mutex_unlock(&bo->wu_mutex);
>>>> - return ret;
>>>> -}
>>>> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>> b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>> index 76eedb963693..505e1787aeea 100644
>>>> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>> @@ -125,31 +125,7 @@ static vm_fault_t ttm_bo_vm_fault(struct
>>>> vm_fault *vmf)
>>>> &bdev->man[bo->mem.mem_type];
>>>> struct vm_area_struct cvma;
>>>> - /*
>>>> - * Work around locking order reversal in fault / nopfn
>>>> - * between mmap_sem and bo_reserve: Perform a trylock operation
>>>> - * for reserve, and if it fails, retry the fault after waiting
>>>> - * for the buffer to become unreserved.
>>>> - */
>>>> - if (unlikely(!dma_resv_trylock(bo->base.resv))) {
>>>> - if (vmf->flags & FAULT_FLAG_ALLOW_RETRY) {
>>>> - if (!(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
>>>> - ttm_bo_get(bo);
>>>> - up_read(&vmf->vma->vm_mm->mmap_sem);
>>>> - (void) ttm_bo_wait_unreserved(bo);
>>>> - ttm_bo_put(bo);
>>>> - }
>>>> -
>>>> - return VM_FAULT_RETRY;
>>>> - }
>>>> -
>>>> - /*
>>>> - * If we'd want to change locking order to
>>>> - * mmap_sem -> bo::reserve, we'd use a blocking reserve here
>>>> - * instead of retrying the fault...
>>>> - */
>>> I think you should justify why the above code is removed, since the
>>> comments actually outlines what to do if we change locking order.
>>>
>>> The code that's removed above is not for adjusting locking orders but
>>> to decrease the mm latency by releasing the mmap_sem while waiting for
>>> bo reserve which in turn might be waiting for GPU. At a minimum we
>>> should have a separate patch with justification.
>>>
>>> Note that the caller here ensures locking progress by adjusting the
>>> RETRY flags after a retry.
> That would be patches 1&2 in this series.
>
Hmm? Those seem to touch only dma-buf and nouveau not ttm? I mean this
patch should look along the lines of (based on an older tree) to
implement the new locking-order mmap_sem->reservation,
but to keep the mm latency optimization using the RETRY functionality:
Thanks,
Thomas
diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
index 85f5bcbe0c76..68482c67b9f7 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
@@ -125,30 +125,20 @@ static vm_fault_t ttm_bo_vm_fault(struct vm_fault *vmf)
&bdev->man[bo->mem.mem_type];
struct vm_area_struct cvma;
- /*
- * Work around locking order reversal in fault / nopfn
- * between mmap_sem and bo_reserve: Perform a trylock operation
- * for reserve, and if it fails, retry the fault after waiting
- * for the buffer to become unreserved.
- */
+ /* Avoid blocking on reservation with mmap_sem held, if possible */
if (unlikely(!reservation_object_trylock(bo->base.resv))) {
- if (vmf->flags & FAULT_FLAG_ALLOW_RETRY) {
- if (!(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
- ttm_bo_get(bo);
- up_read(&vmf->vma->vm_mm->mmap_sem);
- (void) ttm_bo_wait_unreserved(bo);
- ttm_bo_put(bo);
- }
+ if ((vmf->flags & FAULT_FLAG_ALLOW_RETRY) &&
+ !(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
+ ttm_bo_get(bo);
+ up_read(&vmf->vma->vm_mm->mmap_sem);
+ (void) ttm_bo_wait_unreserved(bo);
+ ttm_bo_put(bo);
return VM_FAULT_RETRY;
}
- /*
- * If we'd want to change locking order to
- * mmap_sem -> bo::reserve, we'd use a blocking reserve here
- * instead of retrying the fault...
- */
- return VM_FAULT_NOPAGE;
+ if (reservation_object_lock_interruptible(bo->base.resv, NULL))
+ return VM_FAULT_NOPAGE;
}
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