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Message-ID: <5bf372ea-40ef-4b07-be13-53451828d3e1@damsy.net>
Date: Wed, 5 Nov 2025 11:34:56 +0100
From: Pierre-Eric Pelloux-Prayer <pierre-eric@...sy.net>
To: Christian König <christian.koenig@....com>,
Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@....com>,
Alex Deucher <alexander.deucher@....com>, David Airlie <airlied@...il.com>,
Simona Vetter <simona@...ll.ch>, Huang Rui <ray.huang@....com>,
Matthew Auld <matthew.auld@...el.com>,
Matthew Brost <matthew.brost@...el.com>,
Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
Sumit Semwal <sumit.semwal@...aro.org>
Cc: amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH v1 02/20] drm/ttm: rework pipelined eviction fence
handling
Le 04/11/2025 à 15:12, Christian König a écrit :
> On 11/4/25 09:35, Pierre-Eric Pelloux-Prayer wrote:
>> Until now ttm stored a single pipelined eviction fence which means
>> drivers had to use a single entity for these evictions.
>>
>> To lift this requirement, this commit allows up to 8 entities to
>> be used.
>>
>> Ideally a dma_resv object would have been used as a container of
>> the eviction fences, but the locking rules makes it complex.
>> dma_resv all have the same ww_class, which means "Attempting to
>> lock more mutexes after ww_acquire_done." is an error.
>>
>> One alternative considered was to introduced a 2nd ww_class for
>> specific resv to hold a single "transient" lock (= the resv lock
>> would only be held for a short period, without taking any other
>> locks).
>>
>> The other option, is to statically reserve a fence array, and
>> extend the existing code to deal with N fences, instead of 1.
>>
>> The driver is still responsible to reserve the correct number
>> of fence slots.
>>
>> Lastly ttm_resource_manager.pipelined_eviction.n_fences is
>> initialized to 1, so the new behavior is opt-in.
>>
>> Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@....com>
>> ---
>> drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c | 8 ++-
>> .../gpu/drm/ttm/tests/ttm_bo_validate_test.c | 13 +++--
>> drivers/gpu/drm/ttm/tests/ttm_resource_test.c | 5 +-
>> drivers/gpu/drm/ttm/ttm_bo.c | 56 ++++++++++++-------
>> drivers/gpu/drm/ttm/ttm_bo_util.c | 36 ++++++++++--
>> drivers/gpu/drm/ttm/ttm_resource.c | 45 ++++++++++-----
>> include/drm/ttm/ttm_resource.h | 34 ++++++++---
>> 7 files changed, 139 insertions(+), 58 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
>> index 326476089db3..c66f00434991 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
>> @@ -2156,7 +2156,7 @@ void amdgpu_ttm_set_buffer_funcs_status(struct amdgpu_device *adev, bool enable)
>> {
>> struct ttm_resource_manager *man = ttm_manager_type(&adev->mman.bdev, TTM_PL_VRAM);
>> uint64_t size;
>> - int r;
>> + int r, i;
>>
>> if (!adev->mman.initialized || amdgpu_in_reset(adev) ||
>> adev->mman.buffer_funcs_enabled == enable || adev->gmc.is_app_apu)
>> @@ -2190,8 +2190,10 @@ void amdgpu_ttm_set_buffer_funcs_status(struct amdgpu_device *adev, bool enable)
>> } else {
>> drm_sched_entity_destroy(&adev->mman.high_pr);
>> drm_sched_entity_destroy(&adev->mman.low_pr);
>> - dma_fence_put(man->move);
>> - man->move = NULL;
>> + for (i = 0; i < TTM_FENCES_MAX_SLOT_COUNT; i++) {
>> + dma_fence_put(man->pipelined_eviction.fences[i]);
>> + man->pipelined_eviction.fences[i] = NULL;
>> + }
>> }
>>
>> /* this just adjusts TTM size idea, which sets lpfn to the correct value */
>> diff --git a/drivers/gpu/drm/ttm/tests/ttm_bo_validate_test.c b/drivers/gpu/drm/ttm/tests/ttm_bo_validate_test.c
>> index 3148f5d3dbd6..1396674e1923 100644
>> --- a/drivers/gpu/drm/ttm/tests/ttm_bo_validate_test.c
>> +++ b/drivers/gpu/drm/ttm/tests/ttm_bo_validate_test.c
>> @@ -651,7 +651,8 @@ static void ttm_bo_validate_move_fence_signaled(struct kunit *test)
>> int err;
>>
>> man = ttm_manager_type(priv->ttm_dev, mem_type);
>> - man->move = dma_fence_get_stub();
>> + man->pipelined_eviction.n_fences = 1;
>> + man->pipelined_eviction.fences[0] = dma_fence_get_stub();
>>
>> bo = ttm_bo_kunit_init(test, test->priv, size, NULL);
>> bo->type = bo_type;
>> @@ -668,7 +669,7 @@ static void ttm_bo_validate_move_fence_signaled(struct kunit *test)
>> KUNIT_EXPECT_EQ(test, ctx.bytes_moved, size);
>>
>> ttm_bo_put(bo);
>> - dma_fence_put(man->move);
>> + dma_fence_put(man->pipelined_eviction.fences[0]);
>> }
>>
>> static const struct ttm_bo_validate_test_case ttm_bo_validate_wait_cases[] = {
>> @@ -732,9 +733,10 @@ static void ttm_bo_validate_move_fence_not_signaled(struct kunit *test)
>>
>> spin_lock_init(&fence_lock);
>> man = ttm_manager_type(priv->ttm_dev, fst_mem);
>> - man->move = alloc_mock_fence(test);
>> + man->pipelined_eviction.n_fences = 1;
>> + man->pipelined_eviction.fences[0] = alloc_mock_fence(test);
>>
>> - task = kthread_create(threaded_fence_signal, man->move, "move-fence-signal");
>> + task = kthread_create(threaded_fence_signal, man->pipelined_eviction.fences[0], "move-fence-signal");
>> if (IS_ERR(task))
>> KUNIT_FAIL(test, "Couldn't create move fence signal task\n");
>>
>> @@ -742,7 +744,8 @@ static void ttm_bo_validate_move_fence_not_signaled(struct kunit *test)
>> err = ttm_bo_validate(bo, placement_val, &ctx_val);
>> dma_resv_unlock(bo->base.resv);
>>
>> - dma_fence_wait_timeout(man->move, false, MAX_SCHEDULE_TIMEOUT);
>> + dma_fence_wait_timeout(man->pipelined_eviction.fences[0], false, MAX_SCHEDULE_TIMEOUT);
>> + man->pipelined_eviction.fences[0] = NULL;
>>
>> KUNIT_EXPECT_EQ(test, err, 0);
>> KUNIT_EXPECT_EQ(test, ctx_val.bytes_moved, size);
>> diff --git a/drivers/gpu/drm/ttm/tests/ttm_resource_test.c b/drivers/gpu/drm/ttm/tests/ttm_resource_test.c
>> index e6ea2bd01f07..6dfdf759a491 100644
>> --- a/drivers/gpu/drm/ttm/tests/ttm_resource_test.c
>> +++ b/drivers/gpu/drm/ttm/tests/ttm_resource_test.c
>> @@ -207,6 +207,7 @@ static void ttm_resource_manager_init_basic(struct kunit *test)
>> struct ttm_resource_test_priv *priv = test->priv;
>> struct ttm_resource_manager *man;
>> size_t size = SZ_16K;
>> + int i;
>>
>> man = kunit_kzalloc(test, sizeof(*man), GFP_KERNEL);
>> KUNIT_ASSERT_NOT_NULL(test, man);
>> @@ -216,8 +217,8 @@ static void ttm_resource_manager_init_basic(struct kunit *test)
>> KUNIT_ASSERT_PTR_EQ(test, man->bdev, priv->devs->ttm_dev);
>> KUNIT_ASSERT_EQ(test, man->size, size);
>> KUNIT_ASSERT_EQ(test, man->usage, 0);
>> - KUNIT_ASSERT_NULL(test, man->move);
>> - KUNIT_ASSERT_NOT_NULL(test, &man->move_lock);
>> + for (i = 0; i < TTM_FENCES_MAX_SLOT_COUNT; i++)
>> + KUNIT_ASSERT_NULL(test, man->pipelined_eviction.fences[i]);
>>
>> for (int i = 0; i < TTM_MAX_BO_PRIORITY; ++i)
>> KUNIT_ASSERT_TRUE(test, list_empty(&man->lru[i]));
>> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
>> index f4d9e68b21e7..bc6d4a6c6d70 100644
>> --- a/drivers/gpu/drm/ttm/ttm_bo.c
>> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
>> @@ -658,34 +658,48 @@ void ttm_bo_unpin(struct ttm_buffer_object *bo)
>> EXPORT_SYMBOL(ttm_bo_unpin);
>>
>> /*
>> - * Add the last move fence to the BO as kernel dependency and reserve a new
>> - * fence slot.
>> + * Add the pipelined eviction fencesto the BO as kernel dependency and reserve new
>> + * fence slots.
>> */
>> -static int ttm_bo_add_move_fence(struct ttm_buffer_object *bo,
>> - struct ttm_resource_manager *man,
>> - bool no_wait_gpu)
>> +static int ttm_bo_add_pipelined_eviction_fences(struct ttm_buffer_object *bo,
>> + struct ttm_resource_manager *man,
>> + bool no_wait_gpu)
>> {
>> + struct dma_fence *fences_to_add[TTM_FENCES_MAX_SLOT_COUNT] = {};
>> struct dma_fence *fence;
>> - int ret;
>> + bool all_signaled = true, signaled;
>> + int i, n = 0;
>>
>> - spin_lock(&man->move_lock);
>> - fence = dma_fence_get(man->move);
>> - spin_unlock(&man->move_lock);
>> + spin_lock(&man->pipelined_eviction.lock);
>> + for (i = 0; i < man->pipelined_eviction.n_fences; i++) {
>> + fence = man->pipelined_eviction.fences[i];
>
>> + if (!fence)
>> + continue;
>> + signaled = dma_fence_is_signaled(fence);
>>
>> - if (!fence)
>> + if (signaled) {
>> + dma_fence_put(man->pipelined_eviction.fences[i]);
>> + man->pipelined_eviction.fences[i] = NULL;
>
> Please completely drop that, only check if the fences are signaled when the no_wait_gpu flag is set.
ok.
>
>> + } else {
>> + all_signaled = false;
>> + if (no_wait_gpu) {
>> + spin_unlock(&man->pipelined_eviction.lock);
>> + return -EBUSY;
>> + }
>> + fences_to_add[n++] = dma_fence_get(fence);
>> + }
>> + }
>> + spin_unlock(&man->pipelined_eviction.lock);
>> +
>> + if (all_signaled)
>> return 0;
>>
>> - if (no_wait_gpu) {
>> - ret = dma_fence_is_signaled(fence) ? 0 : -EBUSY;
>> - dma_fence_put(fence);
>> - return ret;
>> + for (i = 0; i < n; i++) {
>> + dma_resv_add_fence(bo->base.resv, fences_to_add[i], DMA_RESV_USAGE_KERNEL);
>> + dma_fence_put(fences_to_add[i]);
>> }
>>
>> - dma_resv_add_fence(bo->base.resv, fence, DMA_RESV_USAGE_KERNEL);
>> -
>> - ret = dma_resv_reserve_fences(bo->base.resv, 1);
>> - dma_fence_put(fence);
>> - return ret;
>> + return dma_resv_reserve_fences(bo->base.resv, TTM_FENCES_MAX_SLOT_COUNT);
>
> Please separate out a patch where the call to dma_resv_reserve_fences() is removed here.
Can you remind me why it's not needed?
>
>> }
>>
>> /**
>> @@ -718,7 +732,7 @@ static int ttm_bo_alloc_resource(struct ttm_buffer_object *bo,
>> int i, ret;
>>
>> ticket = dma_resv_locking_ctx(bo->base.resv);
>> - ret = dma_resv_reserve_fences(bo->base.resv, 1);
>> + ret = dma_resv_reserve_fences(bo->base.resv, TTM_FENCES_MAX_SLOT_COUNT);
>> if (unlikely(ret))
>> return ret;
>>
>> @@ -757,7 +771,7 @@ static int ttm_bo_alloc_resource(struct ttm_buffer_object *bo,
>> return ret;
>> }
>>
>> - ret = ttm_bo_add_move_fence(bo, man, ctx->no_wait_gpu);
>> + ret = ttm_bo_add_pipelined_eviction_fences(bo, man, ctx->no_wait_gpu);
>> if (unlikely(ret)) {
>> ttm_resource_free(bo, res);
>> if (ret == -EBUSY)
>> diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c b/drivers/gpu/drm/ttm/ttm_bo_util.c
>> index acbbca9d5c92..ada8af965acf 100644
>> --- a/drivers/gpu/drm/ttm/ttm_bo_util.c
>> +++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
>> @@ -258,7 +258,7 @@ static int ttm_buffer_object_transfer(struct ttm_buffer_object *bo,
>> ret = dma_resv_trylock(&fbo->base.base._resv);
>> WARN_ON(!ret);
>>
>> - ret = dma_resv_reserve_fences(&fbo->base.base._resv, 1);
>> + ret = dma_resv_reserve_fences(&fbo->base.base._resv, TTM_FENCES_MAX_SLOT_COUNT);
>> if (ret) {
>> dma_resv_unlock(&fbo->base.base._resv);
>> kfree(fbo);
>> @@ -646,6 +646,8 @@ static void ttm_bo_move_pipeline_evict(struct ttm_buffer_object *bo,
>> {
>> struct ttm_device *bdev = bo->bdev;
>> struct ttm_resource_manager *from;
>> + struct dma_fence *tmp;
>> + int i, free_slot = -1;
>>
>> from = ttm_manager_type(bdev, bo->resource->mem_type);
>>
>> @@ -653,13 +655,35 @@ static void ttm_bo_move_pipeline_evict(struct ttm_buffer_object *bo,
>> * BO doesn't have a TTM we need to bind/unbind. Just remember
>> * this eviction and free up the allocation
>> */
>> - spin_lock(&from->move_lock);
>> - if (!from->move || dma_fence_is_later(fence, from->move)) {
>> - dma_fence_put(from->move);
>> - from->move = dma_fence_get(fence);
>> + spin_lock(&from->pipelined_eviction.lock);
>> + for (i = 0; i < from->pipelined_eviction.n_fences; i++) {
>> + tmp = from->pipelined_eviction.fences[i];
>> + if (!tmp) {
>> + if (free_slot < 0)
>> + free_slot = i;
>> + continue;
>
> Just break here.
The logic here is to reuse context slots. Even if slot 0 is empty, I need to use
slot 1 if slot 1's context is the same as fence->context.
This way, we're guaranteed to find a slot for all contexts used by the driver.
>
>> + }
>> + if (fence->context != tmp->context)
>> + continue;
>> + if (dma_fence_is_later(fence, tmp)) {
>> + dma_fence_put(tmp);
>> + free_slot = i;
>> + break;
>> + }
>> + goto unlock;
>> + }
>> + if (free_slot >= 0) {
>
> Drop free_slot and check i here.
>
>> + from->pipelined_eviction.fences[free_slot] = dma_fence_get(fence);
>> + } else {
>> + WARN(1, "not enough fence slots for all fence contexts");
>> + spin_unlock(&from->pipelined_eviction.lock);
>> + dma_fence_wait(fence, false);
>> + goto end;
>> }
>> - spin_unlock(&from->move_lock);
>>
>> +unlock:
>> + spin_unlock(&from->pipelined_eviction.lock);
>> +end:
>> ttm_resource_free(bo, &bo->resource);
>> }
>>
>> diff --git a/drivers/gpu/drm/ttm/ttm_resource.c b/drivers/gpu/drm/ttm/ttm_resource.c
>> index e2c82ad07eb4..ae0d4621cc55 100644
>> --- a/drivers/gpu/drm/ttm/ttm_resource.c
>> +++ b/drivers/gpu/drm/ttm/ttm_resource.c
>> @@ -523,14 +523,19 @@ void ttm_resource_manager_init(struct ttm_resource_manager *man,
>> {
>> unsigned i;
>>
>> - spin_lock_init(&man->move_lock);
>> man->bdev = bdev;
>> man->size = size;
>> man->usage = 0;
>>
>> for (i = 0; i < TTM_MAX_BO_PRIORITY; ++i)
>> INIT_LIST_HEAD(&man->lru[i]);
>> - man->move = NULL;
>> + spin_lock_init(&man->pipelined_eviction.lock);
>> + for (i = 0; i < TTM_FENCES_MAX_SLOT_COUNT; i++)
>> + man->pipelined_eviction.fences[i] = NULL;
>> + /* Can be overridden by drivers that wants to use more than 1 entity
>> + * for moves and evictions (limited to TTM_FENCES_MAX_SLOT_COUNT).
>> + */
>> + man->pipelined_eviction.n_fences = 1;
>> }
>> EXPORT_SYMBOL(ttm_resource_manager_init);
>>
>> @@ -551,7 +556,7 @@ int ttm_resource_manager_evict_all(struct ttm_device *bdev,
>> .no_wait_gpu = false,
>> };
>> struct dma_fence *fence;
>> - int ret;
>> + int ret, i;
>>
>> do {
>> ret = ttm_bo_evict_first(bdev, man, &ctx);
>> @@ -561,18 +566,32 @@ int ttm_resource_manager_evict_all(struct ttm_device *bdev,
>> if (ret && ret != -ENOENT)
>> return ret;
>>
>> - spin_lock(&man->move_lock);
>> - fence = dma_fence_get(man->move);
>> - spin_unlock(&man->move_lock);
>> + ret = 0;
>>
>> - if (fence) {
>> - ret = dma_fence_wait(fence, false);
>> - dma_fence_put(fence);
>> - if (ret)
>> - return ret;
>> - }
>> + do {
>> + fence = NULL;
>>
>> - return 0;
>> + spin_lock(&man->pipelined_eviction.lock);
>> + for (i = 0; i < man->pipelined_eviction.n_fences; i++) {
>> + fence = man->pipelined_eviction.fences[i];
>
>> + man->pipelined_eviction.fences[i] = NULL;
>
> Drop that. We should never set man->pipelined_eviction.fences to NULL.
Why?
>
> Potentially even initialize all move fences with a stub fence.
>
>> + if (fence)
>> + break;
>> + }
>> + spin_unlock(&man->pipelined_eviction.lock);
>> +
>> + if (fence) {
>> + ret = dma_fence_wait(fence, false);
>> + dma_fence_put(fence);
>> +
>> + if (ret)
>> + break;
>> + } else {
>> + break;
>> + }
>> + } while (1);
>> +
>> + return ret;
>> }
>> EXPORT_SYMBOL(ttm_resource_manager_evict_all);
>>
>> diff --git a/include/drm/ttm/ttm_resource.h b/include/drm/ttm/ttm_resource.h
>> index f49daa504c36..898c429b37ad 100644
>> --- a/include/drm/ttm/ttm_resource.h
>> +++ b/include/drm/ttm/ttm_resource.h
>> @@ -50,6 +50,15 @@ struct io_mapping;
>> struct sg_table;
>> struct scatterlist;
>>
>> +/**
>> + * define TTM_FENCES_MAX_SLOT_COUNT - How many entities can be used for evictions
>> + *
>> + * Pipelined evictions can be spread on multiple entities. This
>> + * is the max number of entities that can be used by the driver
>> + * for that purpose.
>> + */
>> +#define TTM_FENCES_MAX_SLOT_COUNT 8
>
> Make that TTM_NUM_MOVE_FENCES.
Ok.
>
>> +
>> /**
>> * enum ttm_lru_item_type - enumerate ttm_lru_item subclasses
>> */
>> @@ -180,8 +189,10 @@ struct ttm_resource_manager_func {
>> * @size: Size of the managed region.
>> * @bdev: ttm device this manager belongs to
>> * @func: structure pointer implementing the range manager. See above
>> - * @move_lock: lock for move fence
>> - * @move: The fence of the last pipelined move operation.
>> + * @pipelined_eviction.lock: lock for eviction fences
>> + * @pipelined_eviction.n_fences: The number of fences allowed in the array. If
>> + * 0, pipelined evictions aren't used.
>> + * @pipelined_eviction.fences: The fences of the last pipelined move operation.
>> * @lru: The lru list for this memory type.
>> *
>> * This structure is used to identify and manage memory types for a device.
>> @@ -195,12 +206,15 @@ struct ttm_resource_manager {
>> struct ttm_device *bdev;
>> uint64_t size;
>> const struct ttm_resource_manager_func *func;
>> - spinlock_t move_lock;
>>
>> - /*
>> - * Protected by @move_lock.
>> + /* This is very similar to a dma_resv object, but locking rules make
>> + * it difficult to use a it in this context.
>> */
>> - struct dma_fence *move;
>> + struct {
>> + spinlock_t lock;
>> + int n_fences;
>> + struct dma_fence *fences[TTM_FENCES_MAX_SLOT_COUNT];
>> + } pipelined_eviction;
>
> Drop the separate structure, just make move an array instead.
IMO pipelined_eviction.fences and pipelined_eviction.lock is clearer when
reading the code than moves and move_lock but if you prefer I'll switch back to
the old names.
>
> And also drop n_fences. Just always take a look at all fences.
OK.
Thanks,
Pierre-Eric
>
> Regards,
> Christian.
>
>>
>> /*
>> * Protected by the bdev->lru_lock.
>> @@ -421,8 +435,12 @@ static inline bool ttm_resource_manager_used(struct ttm_resource_manager *man)
>> static inline void
>> ttm_resource_manager_cleanup(struct ttm_resource_manager *man)
>> {
>> - dma_fence_put(man->move);
>> - man->move = NULL;
>> + int i;
>> +
>> + for (i = 0; i < TTM_FENCES_MAX_SLOT_COUNT; i++) {
>> + dma_fence_put(man->pipelined_eviction.fences[i]);
>> + man->pipelined_eviction.fences[i] = NULL;
>> + }
>> }
>>
>> void ttm_lru_bulk_move_init(struct ttm_lru_bulk_move *bulk);
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