The dmem cgroup v2 controller currently only provides a hard "max"
limit, which causes immediate allocation failures when a cgroup's
device memory usage reaches its quota.  GPU-bound AI workloads need
smoother over-subscription support: a soft limit that temporarily
allows excess usage while applying backpressure through reclaim
rather than outright failure.

Add dmem.high, a soft limit that penalizes over-limit cgroups by
evicting their buffer objects first when eviction is triggered (e.g.
due to a "max" limit hit).  Unlike the rejected v1 approach which
used sleep-on-allocation throttling, this version provides a
meaningful recovery action through prioritized reclaim.

Expose "high" as a new cgroupfs control file per region via
set_resource_high() and get_resource_high(), and initialize it to
PAGE_COUNTER_MAX in reset_all_resource_limits().

Extend dmem_cgroup_state_evict_valuable() with a "try_high"
parameter.  When set, only pools with usage above their high limit
are considered evictable, implementing tier-1 of the prioritized
eviction model.  For the existing low-priority passes, the original
effective-low/effective-min protection logic is unchanged.

Refactor ttm_bo_evict_alloc() into a 3-pass eviction strategy.
Pass 1 uses trylock and targets only BOs whose cgroup exceeds
dmem.high.  Pass 2 falls back to the standard above-elow eviction.
Pass 3+ uses proper locking and repeats while making progress,
with the existing low-watermark fallback.

This adds one extra LRU walk when over-limit cgroups are present,
but avoids any throttling or sleeping in the charge path, which
would be catastrophic for GPU submission pipelines.

Signed-off-by: Qiliang Yuan <realwujing@gmail.com>
---
Introduce a "high" soft limit for the dmem cgroup v2 controller.
When a "max" limit is hit and eviction is triggered, buffer objects
belonging to cgroups that exceed their dmem.high limit are targeted
first, providing a meaningful recovery action through reclaim.

The dmem cgroup currently only supports hard "max" limits, which
cause immediate allocation failures for GPU-bound workloads. A soft
limit enables smoother over-subscription by penalizing over-limit
cgroups via prioritized eviction rather than outright rejection.

The implementation adds a "high" cgroupfs control file per region,
a try_high parameter to dmem_cgroup_state_evict_valuable() for
tier-1 eviction, and a 3-pass strategy in ttm_bo_evict_alloc().
---
V1 -> V2:
- Replace sleep-on-allocation throttling with prioritized eviction.
  When a "max" limit is hit, BOs from cgroups exceeding dmem.high are
  evicted first in a dedicated pass. No throttling or sleeping is
  performed in the charge path.
- Remove task throttling (schedule_timeout_killable, TIF_NOTIFY_RESUME,
  resume_user_mode_work() integration) entirely.
- Add dmem.high cgroupfs control file per region.
- Extend dmem_cgroup_state_evict_valuable() with try_high parameter
  to target over-limit cgroups as tier-1 eviction.
- Refactor ttm_bo_evict_alloc() into a 3-pass eviction strategy:
  (1) trylock: evict only BOs exceeding dmem.high
  (2) trylock: above-elow
  (3) proper-lock: repeat with low fallback.
- Initialize high to PAGE_COUNTER_MAX in reset_all_resource_limits().

v1: https://lore.kernel.org/all/20260520-feature-dmem-high-v1-1-97ca0cb7f95a@gmail.com
---
 drivers/gpu/drm/ttm/ttm_bo.c | 28 +++++++++++++++++++++------
 include/linux/cgroup_dmem.h  |  4 ++--
 kernel/cgroup/dmem.c         | 45 ++++++++++++++++++++++++++++++++++++++++++--
 3 files changed, 67 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
index bcd76f6bb7f02..eefcdb6155d63 100644
--- a/drivers/gpu/drm/ttm/ttm_bo.c
+++ b/drivers/gpu/drm/ttm/ttm_bo.c
@@ -505,6 +505,8 @@ struct ttm_bo_evict_walk {
 
 	/** @limit_pool: Which pool limit we should test against */
 	struct dmem_cgroup_pool_state *limit_pool;
+	/** @try_high: Whether to only evict BO's above the high watermark (first pass) */
+	bool try_high;
 	/** @try_low: Whether we should attempt to evict BO's with low watermark threshold */
 	bool try_low;
 	/** @hit_low: If we cannot evict a bo when @try_low is false (first pass) */
@@ -518,7 +520,8 @@ static s64 ttm_bo_evict_cb(struct ttm_lru_walk *walk, struct ttm_buffer_object *
 	s64 lret;
 
 	if (!dmem_cgroup_state_evict_valuable(evict_walk->limit_pool, bo->resource->css,
-					      evict_walk->try_low, &evict_walk->hit_low))
+					      evict_walk->try_high, evict_walk->try_low,
+					      &evict_walk->hit_low))
 		return 0;
 
 	if (bo->pin_count || !bo->bdev->funcs->eviction_valuable(bo, evict_walk->place))
@@ -577,31 +580,44 @@ static int ttm_bo_evict_alloc(struct ttm_device *bdev,
 	};
 	s64 lret;
 
+	/*
+	 * Pass 1 (trylock): Only evict BOs whose cgroup is above its
+	 * dmem.high soft limit. This penalizes over-limit cgroups first.
+	 */
 	evict_walk.walk.arg.trylock_only = true;
+	evict_walk.try_high = true;
 	lret = ttm_lru_walk_for_evict(&evict_walk.walk, bdev, man, 1);
+	evict_walk.try_high = false;
+	if (lret)
+		goto out;
 
-	/* One more attempt if we hit low limit? */
+	/*
+	 * Pass 2 (trylock): Evict BOs above the effective low watermark.
+	 * Falls back to low-priority eviction if needed.
+	 */
+	lret = ttm_lru_walk_for_evict(&evict_walk.walk, bdev, man, 1);
 	if (!lret && evict_walk.hit_low) {
 		evict_walk.try_low = true;
 		lret = ttm_lru_walk_for_evict(&evict_walk.walk, bdev, man, 1);
 	}
+
 	if (lret || !ticket)
 		goto out;
 
-	/* Reset low limit */
+	/*
+	 * Pass 3+ (properly locked): Evict while making progress.
+	 * Reset flags and retry with try_low if we hit the low watermark.
+	 */
 	evict_walk.try_low = evict_walk.hit_low = false;
-	/* If ticket-locking, repeat while making progress. */
 	evict_walk.walk.arg.trylock_only = false;
 
 retry:
 	do {
-		/* The walk may clear the evict_walk.walk.ticket field */
 		evict_walk.walk.arg.ticket = ticket;
 		evict_walk.evicted = 0;
 		lret = ttm_lru_walk_for_evict(&evict_walk.walk, bdev, man, 1);
 	} while (!lret && evict_walk.evicted);
 
-	/* We hit the low limit? Try once more */
 	if (!lret && evict_walk.hit_low && !evict_walk.try_low) {
 		evict_walk.try_low = true;
 		goto retry;
diff --git a/include/linux/cgroup_dmem.h b/include/linux/cgroup_dmem.h
index dd4869f1d736e..06115d35509b1 100644
--- a/include/linux/cgroup_dmem.h
+++ b/include/linux/cgroup_dmem.h
@@ -23,7 +23,7 @@ int dmem_cgroup_try_charge(struct dmem_cgroup_region *region, u64 size,
 void dmem_cgroup_uncharge(struct dmem_cgroup_pool_state *pool, u64 size);
 bool dmem_cgroup_state_evict_valuable(struct dmem_cgroup_pool_state *limit_pool,
 				      struct dmem_cgroup_pool_state *test_pool,
-				      bool ignore_low, bool *ret_hit_low);
+				      bool try_high, bool ignore_low, bool *ret_hit_low);
 
 void dmem_cgroup_pool_state_put(struct dmem_cgroup_pool_state *pool);
 #else
@@ -54,7 +54,7 @@ static inline void dmem_cgroup_uncharge(struct dmem_cgroup_pool_state *pool, u64
 static inline
 bool dmem_cgroup_state_evict_valuable(struct dmem_cgroup_pool_state *limit_pool,
 				      struct dmem_cgroup_pool_state *test_pool,
-				      bool ignore_low, bool *ret_hit_low)
+				      bool try_high, bool ignore_low, bool *ret_hit_low)
 {
 	return true;
 }
diff --git a/kernel/cgroup/dmem.c b/kernel/cgroup/dmem.c
index 4753a67d0f0f2..3799ecd6d7b52 100644
--- a/kernel/cgroup/dmem.c
+++ b/kernel/cgroup/dmem.c
@@ -156,6 +156,12 @@ set_resource_low(struct dmem_cgroup_pool_state *pool, u64 val)
 	page_counter_set_low(&pool->cnt, val);
 }
 
+static void
+set_resource_high(struct dmem_cgroup_pool_state *pool, u64 val)
+{
+	page_counter_set_high(&pool->cnt, val);
+}
+
 static void
 set_resource_max(struct dmem_cgroup_pool_state *pool, u64 val)
 {
@@ -167,6 +173,11 @@ static u64 get_resource_low(struct dmem_cgroup_pool_state *pool)
 	return pool ? READ_ONCE(pool->cnt.low) : 0;
 }
 
+static u64 get_resource_high(struct dmem_cgroup_pool_state *pool)
+{
+	return pool ? READ_ONCE(pool->cnt.high) : PAGE_COUNTER_MAX;
+}
+
 static u64 get_resource_min(struct dmem_cgroup_pool_state *pool)
 {
 	return pool ? READ_ONCE(pool->cnt.min) : 0;
@@ -186,6 +197,7 @@ static void reset_all_resource_limits(struct dmem_cgroup_pool_state *rpool)
 {
 	set_resource_min(rpool, 0);
 	set_resource_low(rpool, 0);
+	set_resource_high(rpool, PAGE_COUNTER_MAX);
 	set_resource_max(rpool, PAGE_COUNTER_MAX);
 }
 
@@ -289,10 +301,13 @@ dmem_cgroup_calculate_protection(struct dmem_cgroup_pool_state *limit_pool,
  * dmem_cgroup_state_evict_valuable() - Check if we should evict from test_pool
  * @limit_pool: The pool for which we hit limits
  * @test_pool: The pool for which to test
+ * @try_high: Only evict BOs whose usage exceeds the high limit (first pass)
  * @ignore_low: Whether we have to respect low watermarks.
  * @ret_hit_low: Pointer to whether it makes sense to consider low watermark.
  *
  * This function returns true if we can evict from @test_pool, false if not.
+ * When @try_high is set, only pools with usage above their high limit are
+ * evictable, enabling prioritized eviction of over-limit cgroups.
  * When returning false and @ignore_low is false, @ret_hit_low may
  * be set to true to indicate this function can be retried with @ignore_low
  * set to true.
@@ -301,7 +316,7 @@ dmem_cgroup_calculate_protection(struct dmem_cgroup_pool_state *limit_pool,
  */
 bool dmem_cgroup_state_evict_valuable(struct dmem_cgroup_pool_state *limit_pool,
 				      struct dmem_cgroup_pool_state *test_pool,
-				      bool ignore_low, bool *ret_hit_low)
+				      bool try_high, bool ignore_low, bool *ret_hit_low)
 {
 	struct dmem_cgroup_pool_state *pool = test_pool;
 	struct page_counter *ctest;
@@ -331,9 +346,18 @@ bool dmem_cgroup_state_evict_valuable(struct dmem_cgroup_pool_state *limit_pool,
 
 	ctest = &test_pool->cnt;
 
+	used = page_counter_read(ctest);
+
+	/*
+	 * High-priority pass: only evict BOs whose cgroup is above its
+	 * dmem.high soft limit.  This implements tier-1 of the 3-pass
+	 * eviction model, ensuring over-limit cgroups are penalized first.
+	 */
+	if (try_high)
+		return used > READ_ONCE(ctest->high);
+
 	dmem_cgroup_calculate_protection(limit_pool, test_pool);
 
-	used = page_counter_read(ctest);
 	min = READ_ONCE(ctest->emin);
 
 	if (used <= min)
@@ -835,6 +859,17 @@ static ssize_t dmem_cgroup_region_low_write(struct kernfs_open_file *of,
 	return dmemcg_limit_write(of, buf, nbytes, off, set_resource_low);
 }
 
+static int dmem_cgroup_region_high_show(struct seq_file *sf, void *v)
+{
+	return dmemcg_limit_show(sf, v, get_resource_high);
+}
+
+static ssize_t dmem_cgroup_region_high_write(struct kernfs_open_file *of,
+					  char *buf, size_t nbytes, loff_t off)
+{
+	return dmemcg_limit_write(of, buf, nbytes, off, set_resource_high);
+}
+
 static int dmem_cgroup_region_max_show(struct seq_file *sf, void *v)
 {
 	return dmemcg_limit_show(sf, v, get_resource_max);
@@ -868,6 +903,12 @@ static struct cftype files[] = {
 		.seq_show = dmem_cgroup_region_low_show,
 		.flags = CFTYPE_NOT_ON_ROOT,
 	},
+	{
+		.name = "high",
+		.write = dmem_cgroup_region_high_write,
+		.seq_show = dmem_cgroup_region_high_show,
+		.flags = CFTYPE_NOT_ON_ROOT,
+	},
 	{
 		.name = "max",
 		.write = dmem_cgroup_region_max_write,

---
base-commit: ab5fce87a778cb780a05984a2ca448f2b41aafbf
change-id: 20260519-feature-dmem-high-16997148dc38

Best regards,
-- 
Qiliang Yuan <realwujing@gmail.com>

Are you using AI to write these patches? I have some suspicions. AI 
tools and helpers involved in creating patches have to be disclosed.

Regards,
Natalie

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