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(). Like get_resource_max(), get_resource_high() returns PAGE_COUNTER_MAX when the pool is NULL. Extend dmem_cgroup_state_evict_valuable() with a "try_high" parameter. When set, the function walks the page_counter parent chain to check whether any ancestor exceeds its high limit, and verifies that the pool is above its effective minimum to respect dmem.min protection. For the limit-hitting cgroup's own BOs, the ancestry check is skipped but the high threshold still applies. When CONFIG_CGROUP_DMEM is disabled, the stub returns false in try_high mode so the first pass has no effect. Refactor ttm_bo_evict_alloc() into a 3-pass eviction strategy. Pass 1 targets only BOs whose cgroup exceeds dmem.high, using a blocking lock when a ticket is available or trylock otherwise. Pass 2 falls back to the standard above-elow trylock eviction. Pass 3+ uses proper locking and repeats while making progress with the existing low-watermark fallback. Signed-off-by: Qiliang Yuan <[email protected]> --- 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(). --- V5 -> V6: - Guard the try_high dereference of test_pool->cnt with a NULL check to prevent a kernel panic during global memory pressure eviction when a BO has no associated cgroup. - Make the disabled-cgroup stub for dmem_cgroup_state_evict_valuable() return false in try_high mode so the stub does not incorrectly enable Pass 1 when CONFIG_CGROUP_DMEM=n. V4 -> V5: - Restore the original control flow in dmem_cgroup_state_evict_valuable(): test_pool is no longer dereferenced before the ancestry checks, fixing a NULL pointer dereference on BOs without a cgroup. The limit_pool NULL-to-root-cgroup resolution is now performed before the try_high block, fixing a panic during global memory pressure eviction. - Keep the try_high check for limit_pool == test_pool inside the existing early-return branch to avoid bypassing the hierarchy constraint check that prevents cross-cgroup eviction. - Use a blocking lock in Pass 1 only when a ticket is available (trylock otherwise), addressing the deadlock risk of blocking without a valid ww_acquire_ctx. - Explicitly reset trylock_only to true before Pass 2 so it does not inherit Pass 1's blocking behavior. V3 -> V4: - Use a blocking lock in Pass 1 instead of trylock to ensure over-limit cgroups are penalized even when their BOs are actively in use, as requested by Maarten Lankhorst. - Evaluate the try_high condition before the limit_pool == test_pool early-return so that the limit-hitting cgroup's own BOs are also filtered by dmem.high. - Remove the high-priority compensation retry at the start of Pass 3, which is no longer needed now that Pass 1 uses a blocking lock. V2 -> V3: - Walk the page_counter parent chain in the try_high pass to prevent child cgroups from evading the penalty when a parent cgroup exceeds its dmem.high limit. - Check dmem.min protection in the try_high pass to avoid evicting BOs below the effective minimum. - Add a properly-locked high-priority retry at the beginning of Pass 3 so that actively-used over-limit BOs (which failed trylock in Pass 1) are not skipped while innocent cgroups are evicted. - Fix get_resource_high(NULL) returning 0 instead of PAGE_COUNTER_MAX to match the behavior of get_resource_max(). 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(). v5: https://lore.kernel.org/r/[email protected] v4: https://lore.kernel.org/r/[email protected] v3: https://lore.kernel.org/r/[email protected] v2: https://lore.kernel.org/r/[email protected] v1: https://lore.kernel.org/all/[email protected] --- drivers/gpu/drm/ttm/ttm_bo.c | 33 ++++++++++++++---- include/linux/cgroup_dmem.h | 6 ++-- kernel/cgroup/dmem.c | 79 +++++++++++++++++++++++++++++++++++++++++--- 3 files changed, 105 insertions(+), 13 deletions(-) diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c index bcd76f6bb7f02..21fe34fd43eec 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 +580,47 @@ static int ttm_bo_evict_alloc(struct ttm_device *bdev, }; s64 lret; - evict_walk.walk.arg.trylock_only = true; + /* + * Pass 1 (high-priority): Evict only BOs whose cgroup exceeds its + * dmem.high soft limit. A blocking lock is used when a ticket is + * available to ensure over-limit cgroups are penalized even when + * their BOs are actively in use; trylock otherwise. + */ + evict_walk.walk.arg.trylock_only = !ticket; + 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. + */ + evict_walk.walk.arg.trylock_only = true; + 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..3f7278cb290b3 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 @@ -54,8 +54,10 @@ 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) { + if (try_high) + return false; return true; } diff --git a/kernel/cgroup/dmem.c b/kernel/cgroup/dmem.c index 4753a67d0f0f2..4267309e6b01d 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. @@ -301,15 +316,26 @@ 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; u64 used, min, low; - /* Can always evict from current pool, despite limits */ - if (limit_pool == test_pool) + /* + * When the limit-hitting cgroup's own BOs are being considered + * in try_high mode, only evict them if their pool exceeds its + * own dmem.high limit. For non-try_high mode, maintain the + * existing behavior: always evict from the limit-hitting pool. + */ + if (limit_pool == test_pool) { + if (try_high && test_pool) { + ctest = &test_pool->cnt; + used = page_counter_read(ctest); + return used > READ_ONCE(ctest->high); + } return true; + } if (limit_pool) { if (!parent_dmemcs(limit_pool->cs)) @@ -330,10 +356,38 @@ bool dmem_cgroup_state_evict_valuable(struct dmem_cgroup_pool_state *limit_pool, } ctest = &test_pool->cnt; + used = page_counter_read(ctest); + + if (try_high) { + struct page_counter *c; + + /* + * Walk the page_counter parent chain to check whether any + * ancestor cgroup exceeds its dmem.high limit. This prevents + * child cgroups from evading the penalty when a parent cgroup + * is over its high limit. + */ + if (used <= READ_ONCE(ctest->high)) { + for (c = ctest->parent; c; c = c->parent) { + if (page_counter_read(c) > READ_ONCE(c->high)) + break; + } + if (!c) + return false; + } + + /* + * Respect dmem.min protection: do not evict BOs below the + * effective minimum even during the high-priority pass. + */ + dmem_cgroup_calculate_protection(limit_pool, test_pool); + min = READ_ONCE(ctest->emin); + + return used > min; + } dmem_cgroup_calculate_protection(limit_pool, test_pool); - used = page_counter_read(ctest); min = READ_ONCE(ctest->emin); if (used <= min) @@ -835,6 +889,17 @@ 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 +933,12 @@ 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 <[email protected]>
