Re: [Cluster-devel] [PATCH v4 43/48] drm/ttm: introduce pool_shrink_rwsem

[Qi Zheng]

Hi Daniel,

On 2023/8/22 21:56, Daniel Vetter wrote:

On Mon, Aug 07, 2023 at 07:09:31PM +0800, Qi Zheng wrote:

Currently, the synchronize_shrinkers() is only used by TTM pool. It only
requires that no shrinkers run in parallel.

After we use RCU+refcount method to implement the lockless slab shrink,
we can not use shrinker_rwsem or synchronize_rcu() to guarantee that all
shrinker invocations have seen an update before freeing memory.

So we introduce a new pool_shrink_rwsem to implement a private
synchronize_shrinkers(), so as to achieve the same purpose.

Signed-off-by: Qi Zheng <zhengqi.a...@bytedance.com>
Reviewed-by: Muchun Song <songmuc...@bytedance.com>

On the 5 drm patches (I counted 2 ttm and 3 drivers) for merging through
some other tree (since I'm assuming that's how this will land):

Yeah, there are 5 drm patches: PATCH v4 07/48 23/48 24/48 25/48 43/48.

Acked-by: Daniel Vetter <daniel.vet...@ffwll.ch>

Thanks for your review!

Qi

---
  drivers/gpu/drm/ttm/ttm_pool.c | 15 +++++++++++++++
  include/linux/shrinker.h       |  2 --
  mm/shrinker.c                  | 15 ---------------
  3 files changed, 15 insertions(+), 17 deletions(-)

diff --git a/drivers/gpu/drm/ttm/ttm_pool.c b/drivers/gpu/drm/ttm/ttm_pool.c
index c9c9618c0dce..38b4c280725c 100644
--- a/drivers/gpu/drm/ttm/ttm_pool.c
+++ b/drivers/gpu/drm/ttm/ttm_pool.c
@@ -74,6 +74,7 @@ static struct ttm_pool_type global_dma32_uncached[MAX_ORDER + 
1];
  static spinlock_t shrinker_lock;
  static struct list_head shrinker_list;
  static struct shrinker *mm_shrinker;
+static DECLARE_RWSEM(pool_shrink_rwsem);

/* Allocate pages of size 1 << order with the given gfp_flags */

  static struct page *ttm_pool_alloc_page(struct ttm_pool *pool, gfp_t 
gfp_flags,
@@ -317,6 +318,7 @@ static unsigned int ttm_pool_shrink(void)
        unsigned int num_pages;
        struct page *p;

+ down_read(&pool_shrink_rwsem);

        spin_lock(&shrinker_lock);
        pt = list_first_entry(&shrinker_list, typeof(*pt), shrinker_list);
        list_move_tail(&pt->shrinker_list, &shrinker_list);
@@ -329,6 +331,7 @@ static unsigned int ttm_pool_shrink(void)
        } else {
                num_pages = 0;
        }
+       up_read(&pool_shrink_rwsem);

return num_pages;

  }
@@ -572,6 +575,18 @@ void ttm_pool_init(struct ttm_pool *pool, struct device 
*dev,
  }
  EXPORT_SYMBOL(ttm_pool_init);

+/**

+ * synchronize_shrinkers - Wait for all running shrinkers to complete.
+ *
+ * This is useful to guarantee that all shrinker invocations have seen an
+ * update, before freeing memory, similar to rcu.
+ */
+static void synchronize_shrinkers(void)
+{
+       down_write(&pool_shrink_rwsem);
+       up_write(&pool_shrink_rwsem);
+}
+
  /**
   * ttm_pool_fini - Cleanup a pool
   *
diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index c55c07c3f0cb..025c8070dd86 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -103,8 +103,6 @@ struct shrinker *shrinker_alloc(unsigned int flags, const 
char *fmt, ...);
  void shrinker_register(struct shrinker *shrinker);
  void shrinker_free(struct shrinker *shrinker);

-extern void synchronize_shrinkers(void);

-
  #ifdef CONFIG_SHRINKER_DEBUG
  extern int __printf(2, 3) shrinker_debugfs_rename(struct shrinker *shrinker,
                                                  const char *fmt, ...);
diff --git a/mm/shrinker.c b/mm/shrinker.c
index 3ab301ff122d..a27779ed3798 100644
--- a/mm/shrinker.c
+++ b/mm/shrinker.c
@@ -650,18 +650,3 @@ void shrinker_free(struct shrinker *shrinker)
        kfree(shrinker);
  }
  EXPORT_SYMBOL_GPL(shrinker_free);
-
-/**
- * synchronize_shrinkers - Wait for all running shrinkers to complete.
- *
- * This is equivalent to calling unregister_shrink() and register_shrinker(),
- * but atomically and with less overhead. This is useful to guarantee that all
- * shrinker invocations have seen an update, before freeing memory, similar to
- * rcu.
- */
-void synchronize_shrinkers(void)
-{
-       down_write(&shrinker_rwsem);
-       up_write(&shrinker_rwsem);
-}
-EXPORT_SYMBOL(synchronize_shrinkers);
--
2.30.2