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

Qi Zheng
2023-08-23T03:00:20Z

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.arch@bytedance.com>
>> Reviewed-by: Muchun Song <songmuchun@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.vetter@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
>>
>