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

[Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>]

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@xxxxxxxxxxxxx>
> > Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> 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@xxxxxxxx>
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