Re: [PATCH 4/5] drm/ttm: introduce pool_shrink_rwsem

[Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>]

Hi Christian,

On 2023/8/16 17:14, Christian König wrote:
> Am 16.08.23 um 10:34 schrieb Qi Zheng:
> > 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>
> > ---
> >   drivers/gpu/drm/ttm/ttm_pool.c | 15 +++++++++++++++
> >   include/linux/shrinker.h       |  1 -
> >   mm/shrinker.c                  | 15 ---------------
> >   3 files changed, 15 insertions(+), 16 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/ttm/ttm_pool.c 
> > b/drivers/gpu/drm/ttm/ttm_pool.c
> > index cddb9151d20f..713b1c0a70e1 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)
>
> Please rename that function to ttm_pool_synchronize_shrinkers().

OK, will do.

> With that done feel free to add Reviewed-by: Christian König 
> <christian.koenig@xxxxxxx>

Thanks,
Qi

> Regards,
> Christian.
>
> > +{
> > +    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 8dc15aa37410..6b5843c3b827 100644
> > --- a/include/linux/shrinker.h
> > +++ b/include/linux/shrinker.h
> > @@ -103,7 +103,6 @@ extern int __printf(2, 3) register_shrinker(struct 
> > shrinker *shrinker,
> >                           const char *fmt, ...);
> >   extern void unregister_shrinker(struct shrinker *shrinker);
> >   extern void free_prealloced_shrinker(struct shrinker *shrinker);
> > -extern void synchronize_shrinkers(void);
> >   #ifdef CONFIG_SHRINKER_DEBUG
> >   extern int __printf(2, 3) shrinker_debugfs_rename(struct shrinker 
> > *shrinker,
> > diff --git a/mm/shrinker.c b/mm/shrinker.c
> > index 043c87ccfab4..a16cd448b924 100644
> > --- a/mm/shrinker.c
> > +++ b/mm/shrinker.c
> > @@ -692,18 +692,3 @@ void unregister_shrinker(struct shrinker *shrinker)
> >       shrinker->nr_deferred = NULL;
> >   }
> >   EXPORT_SYMBOL(unregister_shrinker);
> > -
> > -/**
> > - * 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);