Re: [PATCH v4 7/8] mm: vmscan: remove shrinker_rwsem from synchronize_shrinkers()

[Christian König]

Am 09.03.23 um 08:06 schrieb Qi Zheng:
> Hi Kirill,
>
> On 2023/3/9 06:39, Kirill Tkhai wrote:
> > On 07.03.2023 09:56, Qi Zheng wrote:
> > > Now there are no readers of shrinker_rwsem, so
> > > synchronize_shrinkers() does not need to hold the
> > > writer of shrinker_rwsem to wait for all running
> > > shinkers to complete, synchronize_srcu() is enough.
> > >
> > > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> > > ---
> > >   mm/vmscan.c | 8 ++------
> > >   1 file changed, 2 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index 7aaf6f94ac1b..ac7ab4aa344f 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -796,15 +796,11 @@ 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.
> > > + * This is useful to guarantee that all shrinker invocations have 
> > > seen an
> > > + * update, before freeing memory.
> > >    */
> > >   void synchronize_shrinkers(void)
> > >   {
> > > -    down_write(&shrinker_rwsem);
> > > -    up_write(&shrinker_rwsem);
> > >       atomic_inc(&shrinker_srcu_generation);
> > >       synchronize_srcu(&shrinker_srcu);
> > >   }
> >
> > Just curious, callers of synchronize_shrinkers() don't want to have 
> > parallel register_shrinker() and unregister_shrink() are completed?
> > Here we only should wait for parallel shrink_slab(), correct?
>
> I think yes.
>
> The synchronize_shrinkers() is currently only used by TTM pool.
>
> In TTM pool, a shrinker named "drm-ttm_pool" is registered, and
> the scan_objects callback will pick an entry from its own shrinker_list:
>
> ttm_pool_shrink
> --> spin_lock(&shrinker_lock);
>     pt = list_first_entry(&shrinker_list, typeof(*pt), shrinker_list);
>     list_move_tail(&pt->shrinker_list, &shrinker_list);
>     spin_unlock(&shrinker_lock);
>
> These entries have been removed from shrinker_list before calling
> synchronize_shrinkers():
>
> ttm_pool_fini
> --> ttm_pool_type_fini
>     --> spin_lock(&shrinker_lock);
>     list_del(&pt->shrinker_list);
>     spin_unlock(&shrinker_lock);
>     synchronize_shrinkers
>
> So IIUC, we only need to wait for the parallel shrink_slab() here. Like
> its comment says:
>
> /* We removed the pool types from the LRU, but we need to also make sure
>  * that no shrinker is concurrently freeing pages from the pool.
>  */

Yes your analyses is completely correct.

I just didn't wanted to add another SRCU into the critical code paths of 
the TTM pool just for device hot plug when I have that functionality 
already.

We just make sure that no shrinker is running in parallel with 
destruction of the pool. Registering and unregistering is harmless.

Regards,
Christian.

> + CC: Christian König :)
>
> Thanks,
> Qi