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

[Qi Zheng]

Hi Christian,

On 2023/3/9 16:11, Christian König wrote:
> 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.

That's great, thanks for confirming.

Thanks,
Qi

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