Re: [PATCH 2/2] drm/ttm: optimize the pool shrinker a bit v2

[Christian König]

Just a gentle ping?

Are you ok with this explanation Andrew or should I look for a different 
approach?

Thanks,
Christian.

Am 16.04.21 um 09:08 schrieb Christian König:
> Am 15.04.21 um 22:33 schrieb Andrew Morton:
> > On Thu, 15 Apr 2021 13:56:24 +0200 "Christian König" 
> > <ckoenig.leichtzumerken@xxxxxxxxx> wrote:
> >
> > > @@ -530,6 +525,11 @@ void ttm_pool_fini(struct ttm_pool *pool)
> > >               for (j = 0; j < MAX_ORDER; ++j)
> > > ttm_pool_type_fini(&pool->caching[i].orders[j]);
> > >       }
> > > +
> > > +    /* 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.
> > > +     */
> > > +    sync_shrinkers();
> > It isn't immediately clear to me how this works. ttm_pool_fini() has
> > already freed all the pages hasn't it?  So why would it care if some
> > shrinkers are still playing with the pages?
>
> Yes ttm_pool_fini() has freed up all pages which had been in the pool 
> when the function was called.
>
> But the problem is it is possible that a parallel running shrinker has 
> taken a page from the pool and is in the process of freeing it up.
>
> When I return here the pool structure and especially the device 
> structure are freed while the parallel running shrinker is still using 
> them.
>
> I could go for a design where we have one shrinker per device instead, 
> but that would put a bit to much pressure on the pool in my opinion.
>
> > Or is it the case that ttm_pool_fini() is assuming that there will be
> > some further action against these pages, which requires that shrinkers
> > no longer be accessing the pages and which further assumes that future
> > shrinker invocations will not be able to look up these pages?
> >
> > IOW, a bit more explanation about the dynamics here would help!
>
> Sorry, I'm not a native speaker of English and sometimes still have a 
> hard time explaining things.
>
> Regards,
> Christian.